TY - JOUR
AU1 - Yoshimoto,, Naoko
AU2 - Saito,, Kazuki
AB - Abstract S-Alk(en)ylcysteine sulfoxides are sulfur-containing natural products characteristic of the genus Allium. Both the flavor and medicinal properties of Allium plants are attributed to a wide variety of sulfur-containing compounds that are generated from S-alk(en)ylcysteine sulfoxides. Previous radiotracer experiments proposed that S-alk(en)ylcysteine sulfoxides are biosynthesized from glutathione. The recent identification of γ-glutamyl transpeptidases and a flavin-containing S-oxygenase involved in the biosynthesis of S-allylcysteine sulfoxide (alliin) in garlic (Allium sativum) provided insights into the reaction order of deglutamylation and S-oxygenation together with the localization of the biosynthesis, although the rest of the enzymes in the pathway still await discovery. In intact plants, S-alk(en)ylcysteine sulfoxides are stored in the cytosol of storage mesophyll cells. During tissue damage, the vacuolar enzyme alliinase contacts and hydrolyzes S-alk(en)ylcysteine sulfoxides to produce the corresponding sulfenic acids, which are further converted into various sulfur-containing bioactive compounds mainly via spontaneous reactions. The formed sulfur-containing compounds exhibit bioactivities related to pathogen defense, the prevention and alleviation of cancer and cardiovascular diseases, and neuroprotection. This review summarizes the current understanding of the occurrence, biosynthesis, and alliinase-triggered chemical conversion of S-alk(en)ylcysteine sulfoxides in Allium plants as well as the impact of S-alk(en)ylcysteine sulfoxides and their derivatives on medicinal, food, and agricultural sciences. Alliin, alliinase, Allium, flavor precursor, garlic, glutathione metabolism, isoalliin, lachrymatory factor, onion, S-alk(en)ylcysteine sulfoxide Introduction The genus Allium is one of the largest plant genera and comprises ~700 species (Jones et al., 2004; Block, 2010). Allium plants have attracted humans because of their flavor, taste, therapeutic properties, and ornamental value; thus, they have been cultivated for thousands of years. Among them, garlic (Allium sativum) is one of the earliest cultivated crops that has been used as a herbal medicine across cultures. The oldest description of garlic as a herbal medicine is found in the Ebers Papyrus, which is an Egyptian medical text of herbal knowledge written in the 16th century BC. In addition to the Egyptian medical text, ancient medical documents from Greece, Rome, China, and India described the therapeutic application of garlic (Rivlin, 2001). Onion (Allium cepa) has also been cultivated since ancient times and is currently one of the most important horticultural crops recognized to have a range of health benefits (Griffiths et al., 2002; Block, 2010). Modern science has confirmed that garlic, onion, and other Allium plants exhibit a variety of medicinal effects that are related to pathogen defense, the prevention and treatment of cancer and cardiovascular diseases, neuroprotection, hepatoprotection, and anti-fatigue effects (Griffiths et al., 2002; Chauhan, 2005; Morihara et al., 2007; Iciek et al., 2009; Block, 2010; Nicastro et al., 2015; Guan et al., 2018; Zhu et al., 2018). Currently, four types of garlic-derived products with different compositions of sulfur-containing compounds are sold as dietary supplements for health promotion and complementary therapy: garlic powder; aged garlic extract; distilled oil of garlic; and garlic macerate (Block, 2010). Molecular studies of the substances responsible for the flavor and medicinal effects of Allium plants were initiated in the middle of the 20th century by the discovery of S-alk(en)ylcysteine sulfoxides, which are non-volatile secondary metabolites in Allium plants, and the enzyme called alliinase (EC 4.4.1.4), which hydrolyzes S-alk(en)ylcysteine sulfoxides (Stoll and Seebeck, 1948, 1949,a, b, 1951). In intact plants, alliinase is physically separated from S-alk(en)ylcysteine sulfoxides (Lancaster and Collin, 1981; Ellmore and Feldberg, 1994; Yamazaki et al., 2002). When tissues are damaged, alliinase cleaves the C–S bond of S-alk(en)ylcysteine sulfoxides. The formed sulfenic acids are chemically unstable and thus undergo spontaneous reactions to yield a variety of sulfur-containing compounds, which give rise to the flavor and medicinal properties of Allium plants (Rose et al., 2005). The diversity of the chemical structures and the bioactivities of the generated sulfur-containing compounds have attracted much attention from both chemists and biologists. To date, many sulfur-containing compounds derived from S-alk(en)ylcysteine sulfoxides have been isolated, and their chemical structures and bioactivities have been characterized extensively (Rose et al., 2005; Block, 2010; Nohara et al., 2017). In contrast, there has been little information on the molecular mechanisms by which Allium plants biosynthesize S-alk(en)ylcysteine sulfoxides. Although the results of radiotracer experiments and chemical analysis performed in the latter part of the 20th century have suggested that S-alk(en)ylcysteine sulfoxides are biosynthesized from glutathione (Suzuki et al., 1961, 1962; Ettala and Virtanen, 1962; Granroth, 1970; Turnbull et al., 1980; Lancaster and Shaw, 1989), the enzymes required for the synthesis of S-alk(en)ylcysteine sulfoxides have long been unidentified at the molecular level. Recently, the genes encoding γ-glutamyl transpeptidases (GGTs), which catalyze the removal of the γ-glutamyl group from the biosynthetic intermediates, and S-oxygenase, which catalyzes the conversion of the intermediate sulfide into sulfoxide, have been identified (Shaw et al., 2005; Cho et al., 2012; Yoshimoto et al., 2015a, b). The characteristics of these genes and their encoded proteins illuminated the molecular basis for the biosynthesis of S-alk(en)ylcysteine sulfoxides in part and the potential for future metabolic engineering. This paper aims to provide an overview of current knowledge on the occurrence and biosynthesis of S-alk(en)ylcysteine sulfoxides and their conversion into various sulfur-containing compounds in Allium plants, especially garlic and onion. It will also summarize the bioactivities of the sulfur-containing compounds derived from S-alk(en)ylcysteine sulfoxides and their potential applications in medical and agricultural areas. S-Alk(en)ylcysteine sulfoxides and their related compounds in Allium plants Studies to discover substances responsible for the medicinal effects and the characteristic flavor of Allium plants have been conducted beginning from the mid-20th century. The first compound reported to be the origin of the medicinal and flavor compounds in Allium plants is S-allylcysteine sulfoxide (alliin) (Fig. 1), which was purified from garlic (Stoll and Seebeck, 1948, 1951). Later studies identified three additional related compounds, S-methylcysteine sulfoxide (methiin), S-1-propenylcysteine sulfoxide (isoalliin), and S-n-propylcysteine sulfoxide (propiin), from onion (Virtanen and Matikkala, 1959; Virtanen and Spåre, 1961). To date, these four compounds have been identified as the major S-alk(en)ylcysteine sulfoxides that accumulate in Allium plants and are the primary sources of the medicinal and flavor compounds produced upon tissue damage. With regard to their distribution among Allium species, alliin and isoalliin are the major S-alk(en)ylcysteine sulfoxides in garlic and onion, respectively; methiin is widely distributed in many Allium plants and is also detected in some Brassica plants, and propiin is found at relatively low levels in onion and several other Allium plants (Fritsch and Keusgen, 2006; Block, 2010). In addition to these four compounds, S-ethylcysteine sulfoxide (ethiin) was detected in several Allium vegetables including shallot and chive at trace levels (Kubec et al., 2000), and S-n-butylcysteine sulfoxide (butiin) was isolated from the ornamental plant Allium siculum (Kubec et al., 2002) as a minor S-alk(en)ylcysteine sulfoxide. In Allium plants, S-alk(en)ylcysteine sulfoxides occur naturally mainly as their (+)-l-enantiomers with (S)-stereochemistry at the sulfur atom, except for (−)-allo alliin [(RCRS)-S-allylcysteine sulfoxide], which is the minor natural form of alliin in garlic (Block, 2010). Fig. 1. View largeDownload slide Chemical structures of S-alk(en)ylcysteine sulfoxides in the genus Allium. Major S-alk(en)ylcysteine sulfoxides include (+)-alliin [(RCSS)-S-allylcysteine sulfoxide], (+)-isoalliin [(RCSS)-S-1-propenylcysteine sulfoxide], (+)-propiin [(RCSS)-S-n-propylcysteine sulfoxide], and (+)-methiin [(RCSS)-S-methylcysteine sulfoxide]. Minor S-alk(en)ylcysteine sulfoxides include (−)-allo alliin [(RCRS)-S-allylcysteine sulfoxide], (+)-ethiin [(RCSS)-S-ethylcysteine sulfoxide], and (+)-butiin [(RCSS)-S-n-butylcysteine sulfoxide]. Fig. 1. View largeDownload slide Chemical structures of S-alk(en)ylcysteine sulfoxides in the genus Allium. Major S-alk(en)ylcysteine sulfoxides include (+)-alliin [(RCSS)-S-allylcysteine sulfoxide], (+)-isoalliin [(RCSS)-S-1-propenylcysteine sulfoxide], (+)-propiin [(RCSS)-S-n-propylcysteine sulfoxide], and (+)-methiin [(RCSS)-S-methylcysteine sulfoxide]. Minor S-alk(en)ylcysteine sulfoxides include (−)-allo alliin [(RCRS)-S-allylcysteine sulfoxide], (+)-ethiin [(RCSS)-S-ethylcysteine sulfoxide], and (+)-butiin [(RCSS)-S-n-butylcysteine sulfoxide]. In addition to S-alk(en)ylcysteine sulfoxides, Allium plants contain γ-glutamyl peptides such as γ-glutamyl-S-alk(en)ylcysteines, γ-glutamyl-S-alk(en)ylcysteine sulfoxides, and S-alk(en)ylglutathione, as sulfur-containing natural products (Granroth, 1970; Whitaker, 1976). These compounds have been suggested to be the biosynthetic intermediates of S-alk(en)ylcysteine sulfoxides but also function as reserves of sulfur and nitrogen (Jones et al., 2004; Rose et al., 2005). Selenium is chemically similar to sulfur and thus can be incorporated into metabolites that usually contain sulfur through the sulfur metabolic pathway in plants (González-Morales et al., 2017). Selenized Allium plants are known to accumulate Se-methylselenocysteine and γ-glutamyl-Se-methylselenocysteine (McSheehy et al., 2000; Dong et al., 2001; Shah et al., 2004), the selenium analogs of S-methylcysteine and γ-glutamyl-S-methylcysteine, respectively, which have been suggested to be related to methiin biosynthesis. These compounds are effective against tumors and sepsis (Whanger, 2004; Angstwurm et al., 2007; Bhattacharya, 2011). The biosynthetic pathway of S-alk(en)ylcysteine sulfoxides The biosynthetic pathway of S-alk(en)ylcysteine sulfoxides in Allium plants, especially alliin, isoalliin, and propiin, has been proposed based on the results of radiotracer experiments and chemical analysis performed in the latter part of the 20th century (Suzuki et al., 1961, 1962; Ettala and Virtanen, 1962; Granroth, 1970; Turnbull et al., 1980; Lancaster and Shaw, 1989). The origin of the cysteine moiety in alliin, isoalliin, propiin, and methiin was determined to be the cysteine residue in glutathione by studies in which plants were fed [35S]sulfate (Suzuki et al., 1961; Lancaster and Shaw, 1989). In contrast, the origin of the S-alk(en)yl group of alliin, isoalliin, and propiin was suggested to be a particular compound biosynthesized from valine by studies in which plants were fed [14C]valine (Suzuki et al., 1962; Granroth, 1970). In [35S]sulfate feeding experiments, 35S-labeled glutathione, glutathione conjugates such as S-(2-carboxypropyl)glutathione and S-methylglutathione, and γ-glutamyl peptides were formed before 35S-labeled S-alk(en)ylcysteine sulfoxides accumulated in onion, garlic, and A. siculum (Suzuki et al., 1961; Lancaster and Shaw, 1989). These observations indicate that glutathione, glutathione conjugates, and γ-glutamyl peptides are the biosynthetic intermediates of S-alk(en)ylcysteine sulfoxides. Production of radiolabeled S-(2-carboxypropyl)glutathione was observed for garlic and onion, but not for A. siculum, which contains methiin as the major S-alk(en)ylcysteine sulfoxide, suggesting that S-(2-carboxypropyl)glutathione is a biosynthetic intermediate of alliin, isoalliin, and propiin (Suzuki et al., 1961; Lancaster and Shaw, 1989). The involvement of glutathione and glutathione conjugates in the biosynthesis of S-alk(en)ylcysteine sulfoxides was also supported by the finding that garlic and onion plants fed with [14C]valine produced 14C-labeled S-(2-carboxypropyl)glutathione (Suzuki et al., 1962; Granroth, 1970). However, the [14C]valine-fed garlic and onion plants also produced 14C-labeled S-(2-carboxypropyl)cysteine (Suzuki et al., 1962; Granroth, 1970), and, moreover, onion leaf tissue rapidly converted S-(2-carboxypropyl)cysteine into isoalliin without incorporation of radiolabel in S-(2-carboxypropyl)glutathione (Granroth, 1970). These observations imply the presence of an alternative biosynthetic pathway of alliin, isoalliin, and propiin without requiring the conjugation of glutathione, in addition to the pathway via glutathione conjugate. The most plausible pathway for the biosynthesis of S-alk(en)ylcysteine sulfoxides in Allium plants at present is shown in Fig. 2. In the beginning of the biosynthetic pathway for alliin, isoalliin, and propiin, S-(2-carboxypropyl)glutathione is generated as a biosynthetic intermediate by the conjugation of glutathione to methacrylic acid or its thioester, methacrylyl-CoA (Suzuki et al., 1962; Granroth, 1970; Lancaster and Shaw, 1989). In higher plants, valine is catabolized to succinyl-CoA with methacrylyl-CoA as an intermediate (Binder et al., 2007). Feeding [14C]valine to Allium tissues resulted in the generation of radiolabeled methacrylic acid (Granroth, 1970), suggesting the in vivo conversion of methacrylyl-CoA into methacrylic acid. The conjugation of glutathione to a methacrylyl compound is a typical Michael addition of the thiol nucleophile of glutathione that may proceed spontaneously. However, the observation that the S-(2-carboxypropyl)cysteine moiety in the naturally occurring S-(2-carboxypropyl)glutathione exclusively has an (S)-configuration at C-2 (Parry and Naidu, 1983) may suggest the involvement of a specific enzyme for this reaction. After the glycyl group of S-(2-carboxypropyl)glutathione is removed, the S-2-carboxypropyl group is converted into the S-1-propenyl and/or S-2-propenyl group by oxidative decarboxylation. The resulting γ-glutamyl-S-1-propenylcysteine and γ-glutamyl-S-allylcysteine (γ-glutamyl-S-2-propenylcysteine) are the precursors of isoalliin and alliin, respectively. γ-Glutamyl-S-1-propenylcysteine and γ-glutamyl-S-allylcysteine have been suggested to be reciprocally converted by isomerization. In addition, the reduction of either the S-1-propenyl or S-2-propenyl group in these compounds theoretically yields γ-glutamyl-S-n-propylcysteine, the precursor of propiin. γ-Glutamyl-S-1-propenylcysteine, γ-glutamyl-S-allylcysteine, and γ-glutamyl-S-n-propylcysteine undergo S-oxygenation and the removal of the γ-glutamyl group to produce isoalliin, alliin, and propiin, respectively. The reaction sequence of the S-oxygenation reaction and removal of the γ-glutamyl group may differ among Allium species and S-alk(en)ylcysteine sulfoxides. The results of tracer experiments suggested that S-oxygenation probably occurs before the removal of the γ-glutamyl group in the biosynthesis of isoalliin in onion (Lancaster and Shaw, 1989), while the catalytic properties of the deglutamylation enzymes and the S-oxygenating enzyme from garlic suggested that S-oxygenation probably occurs after the removal of the γ-glutamyl group in the biosynthesis of alliin in garlic, as described later in this review (Yoshimoto et al., 2015a, b). The biosynthetic pathway for methiin is likely to be similar to those for alliin, isoalliin, and propiin. S-Methylglutathione produced by the S-methylation of glutathione undergoes deglycylation, S-oxygenation, and deglutamylation to yield methiin (Lancaster and Shaw, 1989). The origin of the S-methyl group remains unclear. Fig. 2. View largeDownload slide Putative biosynthetic pathway for alliin, isoalliin, and methiin in garlic. Fig. 2. View largeDownload slide Putative biosynthetic pathway for alliin, isoalliin, and methiin in garlic. In summary, the proposed biosynthetic pathway for S-alk(en)ylcysteine sulfoxides from glutathione includes: (i) the S-conjugation of glutathione; (ii) the removal of the glycyl group; (iii) the modification of the S-alk(en)yl group; (iv) the removal of the γ-glutamyl group; and (v) S-oxygenation. Most of these reactions are presumably catalyzed by specific enzymes. To date, only the enzymes for the removal of the γ-glutamyl group and the enzyme that catalyzes S-oxygenation have been identified at the molecular level. The functions and characteristics of these enzymes are described in the following sections. γ-Glutamyl transpeptidase in the biosynthesis of S-alk(en)ylcysteine sulfoxides The removal of the γ-glutamyl group in the biosynthesis of S-alk(en)ylcysteine sulfoxides is probably catalyzed by GGT (EC 2.3.2.2). GGT is widespread in organisms and catalyzes the transfer of the γ-glutamyl group of a broad range of γ-glutamyl compounds to acceptor molecules such as water, amino acids, and short peptides (Tate and Meister, 1981). Generally, GGT plays a key role in the degradation of glutathione and glutathione S-conjugates of xenobiotics (Grzam et al., 2007; Martin et al., 2007; Ohkama-Ohtsu et al., 2007a, b). GGT also contributes to the biosynthesis of natural products in higher organisms. For example, GGT is responsible for the conversion of leukotriene C4 into leukotriene D4 in rats (Anderson et al., 1982), and is presumably involved in the biosynthesis of camalexin in Arabidopsis thaliana (Su et al., 2011). The catalytic properties of GGT and its contribution to the biosynthesis of natural products observed in non-Allium higher organisms suggest the involvement of GGT in the biosynthesis of S-alk(en)ylcysteine sulfoxides in Allium plants. Indeed, an increase in GGT activity in onion bulbs during sprouting was associated with a decrease in the amounts of biosynthetic intermediates with a γ-glutamyl group (Lancaster and Shaw, 1991). An onion GGT, which was partially purified from sprouting bulbs, exhibited high substrate specificity for the biosynthetic intermediates of S-alk(en)ylcysteine sulfoxides (Lancaster and Shaw, 1994). In contrast, another onion GGT that was purified to homogeneity exhibited high substrate specificity for glutathione and glutathione S-conjugates but not for γ-glutamyl-S-1-propenylcysteine sulfoxide, which is a putative isoalliin biosynthetic intermediate (Shaw et al., 2005), suggesting that at least two GGT proteins with different characteristics exist in onion. In the last decade, three genes encoding GGTs (AsGGT1, AsGGT2, and AsGGT3) were identified from garlic, and the functions of the encoded proteins were analyzed (Cho et al., 2012; Yoshimoto et al., 2015a). In the latter part of the alliin biosynthetic pathway, γ-glutamyl-S-allylcysteine is converted into alliin via S-oxygenation and the removal of the γ-glutamyl group, although the sequence of the S-oxygenation and the removal of the γ-glutamyl group has long been unknown. The recombinant proteins AsGGT1, AsGGT2, and AsGGT3 showed considerable activity for the removal of the γ-glutamyl group of γ-glutamyl-S-allylcysteine to generate S-allylcysteine, with apparent Km values for γ-glutamyl-S-allylcysteine of 86 µM, 1.1 mM, and 9.4 mM, respectively. In contrast, these recombinant proteins exhibited little activity against γ-glutamyl-S-allylcysteine sulfoxide, suggesting that γ-glutamyl-S-allylcysteine is deglutamylated before being S-oxygenated in the biosynthesis of alliin in garlic (Fig. 2; Yoshimoto et al., 2015a). In addition to its involvement in alliin biosynthesis, AsGGT3 would probably function in the production of isoalliin from γ-glutamyl-S-1-propenylcysteine in garlic bulbs during cold storage (Cho et al., 2012). S-Oxygenase in the biosynthesis of S-alk(en)ylcysteine sulfoxides In the biosynthesis of S-alk(en)ylcysteine sulfoxides, the intermediate sulfides are converted into the corresponding sulfoxides by S-oxygenation. This reaction is suggested to proceed in a highly stereoselective fashion because almost all S-alk(en)ylcysteine sulfoxides in Allium plants possess (S)-stereochemistry at their sulfur atom, except for (−)-allo alliin, the minor natural form of alliin in garlic, which has (R)-stereochemistry at its sulfur atom (Block, 2010). The observations that onion leaf tissues converted S-methylcysteine, S-ethylcysteine, S-propylcysteine, and S-propenylcysteine into their corresponding sulfoxides (Granroth, 1970), and that callus tissues of onion, chive (Allium schoenoprasum), and Welsh onion (Allium fistulosum) oxygenated S-allylcysteine into (+)-alliin, although these plants do not produce (+)-alliin (Ohsumi et al., 1993), indicated that S-oxygenase for the biosynthesis of S-alk(en)ylcysteine sulfoxides has broad substrate specificity. Enzymes classified into the plant clade III flavin-containing monooxygenase (FMO) family are most likely to be responsible for the S-oxygenation reaction in the biosynthesis of S-alk(en)ylcysteine sulfoxides. FMO catalyzes the transfer of one atom from molecular O2 to a broad range of substrates that contain a soft nucleophile, using FAD as a prosthetic group and NADPH as a cofactor (Krueger and Williams, 2005; Schlaich, 2007). In general, mammals have a small number of FMOs that contribute to the detoxification of a wide spectrum of xenobiotics. Notably, some mammalian FMOs S-oxygenate S-allylcysteine to form alliin (Ripp et al., 1997; Krause et al., 2002; Novick and Elfarra, 2008). In contrast, plants possess a relatively large number of FMOs, which are classified into three clades according to the similarity of their amino acid sequences (Schlaich, 2007). Plant FMOs appear to catalyze specific oxidation steps in the biosynthesis of natural products including the plant hormone auxin (Mashiguchi et al., 2011) and the lysine catabolite N-hydroxypipecolic acid, which is an essential regulator of systemic acquired resistance to plant pathogens (Hartmann et al., 2018). Notably, the five members from clade III were identified as S-oxygenases of S-methylthioalkyl glucosinolate that contribute to the side chain modification of aliphatic glucosinolates in A. thaliana (Hansen et al., 2007; Li et al., 2008). Recently, AsFMO1, which encodes a clade III FMO sharing high sequence similarity with S-methylthioalkyl glucosinolate S-oxygenases in A. thaliana, was identified in garlic (Yoshimoto et al., 2015b). The recombinant AsFMO1 protein exhibited highly stereoselective S-oxygenation activity toward S-allylcysteine and almost exclusively produced (+)-alliin [(RCSS)-S-allylcysteine sulfoxide], which is the major natural form of alliin found in garlic, with an apparent Km value of 0.25 mM. The catalytic activity of AsFMO1 toward γ-glutamyl-S-allylcysteine was far lower than that toward S-allylcysteine. Taken together with the finding that all three garlic GGTs (AsGGT1, AsGGT2, and AsGGT3) have much higher deglutamylation activity against γ-glutamyl-S-allylcysteine than γ-glutamyl-S-allylcysteine sulfoxide (Yoshimoto et al., 2015a), it is reasonable to assume that γ-glutamyl-S-allylcysteine is first deglutamylated to produce S-allylcysteine by GGTs and subsequently S-oxygenated into (+)-alliin by the function of AsFMO1 during alliin biosynthesis in garlic (Fig. 2). Subcellular localization of the biosynthesis of S-alk(en)ylcysteine sulfoxides Currently, there is limited information about the subcellular localization of the biosynthetic pathway for S-alk(en)ylcysteine sulfoxides (Fig. 3). Cell fractionation analyses indicated that glutathione is located in both the cytosol and chloroplasts, while S-alk(en)ylcysteine sulfoxides and γ-glutamyl peptide intermediates are localized in the cytosol in onion cells (Lancaster and Collin, 1981; Lancaster et al., 1989). Although the subcellular localization of methacrylyl-CoA has not been determined in Allium plants, it has been reported that methacrylyl-CoA is synthesized from valine in mitochondria and possibly in peroxisomes in A. thaliana (Binder, 2010). Fig. 3. View largeDownload slide A schematic illustration of putative subcellular localization of enzymes and compounds in the biosynthesizing and degrading pathways for S-alk(en)ylcysteine sulfoxides in garlic. Enzymes and compounds are indicated in blue and black, respectively. Localization of compounds is surmised from the data from onion and Arabidopsis thaliana. GSH, glutathione. Fig. 3. View largeDownload slide A schematic illustration of putative subcellular localization of enzymes and compounds in the biosynthesizing and degrading pathways for S-alk(en)ylcysteine sulfoxides in garlic. Enzymes and compounds are indicated in blue and black, respectively. Localization of compounds is surmised from the data from onion and Arabidopsis thaliana. GSH, glutathione. Subcellular localization of the enzymes for alliin biosynthesis in garlic was studied by analyzing the subcellular localization of their green fluorescent protein (GFP) fusion proteins. Generally, plant GGTs are found in the vacuole and the extracellular space (Grzam et al., 2007; Ohkama-Ohtsu et al., 2007a, b). Garlic AsGGT2 has a vacuolar-targeting sequence at its N-terminus and displayed a weakly acidic pH optimum for its ability to deglutamylate γ-glutamyl-S-allylcysteine (Yoshimoto et al., 2015a). In contrast, garlic AsGGT1 and AsGGT3 were suggested to possess no signal peptide for targeting to a specific organelle in their N-terminal sequences (Yoshimoto et al., 2015a), and their subcellular localization is still unclear. Garlic AsFMO1 is most probably localized in the cytosol, as indicated by the cytosolic localization of GFP-fused AsFMO1, and produces (+)-alliin in the cytosol where it accumulates (Yoshimoto et al., 2015b). Tissue localization of the biosynthesis of S-alk(en)ylcysteine sulfoxides Tissue-specific localization of the biosynthesis of S-alk(en)ylcysteine sulfoxides is relatively well studied for alliin biosynthesis in garlic (Fig. 4). It was initially studied based on chemical analysis and was recently supported by the analysis of expression profiles of biosynthetic enzymes. In garlic plants, alliin is actively biosynthesized from glutathione predominantly in green foliage leaves before and during the initial stage of bulb formation (Ueda et al., 1991; Koch and Lawson, 1996). Because the presence of chloroplasts is necessary for the biosynthesis of alliin from glutathione (Lancaster et al., 1988), the production of alliin in white foliage leaves would be negligible. Alliin synthesized in green foliage leaves is then transported to developing bulbs through the vascular system (Ueda et al., 1991; Koch and Lawson, 1996; Bloem et al., 2004). Hence, mature garlic bulbs accumulate a large amount of alliin. Mature bulbs also accumulate γ-glutamyl-S-allylcysteine as a storage peptide in their storage leaves. During sprouting, γ-glutamyl-S-allylcysteine in the storage leaves is converted into alliin via two-step enzymatic conversion including the removal of the γ-glutamyl group and S-oxygenation (Ichikawa et al., 2006a). Alliin formed in the storage leaves is probably transported to the emerging foliage leaves (Yoshimoto et al., 2015b), thereby protecting the sprout from attack by microorganisms and herbivorous animals. Fig. 4. View largeDownload slide A schematic illustration of biosynthesis and transport of alliin and γ-glutamyl-S-allylcysteine in garlic during growth. (A) A pre-emergent clove. γ-Glutamyl-S-allylcysteine (GSAC) is abundantly stored in the storage leaf. (B) A sprouting clove. Alliin is synthesized from stored GSAC in the storage leaf and probably transported to the emerging foliage leaves. (C) A garlic plant with expanded green foliage leaves. Alliin is synthesized from glutathione (GSH) in the green foliage leaves. (D) A garlic plants with a developing bulb. Alliin and GSAC are transported from the green foliage leaves to the developing bulb for storage. Fig. 4. View largeDownload slide A schematic illustration of biosynthesis and transport of alliin and γ-glutamyl-S-allylcysteine in garlic during growth. (A) A pre-emergent clove. γ-Glutamyl-S-allylcysteine (GSAC) is abundantly stored in the storage leaf. (B) A sprouting clove. Alliin is synthesized from stored GSAC in the storage leaf and probably transported to the emerging foliage leaves. (C) A garlic plant with expanded green foliage leaves. Alliin is synthesized from glutathione (GSH) in the green foliage leaves. (D) A garlic plants with a developing bulb. Alliin and GSAC are transported from the green foliage leaves to the developing bulb for storage. The spatiotemporal contributions of AsGGT1, AsGGT2, AsGGT3, and AsFMO1 to alliin biosynthesis were proposed based on their gene expression patterns (Yoshimoto et al., 2015b). In garlic plants with green foliage leaves, AsGGT1 mRNA was abundant in green foliage leaves and roots, AsGGT2 mRNA was detected mainly in roots, AsGGT3 mRNA was accumulated in storage leaves and roots, and AsFMO1 mRNA was found in all tissues tested at similar levels. This result suggests that AsGGT1 is the most important GGT for the synthesis of alliin at this stage. The γ-glutamyl-S-allylcysteine content in green foliage leaves is ~10 µg g−1 FW, which is calculated to be 38 µM when the tissue water content is estimated to be 90% (Matsuura et al., 1996; Yoshimoto et al., 2015a). Because AsGGT1 possesses the highest affinity for γ-glutamyl-S-allylcysteine (Km=86 µM; Yoshimoto et al., 2015a) among the three garlic GGTs, it is reasonable to conclude that AsGGT1 is responsible for the conversion of γ-glutamyl-S-allylcysteine into S-allylcysteine in this tissue. The broad tissue-specific accumulation of AsFMO1 mRNA may suggest the possibility that S-allylcysteine is transported among tissues before being converted into alliin. In contrast, in pre-emergent nearly sprouting bulbs, AsGGT1, AsGGT2, AsGGT3, and AsFMO1 exhibited similar mRNA expression profiles. The mRNA levels of these genes were higher in the storage leaves than those in foliage leaves inside the bulbs (Yoshimoto et al., 2015b). The results suggested that AsGGT1, AsGGT2, AsGGT3, and AsFMO1 all contribute to the production of alliin from the abundantly stored γ-glutamyl-S-allylcysteine during sprouting. The γ-glutamyl-S-allylcysteine content is ~5 mg g−1 FW, and its concentration is calculated to be 26 mM when the tissue water content is estimated to be 65% (Matsuura et al., 1996; Ichikawa et al., 2006a, b; Yoshimoto et al., 2015a). At this concentration of γ-glutamyl-S-allylcysteine, all three GGTs, AsGGT1, AsGGT2 (Km=1.1 mM; Yoshimoto et al., 2015a), and AsGGT3 (Km=9.4 mM; Yoshimoto et al., 2015a), would efficiently produce S-allylcysteine, which is subsequently converted into alliin by AsFMO1. Overall, it seems that the distinct spatiotemporal expression patterns of the three GGTs with different affinities for γ-glutamyl-S-allylcysteine enable the optimized production of alliin in each tissue. Effects of sulfur nutrition on the biosynthesis of S-alk(en)ylcysteine sulfoxides Biosynthesis of S-alk(en)ylcysteine sulfoxides both in garlic and in onion is affected by changes in sulfur nutrition. The concentration of alliin in garlic bulbs was markedly increased by sulfur fertilization (Arnault et al., 2003; Bloem et al., 2004, 2010). Similarly, the amounts of S-alk(en)ylcysteine sulfoxides and γ-glutamyl peptide intermediates in onion bulbs were increased by sulfur fertilization (Randle et al., 1995; Bloem et al., 2004). Notably, the ratio of S-alk(en)ylcysteine sulfoxides to total sulfur-containing compounds and the ratio of isoalliin to methiine in onion were varied in response to changes in sulfur nutrition (Randle et al., 1995). The ratio of S-alk(en)ylcysteine sulfoxides to total sulfur-containing compounds was higher in sulfate-deficient conditions than that observed in sulfate-sufficient conditions (Randle et al., 1995), indicating that onion preferentially utilizes a restricted sulfur source for the biosynthesis of S-alk(en)ylcysteine sulfoxides in sulfur-limiting conditions. While isoalliin was the main S-alk(en)ylcysteine sulfoxide in onion bulbs grown under sulfate-sufficient conditions, methiin became the dominant S-alk(en)ylcysteine sulfoxide in onion bulbs that were cultivated under sulfate-deficient conditions (Randle et al., 1995). These observations imply the presence of a molecular system controlling flux in the biosynthesis of S-alk(en)ylcysteine sulfoxide in response to changes in sulfur availability. Hydrolysis of S-alk(en)ylcysteine sulfoxides to generate sulfenic acids by alliinase Intact Allium plants accumulate S-alk(en)ylcysteine sulfoxides and the enzyme alliinase (EC 4.4.1.4; Stoll and Seebeck, 1949a, b), which is a pyridoxal 5′-phosphate (PLP)-dependent C–S lyase capable of hydrolyzing S-alk(en)ylcysteine sulfoxides, in physically separated compartments. Studies including subcellular fractionation and immunostaining revealed that S-alk(en)ylcysteine sulfoxides are stored in the cytosol of storage mesophyll cells, while alliinase is sequestered in the vacuole of vascular bundle sheath cells (Fig. 3; Lancaster and Collin, 1981; Ellmore and Feldberg, 1994; Yamazaki et al., 2002). When plant tissues are disrupted by crushing or cutting, alliinase makes contact with S-alk(en)ylcysteine sulfoxides and cleaves their C–S bond, resulting in the formation of sulfenic acid, pyruvic acid, and ammonia (Fig. 5; Shimon et al., 2007). Sulfenic acids formed by the reaction are highly reactive and thus undergo additional chemical reactions through both non-enzymatic and enzymatic mechanisms, as described in later sections. In brief, in the absence of the enzyme that catalyzes the conversion of sulfenic acids, a series of sulfur-containing compounds attributed to flavor, taste, and the various health-promoting effects of Allium plants are spontaneously generated from sulfenic acids. In contrast, in the presence of the enzyme lachrymatory factor synthase (LFS), which is present in onion macerates, (E)-1-propenylsulfenic acid derived from isoalliin is enzymatically converted into (Z)-propanethial S-oxide, which promotes tear formation and is therefore called the lachrymatory factor (LF). Fig. 5. View largeDownload slide Hydrolysis of S-alk(en)ylcysteine sulfoxides by alliinase. (A) Hydrolysis of alliin in garlic. The 2-propenylsulfenic acid product spontaneously self-condenses to produce allicin. (B) Hydrolysis of isoalliin in onion. (E)-1-Propenylsulfenic acid formed from isoalliin is converted into (Z)-propanethial S-oxide, which promotes tear production, by lachrymatory factor synthase (LFS) or spontaneously self-condenses to produce di-1-propenyl thiosulfinate. Fig. 5. View largeDownload slide Hydrolysis of S-alk(en)ylcysteine sulfoxides by alliinase. (A) Hydrolysis of alliin in garlic. The 2-propenylsulfenic acid product spontaneously self-condenses to produce allicin. (B) Hydrolysis of isoalliin in onion. (E)-1-Propenylsulfenic acid formed from isoalliin is converted into (Z)-propanethial S-oxide, which promotes tear production, by lachrymatory factor synthase (LFS) or spontaneously self-condenses to produce di-1-propenyl thiosulfinate. The structural and functional properties of alliinase have been extensively studied. In the late 20th century, cDNAs encoding alliinase were cloned from several Allium plants including garlic, onion, shallot (Allium ascalonicum), and Chinese chive (Allium tuberosum) that showed that alliinase genes from Allium plants generally encode ~480 amino acid polypeptides, including an ~30 amino acid vacuolar-targeting signal peptide at the N-terminus (Van Damme et al., 1992; Manabe et al., 1998). Mature alliinase is a homodimeric glycoprotein (Tobkin and Mazelis, 1979; Nock and Mazelis, 1986) that belongs to the class I family of PLP-dependent enzymes (Schneider et al., 2000; Kuettner et al., 2002a). The enzyme is located in the vacuole and has a pH optimum of 6.5 (Rabinkov et al., 1994). Alliinase purified from garlic accepted both (+)-alliin and (−)-allo alliin as a substrate, but displayed higher activity against (+)-alliin than (−)-allo alliin (Krest and Keusgen, 1999). The crystal structure of garlic alliinase was solved at the beginning of this century and provided valuable insights into the catalytic mechanism and structural details of alliinase (Fig. 6; Kuettner et al., 2002b; Shimon et al., 2007; Weiner et al., 2009). In the absence of S-alk(en)ylcysteine sulfoxide, PLP covalently binds to Lys251 of garlic alliinase to form an internal aldimine. Upon contact, S-alk(en)ylcysteine sulfoxide displaces Lys251, forming an external aldimine with PLP. Alliinase abstracts a proton attached to the chiral carbon of S-alk(en)ylcysteine sulfoxide, and the C–S bond of S-alk(en)ylcysteine sulfoxide is simultaneously cleaved to yield sulfenic acid and a PLP-bound aminoacrylate. The PLP-bound aminoacrylate is quickly hydrolyzed to release aminoacrylate, which is spontaneously decomposed into pyruvate and ammonia. PLP reforms the internal aldimine with Lys251 and is then ready for the next reaction cycle (Shimon et al., 2007). Fig. 6. View largeDownload slide Schematic representation of the reaction cycle of garlic alliinase with alliin. The catalytic pocket of alliin is shown in thick blue curved lines. In the absence of substrate, PLP forms an internal aldimine with Lys251 of alliinase (upper left). When alliin contacts alliinase, it forms an external aldimine with PLP (upper right). Alliinase cleaves the C–S bond of alliin, releasing 2-propenylsulfenic acid, which is spontaneously converted into allicin. The PLP-bound aminoacrylate in alliinase (bottom) is hydrolyzed to release aminoacrylate, which spontaneously decomposes into pyruvic acid and ammonia. Then, alliinase and PLP again form an internal aldimine and are ready for the next reaction cycle (upper left). Fig. 6. View largeDownload slide Schematic representation of the reaction cycle of garlic alliinase with alliin. The catalytic pocket of alliin is shown in thick blue curved lines. In the absence of substrate, PLP forms an internal aldimine with Lys251 of alliinase (upper left). When alliin contacts alliinase, it forms an external aldimine with PLP (upper right). Alliinase cleaves the C–S bond of alliin, releasing 2-propenylsulfenic acid, which is spontaneously converted into allicin. The PLP-bound aminoacrylate in alliinase (bottom) is hydrolyzed to release aminoacrylate, which spontaneously decomposes into pyruvic acid and ammonia. Then, alliinase and PLP again form an internal aldimine and are ready for the next reaction cycle (upper left). It is known that the catalytic activity of PLP enzymes frequently depends on the thiol groups of their cysteine residues (Weiner et al., 2009). The crystal structure of garlic alliinase showed that 8 of the 10 cysteine residues of one alliinase subunit form four disulfide bridges (Shimon et al., 2007; Weiner et al., 2009). Of these, Cys368 and Cys376 form a disulfide bridge positioned close to the C-terminus of the enzyme and contribute to maintaining the rigidity of the catalytic domain and the relative orientations of the substrate and cofactor (Shimon et al., 2007; Weiner et al., 2009). The two free thiol groups of Cys220 and Cys350 are located far from the active site, and their chemical modification did not affect either the enzyme activity or structure, indicating that free thiol groups are not essential for alliinase activity (Weiner et al., 2009). The crystal structure of garlic alliinase also revealed the existence of branched hexasaccharide chains N-linked to Asn146 and trisaccharide chains N-linked to Asn328 (Kuettner et al., 2002b; Shimon et al., 2007). Non-enzymatic production of various sulfur-containing compounds from sulfenic acids Sulfenic acids formed from S-alk(en)ylcysteine sulfoxides by the action of alliinase are highly reactive and thus converted into thiosulfinates by self-condensation (Fig. 5). The first identified thiosulfinate derived from S-alk(en)ylcysteine sulfoxide by this reaction was diallyl thiosulfinate (allicin), which is generated from alliin (Cavallito and Bailey, 1944). Allicin is a garlic defense compound and is responsible for the characteristic flavor and pungency of freshly cut garlic (Borlinghaus et al., 2014). After the discovery of allicin and alliin, the chemistry of sulfur-containing compounds in Allium plants has been extensively studied. Thiosulfinates formed from sulfenic acids are also highly reactive species and undergo further spontaneous reactions including [3,3]-sigmatropic rearrangement, intramolecular cycloaddition, the Diels–Alder reaction, and the nucleophilic attack of sulfenic acid or other small sulfur-containing compounds to generate a variety of sulfur-containing compounds. The formed compounds include acyclic sulfides, dithiines, and thiolanes, to most of which the various bioactivities of Allium plants are attributable (Fig. 7; Block, 2010; Nohara et al., 2017; Block et al., 2018; Kubec et al., 2018). Fig. 7. View largeDownload slide Representative sulfur-containing compounds spontaneously generated from thiosulfinates in garlic. Fig. 7. View largeDownload slide Representative sulfur-containing compounds spontaneously generated from thiosulfinates in garlic. Thiosulfinates are also converted into colored compounds. When Allium tissues are mechanically disrupted, garlic often tends to turn blue-green in color (Joslyn and Sano, 1956), while onion and leek (Allium ampeloprasum var. porrum) may turn red-pink in color (Joslyn and Peterson, 1958; Körner and Berk, 1967) within several hours. Although the colors formed by this process are markedly different, the mechanisms for the discoloration of garlic and onion/leek are similar (Kubec et al., 2004, 2017; Imai et al., 2006; Kato et al., 2013). 1-Propenyl group-containing thiosulfinates, which are produced by the self-condensation of 1-propenyl sulfenic acids or the intermolecular condensation of 1-propenyl sulfenic acid and other sulfenic acids, react with amino acids to form N-substituted 3,4-dimethylpyrroles. The formed pyrroles are known as pigment precursors and react further with various naturally occurring (thio)carbonyl compounds to generate colored compounds (Kubec et al., 2004, 2017; Imai et al., 2006; Kato et al., 2013). In garlic macerate, purple/blue pigments and yellow pigments are formed in parallel, resulting in a green tone (Kubec and Velíšek, 2007; Kubec et al., 2017), while onion macerate forms pink pigments (Kato et al., 2013). Because in most cases discoloration lowers the market value of processed Allium vegetables, it is a severe problem for food processing companies. Production of the LF from (E)-1-propenylsulfenic acid in onion The substance that causes tear production when cutting onions is called LF and was identified as (Z)-propanethial S-oxide (Brodnitz and Pascale, 1971). It has long been believed that onion LF is non-enzymatically generated from (E)-1-propenylsulfenic acid, which is produced from isoalliin by alliinase. In 2002, LFS was reported to be the enzyme responsible for the conversion of (E)-1-propenylsulfenic acid to LF (Fig. 5B; Imai et al., 2002). For this work, the authors received the 2013 Ig Nobel Prize in Chemistry. Onion LFS produces LF from (E)-1-propenylsulfenic acid that is derived from the natural form of isoalliin but not from its unnatural isomer, (Z)-1-propenylsulfenic acid, indicating that the enzyme is an (E)-1-propenylsulfenic acid isomerase (Masamura et al., 2012). Based on chemical and biochemical analyses, it has been proposed that onion LFS promotes an intramolecular proton substitution between the oxygen (atom 1) and the carbon (atom 4) atoms of (E)-1-propenylsulfenic acid, which is basically equivalent to a [1,4]-sigmatropic rearrangement (Block et al., 1996; Masamura et al., 2012). However, the recently solved crystal structure of LFS suggested that the alternative mechanism involving sequential proton transfer accompanied by the formation of a carbanion intermediate is most likely (Silvaroli et al., 2017). When onion tissues are disrupted, most of the (E)-1-propenylsulfenic acid formed from isoalliin by alliinase is quickly converted into LF before it self-condenses into di-1-propenyl thiosulfinate, an isomer of allicin. With the aim of producing a tearless onion, the expression of the LFS gene was suppressed by RNAi (Eady et al., 2008). In fresh macerates of LF-silenced onion, LF levels were dramatically reduced, as expected. In these macerates, (E)-1-propenylsulfenic acid generated from isoalliin is mainly converted into di-1-propenyl thiosulfinate, which is further converted into thiolane-type compounds through spontaneous reactions (Eady et al., 2008; Aoyagi et al., 2011). The macerates of LF-silenced onion showed reduced cyclooxygenase-1 activity, α-glucosidase activity, and platelet aggregation (Aoyagi et al., 2011; Thomson et al., 2013). Therefore, LF-silenced or LF knockout onion produced by gene-editing technology is a valuable resource not only to prevent irritation when cooking in the kitchen and processing in food companies but also to promote human health. Tearless onions with reduced LF levels were also generated non-transgenically by irradiating onion seeds with neon ions; however, the decrease in LF levels in this onion was caused by the reduction of alliinase activity, and this onion therefore showed non-pungent properties (Kato et al., 2016). Bioactivities of sulfur-containing compounds in Allium plants There have been extensive studies and reviews on the bioactivities of sulfur-containing compounds characteristic of Allium plants. Most of the identified biological activities of Allium-derived sulfur-containing compounds are related to pathogen defense, the prevention and treatment of cancer and cardiovascular diseases, and neuroprotection. Therefore, Allium-derived sulfur-containing compounds may be applicable to a wide range of medical therapies and to agriculture. Here, we briefly summarize several representative garlic- and onion-derived sulfur-containing compounds with useful bioactivities (Table 1). Table 1. Bioactivities of representative Allium-derived sulfur-containing compounds Compound Representative plant origin Bioactivity Reference Allicin Garlic (freshly cut slice) Antibacterial effect Antifungal effect Anticancer effect Cholesterol-lowering effect Antihypertensive effect Powolny and Singh (2008); ,Li et al. (2012); ,Borlinghaus et al. (2014); ,Shouk et al. (2014) Diallyl trisulfide (DATS) Garlic (distilled oil) Anticancer Powolny and Singh (2008); ,Busch et al. (2010); ,Puccinelli and Stan (2017) Diallyl disulfide (DADS) Garlic (distilled oil) Anticancer Powolny and Singh (2008); ,Yi and Su (2013) Diallyl sulfide (DAS) Garlic (distilled oil) Anticancer Powolny and Singh (2008) Ajoene Garlic (macerate) Antibacterial effect Antiviral effect Antithrombosis effect Cholesterol-lowering effect Anticancer effect Hassan (2004); Powolny and Singh (2008); ,Block (2010); ,Kaschula et al. (2010) Onionin A1 Onion (acetone extract) Regulation of macrophage polarization El-Aasr et al. (2010); ,Nohara et al. (2017) S-Allylcysteine Garlic (aged extract) Anticancer effect Cholesterol-lowering effect Antioxidant effect Anti-inflammation Neuroprotection Thomson and Ali (2003); ,Ray et al. (2011); ,Yeh and Liu (2001); ,Colín- González et al. (2012, 2015) S-1-Propenylcysteine Garlic (aged extract) Immunomodulation Antihypertensive effect Suzuki et al. (2016); ,Kodera et al. (2017); ,Matsutomo et al. (2017) Compound Representative plant origin Bioactivity Reference Allicin Garlic (freshly cut slice) Antibacterial effect Antifungal effect Anticancer effect Cholesterol-lowering effect Antihypertensive effect Powolny and Singh (2008); ,Li et al. (2012); ,Borlinghaus et al. (2014); ,Shouk et al. (2014) Diallyl trisulfide (DATS) Garlic (distilled oil) Anticancer Powolny and Singh (2008); ,Busch et al. (2010); ,Puccinelli and Stan (2017) Diallyl disulfide (DADS) Garlic (distilled oil) Anticancer Powolny and Singh (2008); ,Yi and Su (2013) Diallyl sulfide (DAS) Garlic (distilled oil) Anticancer Powolny and Singh (2008) Ajoene Garlic (macerate) Antibacterial effect Antiviral effect Antithrombosis effect Cholesterol-lowering effect Anticancer effect Hassan (2004); Powolny and Singh (2008); ,Block (2010); ,Kaschula et al. (2010) Onionin A1 Onion (acetone extract) Regulation of macrophage polarization El-Aasr et al. (2010); ,Nohara et al. (2017) S-Allylcysteine Garlic (aged extract) Anticancer effect Cholesterol-lowering effect Antioxidant effect Anti-inflammation Neuroprotection Thomson and Ali (2003); ,Ray et al. (2011); ,Yeh and Liu (2001); ,Colín- González et al. (2012, 2015) S-1-Propenylcysteine Garlic (aged extract) Immunomodulation Antihypertensive effect Suzuki et al. (2016); ,Kodera et al. (2017); ,Matsutomo et al. (2017) View Large Table 1. Bioactivities of representative Allium-derived sulfur-containing compounds Compound Representative plant origin Bioactivity Reference Allicin Garlic (freshly cut slice) Antibacterial effect Antifungal effect Anticancer effect Cholesterol-lowering effect Antihypertensive effect Powolny and Singh (2008); ,Li et al. (2012); ,Borlinghaus et al. (2014); ,Shouk et al. (2014) Diallyl trisulfide (DATS) Garlic (distilled oil) Anticancer Powolny and Singh (2008); ,Busch et al. (2010); ,Puccinelli and Stan (2017) Diallyl disulfide (DADS) Garlic (distilled oil) Anticancer Powolny and Singh (2008); ,Yi and Su (2013) Diallyl sulfide (DAS) Garlic (distilled oil) Anticancer Powolny and Singh (2008) Ajoene Garlic (macerate) Antibacterial effect Antiviral effect Antithrombosis effect Cholesterol-lowering effect Anticancer effect Hassan (2004); Powolny and Singh (2008); ,Block (2010); ,Kaschula et al. (2010) Onionin A1 Onion (acetone extract) Regulation of macrophage polarization El-Aasr et al. (2010); ,Nohara et al. (2017) S-Allylcysteine Garlic (aged extract) Anticancer effect Cholesterol-lowering effect Antioxidant effect Anti-inflammation Neuroprotection Thomson and Ali (2003); ,Ray et al. (2011); ,Yeh and Liu (2001); ,Colín- González et al. (2012, 2015) S-1-Propenylcysteine Garlic (aged extract) Immunomodulation Antihypertensive effect Suzuki et al. (2016); ,Kodera et al. (2017); ,Matsutomo et al. (2017) Compound Representative plant origin Bioactivity Reference Allicin Garlic (freshly cut slice) Antibacterial effect Antifungal effect Anticancer effect Cholesterol-lowering effect Antihypertensive effect Powolny and Singh (2008); ,Li et al. (2012); ,Borlinghaus et al. (2014); ,Shouk et al. (2014) Diallyl trisulfide (DATS) Garlic (distilled oil) Anticancer Powolny and Singh (2008); ,Busch et al. (2010); ,Puccinelli and Stan (2017) Diallyl disulfide (DADS) Garlic (distilled oil) Anticancer Powolny and Singh (2008); ,Yi and Su (2013) Diallyl sulfide (DAS) Garlic (distilled oil) Anticancer Powolny and Singh (2008) Ajoene Garlic (macerate) Antibacterial effect Antiviral effect Antithrombosis effect Cholesterol-lowering effect Anticancer effect Hassan (2004); Powolny and Singh (2008); ,Block (2010); ,Kaschula et al. (2010) Onionin A1 Onion (acetone extract) Regulation of macrophage polarization El-Aasr et al. (2010); ,Nohara et al. (2017) S-Allylcysteine Garlic (aged extract) Anticancer effect Cholesterol-lowering effect Antioxidant effect Anti-inflammation Neuroprotection Thomson and Ali (2003); ,Ray et al. (2011); ,Yeh and Liu (2001); ,Colín- González et al. (2012, 2015) S-1-Propenylcysteine Garlic (aged extract) Immunomodulation Antihypertensive effect Suzuki et al. (2016); ,Kodera et al. (2017); ,Matsutomo et al. (2017) View Large Allicin is the primary defense compound for garlic (Borlinghaus et al., 2014). It can kill or inhibit the proliferation of a broad spectrum of bacteria and fungi (Borlinghaus et al., 2014). Allicin induces cell death and inhibits the proliferation of mammalian cells including cancer cells (Powolny and Singh, 2008). When applied to mammals at sublethal concentrations, allicin exhibits cholesterol-lowering and antihypertensive effects, and is thus valuable for the prevention and alleviation of cardiovascular diseases (Borlinghaus et al., 2014; Shouk et al., 2014). It is also effective in attenuating age-related cognitive and memory deficits (Li et al., 2012). Most of the activities of allicin are thought to be mediated via redox-dependent mechanisms (Borlinghaus et al., 2014). Diallyl trisulfide (DATS), diallyl disulfide (DADS), and diallyl sulfide (DAS) are acyclic sulfides formed from allicin. These compounds exhibit anticancer effects via multitargeted mechanisms. For example, apoptosis is induced in cancer cells by DATS, DADS, and DAS, cancer cell cycle progression is inhibited by DATS and DADS, angiogenesis is inhibited by DATS and DADS, histone acetylation is modified by DADS, and carcinogen metabolism is modified by DADS and DAS (Powolny and Singh, 2008; Busch et al., 2010; Yi and Su, 2013; Puccinelli and Stan, 2017). Ajoene is one of the main sulfur-containing compounds generated as a mixture of E- and Z-isomers from allicin when crushed garlic is heated (Kaschula et al., 2010). Ajoene displays a wide range of biological activities such as antibacterial, antiviral, antithrombosis, cholesterol-lowering, and anticancer effects (Block, 2010). The cytotoxicity of ajoene against cancer cells is mediated via multiple mechanisms including the alteration of carcinogen metabolism, cell cycle arrest, apoptosis induction, and angiogenesis suppression (Hassan, 2004; Powolny and Singh, 2008; Kaschula et al., 2010). The Z-isomer of ajoene is moderately more effective than the E-isomer in tumor growth inhibition (Kaschula et al., 2010). Thiolane-type cyclic sulfides are spontaneously generated from thiosulfinates (Nohara et al., 2017; Block et al., 2018; Kubec et al., 2018). Among them, onionin A1, which was isolated from onions, shows promising anticancer activity by modulating macrophage polarization (El-Aasr et al., 2010). Tumor-associated macrophages are macrophages that infiltrate cancer tissues and are involved in the development of the cancer microenvironment (Lewis and Pollard, 2006; Sica et al., 2006). Tumor-associated macrophages have anti-inflammatory activity and thus belong to the alternatively activated class of macrophages (M2) (Gordon, 2003; Hagemann et al., 2009). Onionin A1 inhibits M2 macrophage polarization, thereby suppressing cancer cell proliferation and metastasis (El-Aasr et al., 2010). Garlic extract containing onionin A1-like thiolane-type compounds also showed the ability to control macrophage polarization (Nohara et al., 2017). The chemical structure of onionin A1 was initially proposed to be a cyclic sulfoxide-type compound by El-Aasr et al. (2010) but was recently reassigned as a thiolane-type compound by Block et al. (2018). S-Allylcysteine is a probable biosynthetic intermediate of alliin in garlic, as described above. Although the content of S-allylcysteine in fresh garlic cloves is <0.026 mg g–1 FW (Koch and Lawson, 1996), it is the most abundant organosulfur compound in aged garlic extract (Colín-González et al., 2015) and its concentration in aged garlic extract was reported as 3.95 mg g–1 (Matsutomo et al., 2017). S-Allylcysteine has anticancer (Thomson and Ali, 2003) and cholesterol-lowering effects (Yeh and Liu, 2001). S-Allylcysteine also exhibits a broad spectrum of protective actions including antioxidant, redox-modulatory, and anti-inflammatory activities (Colín-González et al., 2012,, 2015). In this way, it functions as a neuroprotective molecule and is thus potentially applicable to the prevention and treatment of Alzheimer’s disease (Ray et al., 2011). Notably, S-allylcysteine was well absorbed after oral administration in rats and dogs with a bioavailability of 92% (Amano et al., 2015). Therefore, S-allylcysteine is one of the most important orally active components of aged garlic extract with a variety of health-promoting activities. S-1-Propenylcysteine, a stereoisomer of S-allylcysteine, is found at trace levels (<0.006 mg g–1 FW) in fresh garlic cloves (Sugii et al., 1963; Koch and Lawson, 1996). During the aging process of garlic extract, its concentration increases up to levels similar to those of S-allylcysteine (Kodera et al., 2017). The concentration of S-1-propenylcysteine in aged garlic extract was reported as 3.25 mg g–1 (Matsutomo et al., 2017). S-1-Propenylcysteine was readily absorbed after oral administration in rats and dogs, with a bioavailability of 88% and 100%, respectively (Amano et al., 2016), and shows immunomodulatory and antihypertensive effects (Suzuki et al., 2016; Kodera et al., 2017; Matsutomo et al., 2017). These observations suggest that S-1-propenylcysteine is an orally active component of aged garlic extract that is effective in the prevention and alleviation of cardiovascular and immune diseases (Kodera et al., 2017). In situ generation of allicin for clinical and agricultural applications As mentioned above, allicin possesses a range of anticancer, antibacterial, and antifungal activities. However, its clinical and agricultural use is somewhat difficult due to its low chemical stability at room temperature and poor water solubility. Some efforts have been made to generate allicin in situ to solve these problems. With the aim of clinical application, alliinase is conjugated to a monoclonal antibody that recognizes target cells or to a chemical substance recognized by target cells. In the presence of alliin, target cell-localized alliinase generates allicin, which efficiently kills the target cells. For cancer therapy, antibody–alliinase conjugates that target gastric adenocarcinoma cells (Miron et al., 2003) and B-chronic lymphocytic leukemia tumor cells (Arditti et al., 2005), and the conjugate of alliinase and daidzein, a plant isoflavone acting as a weak estrogen, that targets ovarian cancer cells (Appel et al., 2011) were developed. To treat invasive pulmonary aspergillosis, antibody–alliinase conjugates that target Aspergillus fumigatus were created (Appel et al., 2010). In situ alliin generation is also useful to control plant diseases that are caused by pathogens. The binary system consisting of alliinase and alliin prevented germination and appressorium formation in the spores of the highly damaging rice blast fungus Magnaporthe grisea (Fry et al., 2005). This system is potentially applicable to a broad spectrum of pathogenic fungi and bacteria that exhibit sensitivity to allicin. Because of the low toxicity toward humans and low chemical stability of allicin, this system would be an intriguing alternative to modern agricultural chemicals that is usable at a relatively low cost. Future prospects After the discovery of S-alk(en)ylcysteine sulfoxides and alliinase in the middle of the 20th century, numerous studies have been performed to elucidate the chemical structure and properties, mechanism of generation, bioactivity, and pharmacokinetic properties of sulfur-containing compounds, which are produced from S-alk(en)ylcysteine sulfoxides. These efforts have been made with the aim of exploring novel therapeutic agents that are applicable for the prevention and treatment of various human diseases. In contrast, the molecular mechanism for the biosynthesis of S-alk(en)ylcysteine sulfoxides in Allium plants has been relatively unexplored, although a pathway for their biosynthesis has been proposed based on chemical and radiotracer experiments conducted since the early 1960s. The recent identification and characterization of genes for the GGT and S-oxygenase involved in the biosynthesis of S-alk(en)ylcysteine sulfoxides have led to breakthroughs in understanding the molecular basis underlying the production of S-alk(en)ylcysteine sulfoxides in Allium plants. However, enzymes catalyzing the S-conjugation of glutathione, removal of the glycyl group, and the modification of the S-alk(en)yl group and transport proteins facilitating the interorgan movement of S-alk(en)ylcysteine sulfoxides and their biosynthetic intermediates still await identification. Signaling and regulatory networks that control the biosynthetic flux and translocation of S-alk(en)ylcysteine sulfoxides during plant development and in response to changes in the sulfur environment are areas that also require further research. Knowledge of the metabolism and transport of cysteine, γ-glutamylcysteine, glutathione, and glutathione-derived sulfur-containing secondary metabolites in other plants, including glucosinolates and camalexin in A. thaliana, will assist in the molecular understanding of these currently undetermined processes. Moreover, omics technologies and systems biology will be powerful approaches for identifying the components involved in these processes. Molecular understanding of the biosynthesis, transport, and regulation of these compounds will not only provide insights into our basic knowledge but also facilitate the future metabolic engineering of plants using transgenic and gene-editing technologies. Abbreviations Abbreviations DADS diallyl disulfide DAS diallyl sulfide DATS diallyl trisulfide FMO flavin-containing monooxygenase GGT γ-glutamyl transpeptidase LF lachrymatory factor LFS lachrymatory factor synthase PLP pyridoxal 5′-phosphate Acknowledgements This work was supported in part by JSPS KAKENHI grant number 17K08332 and a Lotte Research Promotion grant (to NY). References Amano H , Kazamori D , Itoh K . 2016 . Pharmacokinetics and N-acetylation metabolism of S-methyl-l -cysteine and trans-1-propenyl- l -cysteine in rats and dogs . Xenobiotica 46 , 1017 – 1025 . Google Scholar Crossref Search ADS PubMed WorldCat Amano H , Kazamori D , Itoh K , Kodera Y . 2015 . Metabolism, excretion, and pharmacokinetics of S-allyl- l -cysteine in rats and dogs . Drug Metabolism and Disposition 43 , 749 – 755 . Google Scholar Crossref Search ADS PubMed WorldCat Anderson ME , Allison RD , Meister A . 1982 . Interconversion of leukotrienes catalyzed by purified γ-glutamyl transpeptidase: concomitant formation of leukotriene D4 and γ-glutamyl amino acids . Proceedings of the National Academy of Sciences, USA 79 , 1088 – 1091 . Google Scholar Crossref Search ADS WorldCat Angstwurm MW , Engelmann L , Zimmermann T , et al. . 2007 . Selenium in Intensive Care (SIC): results of a prospective randomized, placebo-controlled, multiple-center study in patients with severe systemic inflammatory response syndrome, sepsis, and septic shock . Critical Care Medicine 35 , 118 – 126 . Google Scholar Crossref Search ADS PubMed WorldCat Aoyagi M , Kamoi T , Kato M , Sasako H , Tsuge N , Imai S . 2011 . Structure and bioactivity of thiosulfinates resulting from suppression of lachrymatory factor synthase in onion . Journal of Agricultural and Food Chemistry 59 , 10893 – 10900 . Google Scholar Crossref Search ADS PubMed WorldCat Appel E , Rabinkov A , Neeman M , Kohen F , Mirelman D . 2011 . Conjugates of daidzein–alliinase as a targeted pro-drug enzyme system against ovarian carcinoma . Journal of Drug Targeting 19 , 326 – 335 . Google Scholar Crossref Search ADS PubMed WorldCat Appel E , Vallon-Eberhard A , Rabinkov A , Brenner O , Shin I , Sasson K , Shadkchan Y , Osherov N , Jung S , Mirelman D . 2010 . Therapy of murine pulmonary aspergillosis with antibody–alliinase conjugates and alliin . Antimicrobial Agents and Chemotherapy 54 , 898 – 906 . Google Scholar Crossref Search ADS PubMed WorldCat Arditti FD , Rabinkov A , Miron T , Reisner Y , Berrebi A , Wilchek M , Mirelman D . 2005 . Apoptotic killing of B-chronic lymphocytic leukemia tumor cells by allicin generated in situ using a rituximab–alliinase conjugate . Molecular Cancer Therapeutics 4 , 325 – 331 . Google Scholar PubMed WorldCat Arnault I , Christidès JP , Mandon N , Haffner T , Kahane R , Auger J . 2003 . High-performance ion-pair chromatography method for simultaneous analysis of alliin, deoxyalliin, allicin and dipeptide precursors in garlic products using multiple mass spectrometry and UV detection . Journal of Chromatography. A 991 , 69 – 75 . Google Scholar Crossref Search ADS PubMed WorldCat Bhattacharya A . 2011 . Methylselenocysteine: a promising antiangiogenic agent for overcoming drug delivery barriers in solid malignancies for therapeutic synergy with anticancer drugs . Expert Opinion on Drug Delivery 8 , 749 – 763 . Google Scholar Crossref Search ADS PubMed WorldCat Binder S . 2010 . Branched-chain amino acid metabolism in Arabidopsis thaliana . The Arabidopsis Book 8 , e0137 . Google Scholar Crossref Search ADS PubMed WorldCat Binder S , Knill T , Schuster J . 2007 . Branched-chain amino acid metabolism in higher plants . Physiologia Plantarum 129 , 68 – 78 . Google Scholar Crossref Search ADS WorldCat Block E . 2010 . Garlic and other alliums: the lore and the science . Cambridge : The Royal Society of Chemistry . Google Preview WorldCat COPAC Block E , Dethier B , Bechand B , et al. . 2018 . Ajothiolanes: 3,4-dimethylthiolane natural products from garlic (Allium sativum) . Journal of Agricultural and Food Chemistry 66 , 10193 – 10204 . Google Scholar Crossref Search ADS PubMed WorldCat Block E , Gillies JZ , Gillies CW , Bazzi AA , Putman D , Revelle LK , Wang D , Zhang X . 1996 . Allium chemistry: microwave spectroscopic identification, mechanism of formation, synthesis, and reactions of (E,Z)-propanethial S-oxide, the lachrymatory factor of the onion (Allium cepa) . Journal of the American Chemical Society 118 , 7492 – 7501 . Google Scholar Crossref Search ADS WorldCat Bloem E , Haneklaus S , Schnug E . 2004 . Influence of nitrogen and sulfur fertilization on the alliin content of onions and garlic . Journal of Plant Nutrition 27 , 1827 – 1839 . Google Scholar Crossref Search ADS WorldCat Bloem E , Haneklaus S , Schnug E . 2010 . Influence of fertilizer practices on containing metabolites in garlic (Allium sativum L.) under field conditions . Journal of Agricultural and Food Chemistry 58 , 10690 – 10696 . Google Scholar Crossref Search ADS PubMed WorldCat Borlinghaus J , Albrecht F , Gruhlke MC , Nwachukwu ID , Slusarenko AJ . 2014 . Allicin: chemistry and biological properties . Molecules 19 , 12591 – 12618 . Google Scholar Crossref Search ADS PubMed WorldCat Brodnitz MH , Pascale JV . 1971 . Thiopropanal S-oxide: a lachrymatory factor in onions . Journal of Agricultural and Food Chemistry 19 , 269 – 272 . Google Scholar Crossref Search ADS PubMed WorldCat Busch C , Jacob C , Anwar A , Burkholz T , Aicha Ba L , Cerella C , Diederich M , Brandt W , Wessjohann L , Montenarh M . 2010 . Diallylpolysulfides induce growth arrest and apoptosis . International Journal of Oncology 36 , 743 – 749 . Google Scholar PubMed WorldCat Cavallito CJ , Bailey JH . 1944 . Allicin, the antibacterial principle of Allium sativum. I. Isolation, physical properties and antibacterial action . Journal of the American Chemical Society 66 , 1950 – 1951 . Google Scholar Crossref Search ADS WorldCat Chauhan NB . 2005 . Multiplicity of garlic health effects and Alzheimer’s disease . Journal of Nutrition, Health & Aging 9 , 421 – 432 . WorldCat Cho J , Park M , Choi D , Lee SK . 2012 . Cloning and expression of γ-glutamyl transpeptidase and its relationship to greening in crushed garlic (Allium sativum) cloves . Journal of the Science of Food and Agriculture 92 , 253 – 257 . Google Scholar Crossref Search ADS PubMed WorldCat Colín-González AL , Ali SF , Túnez I , Santamaría A . 2015 . On the antioxidant, neuroprotective and anti-inflammatory properties of S-allyl cysteine: an update . Neurochemistry International 89 , 83 – 91 . Google Scholar Crossref Search ADS PubMed WorldCat Colín-González AL , Santana RA , Silva-Islas CA , Chánez-Cárdenas ME , Santamaría A , Maldonado PD . 2012 . The antioxidant mechanisms underlying the aged garlic extract- and S-allylcysteine-induced protection . Oxidative Medicine and Cellular Longevity 2012 , 907162 . Google Scholar Crossref Search ADS PubMed WorldCat Dong Y , Lisk D , Block E , Ip C . 2001 . Characterization of the biological activity of γ-glutamyl-Se-methylselenocysteine: a novel, naturally occurring anticancer agent from garlic . Cancer Research 61 , 2923 – 2928 . Google Scholar PubMed WorldCat Eady CC , Kamoi T , Kato M , Porter NG , Davis S , Shaw M , Kamoi A , Imai S . 2008 . Silencing onion lachrymatory factor synthase causes a significant change in the sulfur secondary metabolite profile . Plant Physiology 147 , 2096 – 2106 . Google Scholar Crossref Search ADS PubMed WorldCat El-Aasr M , Fujiwara Y , Takeya M , et al. . 2010 . Onionin A from Allium cepa inhibits macrophage activation . Journal of Natural Products 73 , 1306 – 1308 . Google Scholar Crossref Search ADS PubMed WorldCat Ellmore GS , Feldberg RS . 1994 . Alliin lyase localization in bundle sheaths of the garlic clove (Allium sativum) . American Journal of Botany 81 , 89 – 94 . Google Scholar Crossref Search ADS WorldCat Ettala T , Virtanen AI . 1962 . Labeling of sulfur-containing amino acids and γ-glutamylpeptides after injection of labeled sulfate into onion (Allium cepa) . Acta Chemica Scandinavica 16 , 2061 – 2063 . Google Scholar Crossref Search ADS WorldCat Fritsch RM , Keusgen M . 2006 . Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae) . Phytochemistry 67 , 1127 – 1135 . Google Scholar Crossref Search ADS PubMed WorldCat Fry FH , Okarter N , Baynton-Smith C , Kershaw MJ , Talbot NJ , Jacob C . 2005 . Use of a substrate/alliinase combination to generate antifungal activity in situ . Journal of Agricultural and Food Chemistry 53 , 574 – 580 . Google Scholar Crossref Search ADS PubMed WorldCat González-Morales S , Pérez-Labrada F , García-Enciso EL , Leija-Martínez P , Medrano-Macías J , Dávila-Rangel IE , Juárez-Maldonado A , RivaMartínez EN , BenavideMendoza A . 2017 . Selenium and sulfur to produce Allium functional crops . Molecules 22 , 558 . Google Scholar Crossref Search ADS WorldCat Gordon S . 2003 . Alternative activation of macrophages . Nature Reviews. Immunology 3 , 23 – 35 . Google Scholar Crossref Search ADS PubMed WorldCat Granroth B . 1970 . Biosynthesis and decomposition of cysteine derivatives in onion and other Allium species . Annales Academiae Scientiarum Fennicae. Series A2 154 , 1 – 71 . WorldCat Griffiths G , Trueman L , Crowther T , Thomas B , Smith B . 2002 . Onions—a global benefit to health . Phytotherapy Research 16 , 603 – 615 . Google Scholar Crossref Search ADS PubMed WorldCat Grzam A , Martin MN , Hell R , Meyer AJ . 2007 . γ-Glutamyl transpeptidase GGT4 initiates vacuolar degradation of glutathione conjugates in Arabidopsis . FEBS Letters 581 , 3131 – 3138 . Google Scholar Crossref Search ADS PubMed WorldCat Guan MJ , Zhao N , Xie KQ , Zeng T . 2018 . Hepatoprotective effects of garlic against ethanol-induced liver injury: a mini-review . Food and Chemical Toxicology 111 , 467 – 473 . Google Scholar Crossref Search ADS PubMed WorldCat Hagemann T , Biswas SK , Lawrence T , Sica A , Lewis CE . 2009 . Regulation of macrophage function in tumors: the multifaceted role of NF-κB . Blood 113 , 3139 – 3146 . Google Scholar Crossref Search ADS PubMed WorldCat Hansen BG , Kliebenstein DJ , Halkier BA . 2007 . Identification of a flavin-monooxygenase as the oxygenating enzyme in aliphatic glucosinolate biosynthesis in Arabidopsis . The Plant Journal 50 , 902 – 910 . Google Scholar Crossref Search ADS PubMed WorldCat Hartmann M , Zeier T , Bernsdorff F , et al. . 2018 . Flavin monooxygenase-generated N-hydroxypipecolic acid is a critical element of plant systemic immunity . Cell 173 , 456 – 469.e16 . Google Scholar Crossref Search ADS PubMed WorldCat Hassan HT . 2004 . Ajoene (natural garlic compound): a new anti-leukaemia agent for AML therapy . Leukemia Research 28 , 667 – 671 . Google Scholar Crossref Search ADS PubMed WorldCat Ichikawa M , Ide N , Ono K . 2006b. Changes in organosulfur compounds in garlic cloves during storage . Journal of Agricultural and Food Chemistry 54 , 4849 – 4854 . Google Scholar Crossref Search ADS PubMed WorldCat Ichikawa M , Ide N , Yoshida J , Yamaguchi H , Ono K . 2006a. Determination of seven organosulfur compounds in garlic by high-performance liquid chromatography . Journal of Agricultural and Food Chemistry 54 , 1535 – 1540 . Google Scholar Crossref Search ADS PubMed WorldCat Iciek M , Kwiecień I , Włodek L . 2009 . Biological properties of garlic and garlic-derived organosulfur compounds . Environmental and Molecular Mutagenesis 50 , 247 – 265 . Google Scholar Crossref Search ADS PubMed WorldCat Imai S , Akita K , Tomotake M , Sawada H . 2006 . Model studies on precursor system generating blue pigment in onion and garlic . Journal of Agricultural and Food Chemistry 54 , 848 – 852 . Google Scholar Crossref Search ADS PubMed WorldCat Imai S , Tsuge N , Tomotake M , Nagatome Y , Sawada H , Nagata T , Kumagai H . 2002 . Plant biochemistry: an onion enzyme that makes the eyes water . Nature 419 , 685 . Google Scholar Crossref Search ADS PubMed WorldCat Jones MG , Hughes J , Tregova A , Milne J , Tomsett AB , Collin HA . 2004 . Biosynthesis of the flavour precursors of onion and garlic . Journal of Experimental Botany 55 , 1903 – 1918 . Google Scholar Crossref Search ADS PubMed WorldCat Joslyn MA , Peterson RG . 1958 . Redding of white onion bulb puree . Journal of Agricultural and Food Chemistry 6 , 754 – 765 . Google Scholar Crossref Search ADS WorldCat Joslyn MA , Sano T . 1956 . The formation and decomposition of green pigment in crushed garlic tissue . Journal of Food Science 21 , 170 – 183 . Google Scholar Crossref Search ADS WorldCat Kaschula CH , Hunter R , Parker MI . 2010 . Garlic-derived anticancer agents: structure and biological activity of ajoene . BioFactors 36 , 78 – 85 . Google Scholar PubMed WorldCat Kato M , Kamoi T , Sasaki R , Sakurai N , Aoki K , Shibata D , Imai S . 2013 . Structures and reactions of compounds involved in pink discolouration of onion . Food Chemistry 139 , 885 – 892 . Google Scholar Crossref Search ADS PubMed WorldCat Kato M , Masamura N , Shono J , Okamoto D , Abe T , Imai S . 2016 . Production and characterization of tearless and non-pungent onion . Scientific Reports 6 , 23779 . Google Scholar Crossref Search ADS PubMed WorldCat Koch HP , Lawson LD . 1996 . Garlic: the science and therapeutic application of Allium sativum L. and related species, 2 nd edn. Baltimore : Williams & Wilkins . Google Preview WorldCat COPAC Kodera Y , Ushijima M , Amano H , Suzuki J , Matsutomo T . 2017 . Chemical and biological properties of S-1-propenyl- l -cysteine in aged garlic extract . Molecules 22 , 570 . Google Scholar Crossref Search ADS WorldCat Körner B , Berk Z . 1967 . The mechanism of pink-red pigment formation in leeks . Advancing Frontiers of Plant Sciences 18 , 39 – 52 . WorldCat Krause RJ , Glocke SC , Elfarra AA . 2002 . Sulfoxides as urinary metabolites of S-allyl- l -cysteine in rats: evidence for the involvement of flavin-containing monooxygenases . Drug Metabolism and Disposition 30 , 1137 – 1142 . Google Scholar Crossref Search ADS PubMed WorldCat Krest I , Keusgen M . 1999 . Quality of herbal remedies from Allium sativum: differences between alliinase from garlic powder and fresh garlic . Planta Medica 65 , 139 – 143 . Google Scholar Crossref Search ADS PubMed WorldCat Krueger SK , Williams DE . 2005 . Mammalian flavin-containing monooxygenases: structure/function, genetic polymorphisms and role in drug metabolism . Pharmacology & Therapeutics 106 , 357 – 387 . Google Scholar Crossref Search ADS PubMed WorldCat Kubec R , Curko P , Urajová P , Rubert J , Hajšlová J . 2017 . Allium discoloration: color compounds formed during greening of processed garlic . Journal of Agricultural and Food Chemistry 65 , 10615 – 10620 . Google Scholar Crossref Search ADS PubMed WorldCat Kubec R , Hrbácová M , Musah RA , Velísek J . 2004 . Allium discoloration: precursors involved in onion pinking and garlic greening . Journal of Agricultural and Food Chemistry 52 , 5089 – 5094 . Google Scholar Crossref Search ADS PubMed WorldCat Kubec R , Kim S , McKeon DM , Musah RA . 2002 . Isolation of n-butylcysteine sulfoxide and six n-butyl-containing thiosulfinates from Allium siculum . Journal of Natural Products 65 , 960 – 964 . Google Scholar Crossref Search ADS PubMed WorldCat Kubec R , Štefanová I , Moos M , Urajová P , Kuzma M , Zápal J . 2018 . Allithiolanes: nine groups of a newly discovered family of sulfur compounds responsible for the bitter off-taste of processed onion . Journal of Agricultural and Food Chemistry 66 , 8783 – 8794 . Google Scholar Crossref Search ADS PubMed WorldCat Kubec R , Svobodová M , Velísek J . 2000 . Distribution of S-alk(en)ylcysteine sulfoxides in some Allium species. Identification of a new flavor precursor: ethylcysteine sulfoxide (Ethiin) . Journal of Agricultural and Food Chemistry 48 , 428 – 433 . Google Scholar Crossref Search ADS PubMed WorldCat Kubec R , Velísek J . 2007 . Allium discoloration: the color-forming potential of individual thiosulfinates and amino acids: structural requirements for the color-developing precursors . Journal of Agricultural and Food Chemistry 55 , 3491 – 3497 . Google Scholar Crossref Search ADS PubMed WorldCat Kuettner EB , Hilgenfeld R , Weiss MS . 2002a. Purification, characterization, and crystallization of alliinase from garlic . Archives of Biochemistry and Biophysics 402 , 192 – 200 . Google Scholar Crossref Search ADS PubMed WorldCat Kuettner EB , Hilgenfeld R , Weiss MS . 2002b. The active principle of garlic at atomic resolution . Journal of Biological Chemistry 277 , 46402 – 46407 . Google Scholar Crossref Search ADS PubMed WorldCat Lancaster JE , Collin HA . 1981 . Presence of alliinase in isolated vacuoles and of alkyl cysteine sulphoxides in the cytoplasm of bulbs of onion (Allium cepa) . Plant Science Letters 22 , 169 – 176 . Google Scholar Crossref Search ADS WorldCat Lancaster JE , Dommisse EM , Shaw ML . 1988 . Production of flavour precursors [S-alk(en)yl- l -cysteine sulphoxides] in photomixotrophic callus of garlic . Phytochemistry 27 , 2123 – 2124 . Google Scholar Crossref Search ADS WorldCat Lancaster JE , Reynolds PHS , Shaw ML , Dommisse EM , Munro J . 1989 . Intra-cellular localization of the biosynthetic pathway to flavour precursors in onion . Phytochemistry 28 , 461 – 464 . Google Scholar Crossref Search ADS WorldCat Lancaster JE , Shaw ML . 1989 . γ-Glutamyl peptides in the biosynthesis of S-alk(en)yl- l -cysteine sulphoxides (flavor precursors) in Allium . Phytochemistry 28 , 455 – 460 . Google Scholar Crossref Search ADS WorldCat Lancaster JE , Shaw ML . 1991 . Metabolism of γ-glutamyl peptides during development, storage and sprouting of onion bulbs . Phytochemistry 30 , 2857 – 2859 . Google Scholar Crossref Search ADS WorldCat Lancaster JE , Shaw ML . 1994 . Characterization of purified γ-glutamyl transpeptidase in onions: evidence for in vivo role as a peptidase . Phytochemistry 36 , 1351 – 1358 . Google Scholar Crossref Search ADS WorldCat Lewis CE , Pollard JW . 2006 . Distinct role of macrophages in different tumor microenvironments . Cancer Research 66 , 605 – 612 . Google Scholar Crossref Search ADS PubMed WorldCat Li J , Hansen BG , Ober JA , Kliebenstein DJ , Halkier BA . 2008 . Subclade of flavin-monooxygenases involved in aliphatic glucosinolate biosynthesis . Plant Physiology 148 , 1721 – 1733 . Google Scholar Crossref Search ADS PubMed WorldCat Li XH , Li CY , Lu JM , Tian RB , Wei J . 2012 . Allicin ameliorates cognitive deficits ageing-induced learning and memory deficits through enhancing of Nrf2 antioxidant signaling pathways . Neuroscience Letters 514 , 46 – 50 . Google Scholar Crossref Search ADS PubMed WorldCat Manabe T , Hasumi A , Sugiyama M , Yamazaki M , Saito K . 1998 . Alliinase [S-alk(en)yl- l -cysteine sulfoxide lyase] from Allium tuberosum (Chinese chive)—purification, localization, cDNA cloning and heterologous functional expression . European Journal of Biochemistry 257 , 21 – 30 . Google Scholar Crossref Search ADS PubMed WorldCat Martin MN , Saladores PH , Lambert E , Hudson AO , Leustek T . 2007 . Localization of members of the γ-glutamyl transpeptidase family identifies sites of glutathione and glutathione S-conjugate hydrolysis . Plant Physiology 144 , 1715 – 1732 . Google Scholar Crossref Search ADS PubMed WorldCat Masamura N , Aoyagi M , Tsuge N , Kamoi T , Imai S . 2012 . Proton transfer in a reaction catalyzed by onion lachrymatory factor synthase . Bioscience, Biotechnology, and Biochemistry 76 , 1799 – 1801 . Google Scholar Crossref Search ADS PubMed WorldCat Mashiguchi K , Tanaka K , Sakai T , et al. 2011 . The main auxin biosynthesis pathway in Arabidopsis . Proceedings of the National Academy of Sciences, USA 108 , 18512 – 18517 . Google Scholar Crossref Search ADS WorldCat Matsutomo T , Ushijima M , Kodera Y , Nakamoto M , Takashima M , Morihara N , Tamura K . 2017 . Metabolomic study on the antihypertensive effect of S-1-propenylcysteine in spontaneously hypertensive rats using liquid chromatography coupled with quadrupole-Orbitrap mass spectrometry . Journal of Chromatography. B, Analytical Technologies in the Biomedical and Life Sciences 1046 , 147 – 155 . Google Scholar Crossref Search ADS PubMed WorldCat Matsuura H , Inagaki M , Maeshige K , Ide N , Kajimura Y , Itakura Y . 1996 . Changes in contents of γ-glutamyl peptides and fructan during growth of Allium sativum . Planta Medica 62 , 70 – 71 . Google Scholar Crossref Search ADS PubMed WorldCat McSheehy S , Yang W , Pannier F , Szpunar J , Łobiński R , Auger J , Potin-Gautier M . 2000 . Speciation analysis of selenium in garlic by two-dimensional high-performance liquid chromatography with parallel inductively coupled plasma mass spectrometric and electrospray tandem mass spectrometric detection . Analytica Chimica Acta 421 , 147 – 153 . Google Scholar Crossref Search ADS WorldCat Miron T , Mironchik M , Mirelman D , Wilchek M , Rabinkov A . 2003 . Inhibition of tumor growth by a novel approach: in situ allicin generation using targeted alliinase delivery . Molecular Cancer Therapeutics 2 , 1295 – 1301 . Google Scholar PubMed WorldCat Morihara N , Nishihama T , Ushijima M , Ide N , Takeda H , Hayama M . 2007 . Garlic as an anti-fatigue agent . Molecular Nutrition & Food Research 51 , 1329 – 1334 . Google Scholar Crossref Search ADS PubMed WorldCat Nicastro HL , Ross SA , Milner JA . 2015 . Garlic and onions: their cancer prevention properties . Cancer Prevention Research 8 , 181 – 189 . Google Scholar Crossref Search ADS PubMed WorldCat Nock LP , Mazelis M . 1986 . The C–S lyases of higher plants: preparation and properties of homogeneous alliin lyase from garlic (Allium sativum) . Archives of Biochemistry and Biophysics 249 , 27 – 33 . Google Scholar Crossref Search ADS PubMed WorldCat Nohara T , Fujiwara Y , El-Aasr M , Ikeda T , Ono M , Nakano D , Kinjo J . 2017 . Antitumor Allium sulfides . Chemical & Pharmaceutical Bulletin 65 , 209 – 217 . Google Scholar Crossref Search ADS PubMed WorldCat Novick RM , Elfarra AA . 2008 . Purification and characterization of flavin-containing monooxygenase isoform 3 from rat kidney microsomes . Drug Metabolism and Disposition 36 , 2468 – 2474 . Google Scholar Crossref Search ADS PubMed WorldCat Ohkama-Ohtsu N , Radwan S , Peterson A , Zhao P , Badr AF , Xiang C , Oliver DJ . 2007a. Characterization of the extracellular γ-glutamyl transpeptidases, GGT1 and GGT2, in Arabidopsis . The Plant Journal 49 , 865 – 877 . Google Scholar Crossref Search ADS PubMed WorldCat Ohkama-Ohtsu N , Zhao P , Xiang C , Oliver DJ . 2007b. Glutathione conjugates in the vacuole are degraded by γ-glutamyl transpeptidase GGT3 in Arabidopsis . The Plant Journal 49 , 878 – 888 . Google Scholar Crossref Search ADS PubMed WorldCat Ohsumi C , Hayashi T , Sano K . 1993 . Formation of alliin in the culture tissues of Allium sativum. Oxidation of S-allyl- l -cysteine . Phytochemistry 33 , 107 – 111 . Google Scholar Crossref Search ADS WorldCat Parry RJ , Naidu MV . 1983 . Determination of the absolute configuration of (−)-S-(2-carboxypropyl)- l -cysteine . Tetrahedron Letters 24 , 1133 – 1134 . Google Scholar Crossref Search ADS WorldCat Powolny AA , Singh SV . 2008 . Multitargeted prevention and therapy of cancer by diallyl trisulfide and related Allium vegetable-derived organosulfur compounds . Cancer Letters 269 , 305 – 314 . Google Scholar Crossref Search ADS PubMed WorldCat Puccinelli MT , Stan SD . 2017 . Dietary bioactive diallyl trisulfide in cancer prevention and treatment . International Journal of Molecular Sciences 18 , 1645 . Google Scholar Crossref Search ADS WorldCat Rabinkov A , Zhu XZ , Grafi G , Galili G , Mirelman D . 1994 . Alliin lyase (Alliinase) from garlic (Allium sativum). Biochemical characterization and cDNA cloning . Applied Biochemistry and Biotechnology 48 , 149 – 171 . Google Scholar Crossref Search ADS PubMed WorldCat Randle WM , Lancaster JE , Shaw ML , Sutton KH , Hay RL , Bussard ML . 1995 . Quantifying onion flavor compounds responding to sulfur fertility—sulfur increases levels of alk(en)yl cysteine sulfoxides and biosynthetic intermediates . Journal of the American Society for Horticultural Science 120 , 1075 – 1081 . Google Scholar Crossref Search ADS WorldCat Ray B , Chauhan NB , Lahiri DK . 2011 . The ‘aged garlic extract’: (AGE) and one of its active ingredients S-allyl- l -cysteine (SAC) as potential preventive and therapeutic agents for Alzheimer’s disease (AD) . Current Medicinal Chemistry 18 , 3306 – 3313 . Google Scholar Crossref Search ADS PubMed WorldCat Ripp SL , Overby LH , Philpot RM , Elfarra AA . 1997 . Oxidation of cysteine conjugates by rabbit liver microsomes and cDNA-expressed flavin-containing mono-oxygenases: studies with S-(1,2-dichlorovinyl)- l -cysteine, S-(1,2,2-trichlorovinyl)- l -cysteine, S-allyl- l -cysteine, and S-benzyl- l -cysteine . Molecular Pharmacology 51 , 507 – 515 . Google Scholar PubMed WorldCat Rivlin RS . 2001 . Historical perspective on the use of garlic . Journal of Nutrition 131 , 951S – 954S . Google Scholar Crossref Search ADS PubMed WorldCat Rose P , Whiteman M , Moore PK , Zhu YZ . 2005 . Bioactive S-alk(en)yl cysteine sulfoxide metabolites in the genus Allium: the chemistry of potential therapeutic agents . Natural Product Reports 22 , 351 – 368 . Google Scholar Crossref Search ADS PubMed WorldCat Schlaich NL . 2007 . Flavin-containing monooxygenases in plants: looking beyond detox . Trends in Plant Science 12 , 412 – 418 . Google Scholar Crossref Search ADS PubMed WorldCat Schneider G , Käck H , Lindqvist Y . 2000 . The manifold of vitamin B6 dependent enzymes . Structure 8 , R1 – R6 . Google Scholar Crossref Search ADS PubMed WorldCat Shah M , Kannamkumarath SS , Wuilloud JCA , Wuilloud RG , Caruso JA . 2004 . Identification and characterization of selenium species in enriched green onion (Allium fistulosum) by HPLC-ICP-MS and ESI-ITMS . Journal of Analytical Atomic Spectrometry 19 , 381 – 386 . Google Scholar Crossref Search ADS WorldCat Shaw ML , Pither-Joyce MD , McCallum JA . 2005 . Purification and cloning of a γ-glutamyl transpeptidase from onion (Allium cepa) . Phytochemistry 66 , 515 – 522 . Google Scholar Crossref Search ADS PubMed WorldCat Shimon LJ , Rabinkov A , Shin I , Miron T , Mirelman D , Wilchek M , Frolow F . 2007 . Two structures of alliinase from Alliium sativum L.: apo form and ternary complex with aminoacrylate reaction intermediate covalently bound to the PLP cofactor . Journal of Molecular Biology 366 , 611 – 625 . Google Scholar Crossref Search ADS PubMed WorldCat Shouk R , Abdou A , Shetty K , Sarkar D , Eid AH . 2014 . Mechanisms underlying the antihypertensive effects of garlic bioactives . Nutrition Research 34 , 106 – 115 . Google Scholar Crossref Search ADS PubMed WorldCat Sica A , Schioppa T , Mantovani A , Allavena P . 2006 . Tumour-associated macrophages are a distinct M2 polarised population promoting tumour progression: potential targets of anti-cancer therapy . European Journal of Cancer 42 , 717 – 727 . Google Scholar Crossref Search ADS PubMed WorldCat Silvaroli JA , Pleshinger MJ , Banerjee S , Kiser PD , Golczak M . 2017 . Enzyme that makes you cry—crystal structure of lachrymatory factor synthase from Allium cepa . ACS Chemical Biology 12 , 2296 – 2304 . Google Scholar Crossref Search ADS PubMed WorldCat Stoll A , Seebeck E . 1948 . Allium compounds. I. Alliin, the true mother compound of garlic oil . Helvetica Chimica Acta 31 , 189 – 210 . Google Scholar Crossref Search ADS PubMed WorldCat Stoll A , Seebeck E . 1949a. Allium compounds. II. Enzymic degradation of alliin and the properties of alliinase . Helvetica Chimica Acta 32 , 197 – 205 . Google Scholar Crossref Search ADS PubMed WorldCat Stoll A , Seebeck E . 1949b. Allium compounds. III. Specificity of alliinase and synthesis of compounds related to alliin . Helvetica Chimica Acta 32 , 866 – 876 . Google Scholar Crossref Search ADS PubMed WorldCat Stoll A , Seebeck E . 1951 . Chemical investigations on alliin, the specific principle of garlic . Advances in Enzymology and Related Subjects of Biochemistry 11 , 377 – 400 . Google Scholar PubMed WorldCat Su T , Xu J , Li Y , Lei L , Zhao L , Yang H , Feng J , Liu G , Ren D . 2011 . Glutathione-indole-3-acetonitrile is required for camalexin biosynthesis in Arabidopsis thaliana . The Plant Cell 23 , 364 – 380 . Google Scholar Crossref Search ADS PubMed WorldCat Sugii M , Suzuki T , Nagasawa S . 1963 . Isolation of (–) S-propenyl- l -cysteine from garlic . Chemical & Pharmaceutical Bulletin 11 , 548 – 549 . Google Scholar Crossref Search ADS PubMed WorldCat Suzuki T , Sugii M , Kakimoto T . 1961 . New γ-glutamyl peptides in garlic . Chemical and Pharmaceutical Bulletin 9 , 77 – 78 . Google Scholar Crossref Search ADS WorldCat Suzuki T , Sugii M , Kakimoto T . 1962 . Incorporation of l -valine-[14C] into S-(2-carboxypropyl)glutathione and S-(2-carboxypropyl)cysteine in garlic . Chemical and Pharmaceutical Bulletin 10 , 328 – 331 . Google Scholar Crossref Search ADS PubMed WorldCat Suzuki J , Yamaguchi T , Matsutomo T , Amano H , Morihara N , Kodera Y . 2016 . S-1-Propenylcysteine promotes the differentiation of B cells into IgA-producing cells by the induction of Erk1/2-dependent Xbp1 expression in Peyer’s patches . Nutrition 32 , 884 – 889 . Google Scholar Crossref Search ADS PubMed WorldCat Tate SS , Meister A . 1981 . γ-Glutamyl transpeptidase: catalytic, structural and functional aspects . Molecular and Cellular Biochemistry 39 , 357 – 368 . Google Scholar Crossref Search ADS PubMed WorldCat Thomson M , Ali M . 2003 . Garlic [Allium sativum]: a review of its potential use as an anti-cancer agent . Current Cancer Drug Targets 3 , 67 – 81 . Google Scholar Crossref Search ADS PubMed WorldCat Thomson SJ , Rippon P , Butts C , Olsen S , Shaw M , Joyce NI , Eady CC . 2013 . Inhibition of platelet activation by lachrymatory factor synthase (LFS)-silenced (tearless) onion juice . Journal of Agricultural and Food Chemistry 61 , 10574 – 10581 . Google Scholar Crossref Search ADS PubMed WorldCat Tobkin HE Jr , Mazelis M . 1979 . Alliin lyase: preparation and characterization of the homogeneous enzyme from onion bulbs . Archives of Biochemistry and Biophysics 193 , 150 – 157 . Google Scholar Crossref Search ADS PubMed WorldCat Turnbull A , Galpin IJ , Collin HA . 1980 . Comparison of the onion plant (Allium cepa) and onion tissue culture. III. Feeding of 14C labeled precursors of the flavor precursor compounds . New Phytologist 85 , 483 – 487 . Google Scholar Crossref Search ADS WorldCat Ueda Y , Kawajiri H , Miyamura N , Miyajima R . 1991 . Content of some sulfur-containing components and free amino acids in various strains of garlic . Nippon Shokuhin Kogyo Gakkaishi 38 , 429 – 434 . Google Scholar Crossref Search ADS WorldCat Van Damme EJ , Smeets K , Torrekens S , Van Leuven F , Peumans WJ . 1992 . Isolation and characterization of alliinase cDNA clones from garlic (Allium sativum L.) and related species . European Journal of Biochemistry 209 , 751 – 757 . Google Scholar Crossref Search ADS PubMed WorldCat Virtanen AI , Matikkala EJ . 1959 . The isolation of S-methylcysteine sulphoxide and S-n-propylcysteine sulphoxide from onion (Allium cepa) and the antibiotic activity of crushed onion . Acta Chemica Scandinavica 13 , 1898 – 1900 . Google Scholar Crossref Search ADS WorldCat Virtanen AI , Spåre CG . 1961 . Isolation of the precursor of the lachrymatory factor in onion (Allium cepa) . Suomen Kemistilehti B 34 , 72 . WorldCat Weiner L , Shin I , Shimon LJ , Miron T , Wilchek M , Mirelman D , Frolow F , Rabinkov A . 2009 . Thiol-disulfide organization in alliin lyase (alliinase) from garlic (Allium sativum) . Protein Science 18 , 196 – 205 . Google Scholar PubMed WorldCat Whanger PD . 2004 . Selenium and its relationship to cancer: an update . British Journal of Nutrition 91 , 11 – 28 . Google Scholar Crossref Search ADS PubMed WorldCat Whitaker JR . 1976 . Development of flavor, odor and pungency in onion and garlic . Advances in Food Research 22 , 73 – 133 . Google Scholar Crossref Search ADS WorldCat Yamazaki M , Sugiyama M , Saito K . 2002 . Intercellular localization of cysteine synthase and alliinase in bundle sheaths of Allium plants . Plant Biotechnology 19 , 7 – 10 . Google Scholar Crossref Search ADS WorldCat Yeh YY , Liu L . 2001 . Cholesterol-lowering effect of garlic extracts and organosulfur compounds: human and animal studies . Journal of Nutrition 131 , 989S – 993S . Google Scholar Crossref Search ADS PubMed WorldCat Yi L , Su Q . 2013 . Molecular mechanisms for the anti-cancer effects of diallyl disulfide . Food and Chemical Toxicology 57 , 362 – 370 . Google Scholar Crossref Search ADS PubMed WorldCat Yoshimoto N , Onuma M , Mizuno S , Sugino Y , Nakabayashi R , Imai S , Tsuneyoshi T , Sumi S , Saito K . 2015b. Identification of a flavin-containing S-oxygenating monooxygenase involved in alliin biosynthesis in garlic . The Plant Journal 83 , 941 – 951 . Google Scholar Crossref Search ADS PubMed WorldCat Yoshimoto N , Yabe A , Sugino Y , Murakami S , Sai-Ngam N , Sumi S , Tsuneyoshi T , Saito K . 2015a. Garlic γ-glutamyl transpeptidases that catalyze deglutamylation of biosynthetic intermediate of alliin . Frontiers in Plant Science 5 , 758 . Google Scholar Crossref Search ADS PubMed WorldCat Zhu Y , Anand R , Geng X , Ding Y . 2018 . A mini review: garlic extract and vascular diseases . Neurological Research 40 , 421 – 425 . Google Scholar Crossref Search ADS PubMed WorldCat © The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - S-Alk(en)ylcysteine sulfoxides in the genus Allium: proposed biosynthesis, chemical conversion, and bioactivities
JF - Journal of Experimental Botany
DO - 10.1093/jxb/erz243
DA - 2019-08-19
UR - https://www.deepdyve.com/lp/oxford-university-press/s-alk-en-ylcysteine-sulfoxides-in-the-genus-allium-proposed-DI09UOAUIu
SP - 4123
VL - 70
IS - 16
DP - DeepDyve
ER -