TY - JOUR
AU - Wang, Min
AB - Abstract Cyclodextrins (CDs) can improve the productivity of steroid biotransformation by enhancing substrate solubility. CDs can be recycled by grafting them with appropriate carriers. Loofah fiber is an excellent grafting material for CDs, and can be applied to the biotransformation and recycling of β-cyclodextrin (β-CD). In this work, a technique for recycling β-CD in cortisone acetate (CA) biotransformation by Arthrobacter simplex CPCC 140451 was studied. Loofah fiber-grafted β-CD (LF-β-CD) was prepared using epichlorohydrin, which is a cross-linking agent. The grafting yield of β-CD was 74.8 mg g−1 dried fibers. LF-β-CD could increase the solubility of CA and enhance biotransformation. The initial conversion rate of CA was 1.5-fold higher than that of the blank group. LF-β-CD was also used in biocatalytic reactions for eight cycles, and it maintained the conversion ratio of CA at approximately 90%. Given the above positive results, LF-β-CD can be utilized in biotechnological recycling applications. This method can also be applied to CD derivatives and hydrophobic compounds. Introduction Microbial steroid transformation is a powerful tool for generating novel steroidal drugs, while low substrate solubility and cell uptake are serious challenges that influence biotransformation efficiency [6]. Cyclodextrins (CDs) are water-soluble cyclic oligosaccharides containing six, seven, or eight α-1, 4-linked D-glucopyranose units (α-, β-, and γ-cyclodextrins). These compounds can selectively carry a wide range of guest molecules into their hydrophobic cavity by van der Waals interactions and hydrogen-bond formation [1]. At present, CDs are extensively used as stabilizers and solubilizers for several steroid drugs during biotransformation to increase the reaction rate and degree of conversion by forming a host–guest complex [12, 16]. However, the extensive application of CDs is limited because of their high cost. Grafting new specific groups on cellulose molecules through functionalization can combine the advantageous properties of cellulose to form synthetic polymers. In this way, cellulose can serve as a backbone for chemically bonded groups, which can be functionalized permanently [7]. As environmentally friendly auxiliaries, CDs can also be grafted onto macromolecules with cross-linkers for modification. Numerous cross-linking agents, such as epichlorohydrin [22], cyanuric chloride [8], N-methylol acrylamide [14, 15], and polycarboxylic acids [11, 28], have been utilized to graft β-cyclodextrin (β-CD) onto natural fibers. Functional agents can be introduced to CDs using cellulose fibers as immobilized carriers, which can readily form inclusion complexes with various small molecules [9]. In addition, loofah fiber is a natural, biodegradable, low-cost, non-toxic, and useful lignocellulosic material [2]. The fiber mainly consists of cellulose, hemicellulose, and lignin [13]. Loofah fiber is also an excellent carrier for immobilizing microorganisms and plant and animal cells [4, 19]. This lignocellulosic material has also been utilized for practical applications in food and wastewater treatment industries. Furthermore, immobilization can be achieved by simply inoculating the cells into the reactor containing the loofa sponge bed [18]. This technique can produce CD-grafted loofah fiber, fully utilize CDs in bioconversion reactions, and implement recycling of CDs. The present work aims to use a low-cost loofah fiber as carrier-grafted β-CD to achieve the recycling of β-CD and to reduce the cost of industrial applications. In this study, 1-dehydrogenation of cortisone acetate (CA) by Arthrobacter simplex CPCC 140451 (ASP) [21] was selected as an experimental model. Loofah fiber-grafted β-CD (LF-β-CD) was prepared and used as a medium for CA biotransformation and recycling. The LF-β-CD in biotransformation solutions was studied first, and optimum grafting conditions were then investigated. The effect in solubility of CA, biotransformation yields, and efficiency of the grafted β-CD was evaluated. This method can be used in recovering CD derivatives for hydrophobic compound biotransformation and improving economic efficiency. Materials and methods Chemicals CA standard with purity of ≥98% was kindly provided by Xianju Pharmaceutical Company Ltd. (Zhejiang, China). Loofah fiber was obtained from the ripped dried vegetable of Luffa aegyptica purchased from a local shop in Tianjin, China. All reagents were chromatographically pure or of analytical grade. Bacterial strain and cultivation ASP was stored in the laboratory and prepared in two cultivation steps in shake flasks, as previously described [17]. The ASP cells were collected by centrifugation at 4 °C, and the resultant was then rinsed and resuspended with 100 mmol KH2PO4–NaOH buffer (pH 7.2). LF-β-CD preparation The process for the preparation of LF-β-CD is illustrated in Fig. 1. Loofah fiber was cut into discs with a diameter of approximately 2.5 cm and thickness of 3–4 mm. The discs were soaked in boiling water for 30 min, thoroughly washed with tap water, and left for 24 h in distilled water. The water was replaced three to four times. The discs were then oven dried at 70 °C [23]. Fig. 1 Open in new tabDownload slide Scheme of LF-β-CD composite preparation. (LF-β-CD was prepared using epichlorohydrin as a cross-linking agent.) Epoxidized loofah fiber was first obtained by reacting epichlorohydrin with loofah fiber and then grafted with β-CD. The pretreated loofah fiber with a certain weight was soaked in distilled water, and NaOH solution (0.1 mol L−1) was then added for 1 h to obtain sufficient swelling. The fibers were placed in a flask filled with epichlorohydrin and NaOH solution (10 mol L−1). The mixture was reacted at 45 °C for 3 h and then washed with water until epichlorohydrin was removed. The material was washed for two times with acetone and dried at 50 °C. Epoxidized loofah fiber and various β-CD concentrations were dissolved in 10 mol L−1 NaOH solution. The mass ratio of the epoxidized loofah fiber was 1:40. The mixture was shaken in a water bath for 3 h at a constant temperature of 40 °C. The obtained fibers were washed in distilled water until the filtrate was neutral. The fibers were then dried at 50 °C. Effect of LF-β-CD on CA solubility Aqueous solution and a certain amount of grafted β-CD were added to a final volume of 20 mL in 250 mL shake flasks. After the addition of CA, the shake flasks were incubated under the conditions identical to those employed in bioconversion (32 °C, 180 rpm). The flasks were tightly sealed to avoid evaporation. After 24 h, 1 mL aliquot of the slurry was withdrawn and filtered through a 0.2 µm filter. The filtrate was dried in vacuum and redissolved in mobile phase (methanol/water, 80:20, v/v), and the total concentration of CA in the filtrate was analyzed by high-performance liquid chromatography (HPLC), as previously reported [29]. The concentration of CA was determined from the calibration curves, which were obtained from the eluent solutions of standard CA. Recovery of β-CD and biotransformation CA (0.06 g), 1 g LF-β-CD, and 1 mL of ASP cells (10 g dry weight per liter) were added to 19 mL KH2PO4–NaOH buffer at 34 °C and 180 rpm for 8 h, as previously described [24]. During incubation, the samples were withdrawn and extracted using ethyl acetate. These samples were then analyzed by HPLC (Agilent 1100, USA). The experiments were performed in triplicates. LF-β-CD was reused for several cycles to investigate reusability. Analytical methods Scanning electron microscopy (SEM, SU-1510) operated at 2 keV was used to observe the changes in the loofah fiber samples before and after treatment with alkaline solution. The infrared spectroscopy curves of loofah fiber, β-CD, and grafted β-CD were measured by Fourier transform infrared spectroscopy (FTIR, Tensor-27). The dried sample was mixed with KBr powder for tableting. The analysis results ranged from 400 to 4000 cm−1. The content of β-CD grafted with loofah fiber was determined using a UV–visible spectrophotometer at 552 nm. Phenolphthalein method was performed on the basis of the decrease in the absorbance of phenolphthalein caused by the presence of β-CD in the alkaline solution [5]. The samples were withdrawn and extracted by ethyl acetate and dried in vacuum. The solid extracts were then redissolved in mobile phase (dichloromethane/ether/methanol at a ratio of 86:12:3.6, v/v/v) and filtered through a 0.45 µm filter. The extracts were assayed by HPLC (Agilent 1100, USA), and their absorbance was measured at 240 nm, as previously described [17]. Results and discussion Characterization of LF-β-CD To recycle CD in CA biotransformation, the structure of the cellulose after treatment was maintained and CD was grafted with loofah fiber. The SEM images of original cellulose fibers and alkaline solution-treated fibers are shown in Fig. 2. The SEM image in Fig. 2a reveals rough surface and an outer lignin rich layer around the fibers [3, 27]. In Fig. 2b, NaOH-treated fibers contained an irregular surface; the inner layers of the fiber were exposed, because a large area of the surface material was removed. Therefore, inner single fibers were exposed to some extent, because loofah fiber was treated with alkali solution. This scenario is beneficial for the β-CD grafted with loofah fiber. The SEM images reveal that the ordered structure of cellulose was not disrupted. Therefore, the fiber can serve as a carrier for grafting. Fig. 2 Open in new tabDownload slide SEM images of loofah fiber. a untreated loofah fiber; b loofah fiber treated by alkaline solution FTIR was used to examine the functional groups of LF-β-CD (Fig. 3). The adsorption bands at 3403, 1418, 1081, 941, and 707 cm−1 in the spectrum corresponded to β-CD [20]. The region between 1335 and 1165 cm−1 was related to the C–H and C–O bond stretching frequencies. A peak at 2925 cm−1 was assigned to C–H vibration. The band ranging from 3200 to 3400 cm−1 corresponded to the vibration stretching of intermolecular and intramolecular hydrogen bonds of β-CD. Compared with the spectra of the untreated cellulose fibers, those of LF-β-CD reveal a new vibration at 668 cm−1, which can be assigned to the presence of a new pyran ring. Thus, CD is successfully grafted with cellulose. Fig. 3 Open in new tabDownload slide FTIR spectra of β-CD, loofah fiber, and LF-β-CD. a β-CD; b loofah fiber; c β-CD graft loofah fiber Effect of LF-β-CD on biotransformation Biotransformation experiments were conducted using LF-β-CD. The grafting conditions shown in Table 1 were optimized to maximize the use of LF-β-CD. These data reveal the following optimal reaction conditions: 1:20 weight ratio of loofah fiber and epichlorohydrin and 1:1.3 weight ratio of the epoxidized loofah fiber and β-CD. The grafting yield of the grafted CD was 74.8 mg/g of oven-dried fibers (Table 1). These conditions produced high initial and final conversion rates. The reason can be due to that high amount of epichlorohydrin which could enhance the amount of grafting β-CD, thereby improving the conversion of CA. Moreover, the amounts of β-CD and epichlorohydrin influenced the grafting conditions. Prior studies have shown that the final CA conversion ratio in hydroxypropyl-β-CD free medium can reach more than 90% [29]. Similar result is observed in that of grafted β-CD. Amount of grafting β-CD, initial conversion rate and conversion ratio of CA at different graft conditions (20 mL cell suspension, cortisone acetate 0.06 g, adding a certain amount of grafted β-CD under different graft conditions) The weight ratio of loofah fiber and epichlorohydrin . The weight ratio of epoxidized loofah fiber and β-CD . Amount of grafting β-CD (mg g−1-oven dried fibers) . The initial conversion rate of CA (×10−2 g L−1 min−1) . Conversion ratio of CA (%) . Control Control – 1.05 ± 0.03 86.9 ± 0.2 1:10 1:0 – 1.06 ± 0.02 91.2 ± 0.6 1:10 1:1.3 47.1 ± 0.03 1.24 ± 0.05 93.2 ± 0.3 1:10 1:1.5 40.9 ± 0.02 1.14 ± 0.08 92.6 ± 0.2 1:10 1:2.0 42.5 ± 0.02 1.13 ± 0.01 92.4 ± 0.3 1:15 1:0 – 1.07 ± 0.02 92.2 ± 0.5 1:15 1:1.3 61.0 ± 0.11 1.28 ± 0.04 93.4 ± 0.8 1:15 1:1.5 57.1 ± 0.01 1.09 ± 0.02 92.1 ± 0.2 1:15 1:2.0 58.2 ± 0.01 1.03 ± 0.05 92.6 ± 0.3 1:20 1:0 – 1.09 ± 0.03 91.3 ± 0.7 1:20 1:1.3 74.8 ± 0.01 1.69 ± 0.04 93.7 ± 0.7 1:20 1:1.5 60.7 ± 0.02 1.62 ± 0.02 93.2 ± 0.8 1:20 1:2.0 67.9 ± 0.03 1.55 ± 0.01 91.3 ± 0.6 The weight ratio of loofah fiber and epichlorohydrin . The weight ratio of epoxidized loofah fiber and β-CD . Amount of grafting β-CD (mg g−1-oven dried fibers) . The initial conversion rate of CA (×10−2 g L−1 min−1) . Conversion ratio of CA (%) . Control Control – 1.05 ± 0.03 86.9 ± 0.2 1:10 1:0 – 1.06 ± 0.02 91.2 ± 0.6 1:10 1:1.3 47.1 ± 0.03 1.24 ± 0.05 93.2 ± 0.3 1:10 1:1.5 40.9 ± 0.02 1.14 ± 0.08 92.6 ± 0.2 1:10 1:2.0 42.5 ± 0.02 1.13 ± 0.01 92.4 ± 0.3 1:15 1:0 – 1.07 ± 0.02 92.2 ± 0.5 1:15 1:1.3 61.0 ± 0.11 1.28 ± 0.04 93.4 ± 0.8 1:15 1:1.5 57.1 ± 0.01 1.09 ± 0.02 92.1 ± 0.2 1:15 1:2.0 58.2 ± 0.01 1.03 ± 0.05 92.6 ± 0.3 1:20 1:0 – 1.09 ± 0.03 91.3 ± 0.7 1:20 1:1.3 74.8 ± 0.01 1.69 ± 0.04 93.7 ± 0.7 1:20 1:1.5 60.7 ± 0.02 1.62 ± 0.02 93.2 ± 0.8 1:20 1:2.0 67.9 ± 0.03 1.55 ± 0.01 91.3 ± 0.6 Open in new tab Amount of grafting β-CD, initial conversion rate and conversion ratio of CA at different graft conditions (20 mL cell suspension, cortisone acetate 0.06 g, adding a certain amount of grafted β-CD under different graft conditions) The weight ratio of loofah fiber and epichlorohydrin . The weight ratio of epoxidized loofah fiber and β-CD . Amount of grafting β-CD (mg g−1-oven dried fibers) . The initial conversion rate of CA (×10−2 g L−1 min−1) . Conversion ratio of CA (%) . Control Control – 1.05 ± 0.03 86.9 ± 0.2 1:10 1:0 – 1.06 ± 0.02 91.2 ± 0.6 1:10 1:1.3 47.1 ± 0.03 1.24 ± 0.05 93.2 ± 0.3 1:10 1:1.5 40.9 ± 0.02 1.14 ± 0.08 92.6 ± 0.2 1:10 1:2.0 42.5 ± 0.02 1.13 ± 0.01 92.4 ± 0.3 1:15 1:0 – 1.07 ± 0.02 92.2 ± 0.5 1:15 1:1.3 61.0 ± 0.11 1.28 ± 0.04 93.4 ± 0.8 1:15 1:1.5 57.1 ± 0.01 1.09 ± 0.02 92.1 ± 0.2 1:15 1:2.0 58.2 ± 0.01 1.03 ± 0.05 92.6 ± 0.3 1:20 1:0 – 1.09 ± 0.03 91.3 ± 0.7 1:20 1:1.3 74.8 ± 0.01 1.69 ± 0.04 93.7 ± 0.7 1:20 1:1.5 60.7 ± 0.02 1.62 ± 0.02 93.2 ± 0.8 1:20 1:2.0 67.9 ± 0.03 1.55 ± 0.01 91.3 ± 0.6 The weight ratio of loofah fiber and epichlorohydrin . The weight ratio of epoxidized loofah fiber and β-CD . Amount of grafting β-CD (mg g−1-oven dried fibers) . The initial conversion rate of CA (×10−2 g L−1 min−1) . Conversion ratio of CA (%) . Control Control – 1.05 ± 0.03 86.9 ± 0.2 1:10 1:0 – 1.06 ± 0.02 91.2 ± 0.6 1:10 1:1.3 47.1 ± 0.03 1.24 ± 0.05 93.2 ± 0.3 1:10 1:1.5 40.9 ± 0.02 1.14 ± 0.08 92.6 ± 0.2 1:10 1:2.0 42.5 ± 0.02 1.13 ± 0.01 92.4 ± 0.3 1:15 1:0 – 1.07 ± 0.02 92.2 ± 0.5 1:15 1:1.3 61.0 ± 0.11 1.28 ± 0.04 93.4 ± 0.8 1:15 1:1.5 57.1 ± 0.01 1.09 ± 0.02 92.1 ± 0.2 1:15 1:2.0 58.2 ± 0.01 1.03 ± 0.05 92.6 ± 0.3 1:20 1:0 – 1.09 ± 0.03 91.3 ± 0.7 1:20 1:1.3 74.8 ± 0.01 1.69 ± 0.04 93.7 ± 0.7 1:20 1:1.5 60.7 ± 0.02 1.62 ± 0.02 93.2 ± 0.8 1:20 1:2.0 67.9 ± 0.03 1.55 ± 0.01 91.3 ± 0.6 Open in new tab Compared with the control, the solubility of CA increased by twofold to 0.0396 mg mL−1 (data not shown). The grafting ratio of β-CD was 7.5%. This ratio is different from that in literature, which is 9.7% by covalently bonding β-CD with cellulose fiber using citric acid as cross-linking agent [10]. The dissimilarity may be due to the difference in content of cellulose and materials used. This study uses loofah fiber, whereas the previous studies use cellulose fiber. The effect of LF-β-CD on the biotransformation of CA was also evaluated (Fig. 4). The group with LF-β-CD yielded a higher conversion ratio than that of the control group. Unlike the latter, the former could shorten the duration of fermentation. The conversion observed in this experiment was slightly enhanced because of the dense network structure of loofah fiber. The efficiency of substrate utilization and the productivities of various fermentation processes were improved by providing a protective microenvironment system [25]. The β-CD rigid ring might open after grafting with loofah fiber. As a result, the solubility of CA was improved, the initial conversion rate was increased, and the final conversion rate was enhanced. Cell contraction was facilitated by loofah fiber with a substrate, and viability was maintained by fixing the cells [26]. However, the complex mechanisms of the increase in biocatalytic reactions caused by loofah fiber grafted with β-CD must be further studied. Fig. 4 Open in new tabDownload slide Transformation curve of CA at the optimum graft conditions. (20 mL cell suspension, cortisone acetate 0.06 g, adding a certain amount of grafted β-CD under optimum graft conditions, without addition of grafted β-CD system was used as a control.) Cyclic utilization of LF-β-CD Biotransformation experiments were performed using the same LF-β-CD for eight times. After each experiment, LF-β-CD was separated, washed, and recycled. As shown in Fig. 5, the grafted CD maintained a good conversion ratio even after eight cycles of reuse. The first conversion ratio was high, possibly because grafted β-CD formed better inclusion complexes with CA, and the substrate/product mass transfer resistance was small during the first trial. With the increase in conversion number, the conversion ratio of CA slightly decreased. After multiple cycles, the capacity of CD inclusion diminished. The effectiveness of loofah fiber-fixed bacteria also gradually weaken. Fig. 5 Open in new tabDownload slide The recycle times and conversion ratio of CA using LF-β-CD as reaction media (20 mL cell suspension, cortisone acetate 0.06 g, adding a certain amount of LF-β-CD under optimum graft conditions) After increasing the circulation of LF-β-CD, the conversion ratio of CA was nearly the same. Assuming that the loofah fiber was linked with CD through chemical bonding and that β-CD was barely lost during each circulation process is reasonable. In addition, the methods of grafting β-CD on loofah fiber for recycling β-CD omitted the extraction steps of CD and did not replenish CD in the beginning of each circulation, as reported in literature [24]. In industrial applications, the desired products can be isolated from the by-products and substrate when the conversion ratio of CA is over 90% by the recrystallization method. Therefore, this finding suggests that LF-β-CD exhibits excellent reuse capability. Conclusion As a potential carrier, loofah fiber is natural, low-cost, and non-toxic. β-CD can be grafted successfully on loofah fiber, and this fiber can be used as a solid support for biotransformation. The maximum graft quantity of 74.8 mg-β-CD/g of oven-dried fibers is obtained under optimal conditions. LF-β-CD can promote CA biotransformation. This molecule provides several advantages, including easy separation and repeatability. Future work will extend this technique to CD derivatives and the biotransformation of other hydrophobic compounds. The specific mechanism of the improved efficiency of bioconversion by grafting CDs on loofah fiber will be determined. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21276196, 21406167, and 21306138), the Key Project of Chinese Ministry of Education (Grant No. 213004A), and the Tianjin Programs for Science and Technology Development (Grant No. 15ZCZDSY00510). References 1. Auzely-Velty R , Rinaudo M New supramolecular assemblies of a cyclodextrin-grafted chitosan through specific complexation Macromolecules 2002 35 7955 7962 10.1021/ma020664o Google Scholar Crossref Search ADS WorldCat 2. Bou-Saab H , Boulanger A, Schellenbaum P, Neunlist S Performance of Loofah fiber as immobilization matrix in bioconversion reactions by Nicotiana tabacum BY-2 J Biosci Bioeng 2013 116 506 508 10.1016/j.jbiosc.2013.04.017 Google Scholar Crossref Search ADS PubMed WorldCat 3. 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TI - A new technique for promoting cyclic utilization of cyclodextrins in biotransformation
JF - Journal of Industrial Microbiology and Biotechnology
DO - 10.1007/s10295-016-1856-1
DA - 2017-01-01
UR - https://www.deepdyve.com/lp/oxford-university-press/a-new-technique-for-promoting-cyclic-utilization-of-cyclodextrins-in-zd7RXJvGUj
SP - 1
EP - 7
VL - 44
IS - 1
DP - DeepDyve
ER -