Plant Science 139 (1998) 233–242
Purification and preliminary characterization of the soybean
glyoxysomal aspartate aminotransferase isozyme
Amy S. Stephens
a
, Joan S. Gebhardt
b
, Benjamin F. Matthews
b
,
Gregory J. Wadsworth
a,
*
a
Buffalo State College, Department of Biology,
1300
Elmwood A6enue, Buffalo, NY
14222
, USA
b
US Department of Agriculture, Agricultural Research Ser6ice, Soybean and Alfalfa Research Laboratory, Belts6ille,
MD
20705
, USA
Received 13 July 1998; received in revised form 23 September 1998; accepted 24 September 1998
Abstract
The glyoxysomal isozyme of aspartate aminotransferase (EC 2.6.1.1: AAT1) was purified from soybean cotyledons.
AAT1 was separated from other soybean AAT isozymes by DEAE ion exchange chromatography and then purified
to electrophoretic homogeneity by immunoaffinity chromatography. The purified isozyme contained a single
polypeptide as detected on SDS PAGE with an estimated M
r
of 42 000 Da. The glyoxysomal AAT had a broad pH
optimum from 7.5 to 10.5 with maximal activity found at 9.5. The apparent K
m
values were 4.7 mM for aspartate,
0.11 mM for 2-oxoglutarate, 49 mM for glutamate, and 0.11 mM for oxaloacetate. Kinetic parameters and size of the
AAT1 polypeptide suggest this glyoxysomal form of AAT is similar to AAT isozymes targeted to other subcellular
compartments in plants. © 1998 Elsevier Science Ireland Ltd. All rights reserved.
Keywords
:
Aspartate aminotransferase; Isozyme; Glyoxysome; Soybean
1. Introduction
Aspartate aminotransferase (EC 2.6.1.1:AAT)
catalyzes the reversible transamination of aspar-
tate and 2-oxoglutarate to form oxaloacetate and
glutamate. AAT’s substrates are central to the
carbon/nitrogen metabolism of cells and numer-
ous metabolic roles have been suggested for it. In
plants, much of the research has focused on its
role in ammonia assimilation [1,2]. It is proposed
to function by recycling carbon skeletons in the
glutamine synthetase/2-oxoglutarate aminotrans-
ferase cycle. AAT is also the major source of
aspartate in cells which is itself a precursor for the
aspartate family of amino acids [3]. In some C4
plants, AAT also functions in a carbon shuttle
* Corresponding author..
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