NADPH/NADH‐dependent cold‐labile glutamate dehydrogenase in Azospirillum brasilense

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Abstract
A cord-labile glutamate dehydrogenase (GDH, EC 1.4.1.3) has been purified to homogeneity from the crude extracts of Azospirillum brasilense. The purified enzyme shows a dual coenzyme specificity, and both the NADPH and NADH-dependent activities are equally cold-sensitive. The enzyme is highly specific for the substrates 2-oxoglutarate and glutamate. Kinetic studies with GDH indicate that the enzyme is primarily designed to catalyze the reductive amination of 2-oxoglutarate. The NADP+-linked activity of GDH showed Km values 2.5 .times. 10-4 M and 1.0 .times. 10-2 M for 2-oxoglutarate and glutamate respectively. NAD+-linked activity of GDH could be demonstrated only for the amination of 2-oxoglutarate but not for the deamination of glutamate. The Lineweaver-Burk plot with ammonia as substrate for NADPH-dependent activity shows a biphasic curve, indicating two apparent Km values (0.38 mM and 100 mM) for ammonia; the same plot for NADH-dependent activity shows only one apparent Km value (66 mM) for ammonia. The NADPH-dependent activity shows an optimum pH from 8.5 to 8.6 in Tris/HCl buffer, whereas in potassium phosphate buffer the activity shows a plateau from pH 8.4 to 10.0. At high pH (> 9.5) amino acids in general strongly inhibit the reductive amination reaction by their competition with 2-oxoglutarate for the binding site on GDH. The native enzyme has a Mr = 285,000 .+-. 20,000 and appears to be composed of six identical subunits of Mr = 48,000 .+-. 2000. The GDH level in A. brasilense is strongly regulated by the nitrogen source in the growth medium.