Control of Glutamate Dehydrogenase from Green Tobacco Callus Mitochondria by Ca2$ and pH

Glutamate dehydrogenase (GDH) (EC 1.4.1.3 EC] .) purified from greentobacco callus mitochondria was activated markedly by Ca^2$ inthe amination reaction. This activation was detectable evenat concentrations below 5 µM Ca^2$. Saturation curves for the three substrates of the aminationreaction showed normal Michaelis-Menten kinetics in the presenceof 1 mM of Ca^2$, but pronounced substrate inhibition occurredwithout Ca^2$. The effect of Ca^2$ was chiefly on the maximalvelocity. The saturation curve for NH₄Cl in the presence of Ca^2$ was modulatedby a change in pH. The apparent K_m value for NH₄Cl markedlydecreased whereas that for -ketoglutarate increased slightlywhen the pH was raised from 7.3 to 9.0. In contrast, the K_mfor NADH was little affected by raising the pH. The characteristicof GDH which increases its affinity for NH₄Cl when the pH israised may be compatible with the detoxification of ammonia. ¹ Present address: Mochida Pharmaceutical Co., Ltd. (Received August 24, 1981; Accepted November 28, 1981)