Glutamic Dehydrogenases of Oomycetes

Purified NAD-specified glutamic dehydrogenases from two fungi, Pythium debaryanum and Achlya sp. (1969), have been studied kinetically, and some of their physical properties elucidated. The enzymes are regulated by a multitude of allosteric effectors: NADP+, GTP, short chain acyl-CoA derivatives, P-enolpyruvate, and ATP as activators; citrate, AMP, and long chain CoA derivatives as inhibitors. The kinetic mechanism was determined for activated and non-activated enzymes. In both cases, an ordered binary-ternary mechanism prevailed. The order of substrate addition and product release is identical with the order found for all NADP-specific glutamic dehydrogenases although NADP+ is not a substrate of the enzyme. It is proposed that the enzyme may have evolved as a catalyst that was initially non-specific for both coenzymes and later became modified to accommodate NADP+ (or NADPH) as an allosteric activator. The enzymes are extremely heat labile. They denature rapidly above 37°. The activators and inhibitors fail to stabilize the proteins against heat denaturation. A novel effect of pH influence on the degree of activation and inhibition was observed. The enzymes are activated and inhibited best at the pH minima and least of all at the optimum pH. The relevance of these pH effects to the mitochondrial location of the enzymes is discussed.