Full-time course studies of bovine liver glutamate dehydrogenase. Simulation of inhibition by pyridoxal-5'-phosphate.

Inhibition of bovine liver glutamate dehydrogenase by pyridoxal-5'-phosphate was studied by measuring the full time course of the oxidation of NADPH. Progress curves were determined before and after incubation of the enzyme with PLP in the presence of saturating concentrations of alpha-ketoglutarate and ammonium ion, at pH 7.4 and 25 degrees C. The data were fitted to the integrated Michaelis-Menten equation and an inhibition model derived. According to the model, PLP inhibits the enzyme non-competitively, by reversible formation of the complexes E--PLP and E--PLP--NADPH; the oxidation of NADPH is also inhibited by NADP+. After incubation with PLP, the dissociation constants of E--NADPH and E--NADP+ (Km and Kp) show a very definite decrease, while the maximum rate of oxidation (Vm) is increased. The inhibition constants for PLP were also computed.