Regulation of Nicotinamide Adenine Dinucleotide Phosphate-specific Glutamate Dehydrogenase in Germinated Spores of Geotrichum candidum

Germinating spores of Geotrichum candidum produce only a nicotinamide adenine dinucleotide phosphate-linked glutamate dehydrogenase. Synthesis of glutamate dehydrogenase was repressed by the presence of ammonia, whereas urea, glutamate, or glutamine were ineffective. The enzyme was not subject to catabolite repression and was localized in the cell sap fraction. The glutamate dehydrogenase has been purified 93-fold and showed maximal activity at pH 8.2 in the forward and reverse directions. When measuring the initial reaction rate at pH 7.2, a variety of tricarboxylic acid cycle intermediates displayed additive and unidirectional activation of the reductive amination reaction and inhibition of the oxidative deamination reaction. The modulating effects were pH-dependent and diminished at alkaline pH values. Substrate inhibition exerted by alpha-ketoglutarate was strongest at neutral pH.Rate-concentration plot with respect to alpha-ketoglutarate at pH 7.2 was sigmoidal, and the enzyme was entirely inactive at substrate concentrations lower than 1 mm. In the presence of fumarate, the enzyme did not exhibit the cooperative interaction described above and was markedly activated at alpha-ketoglutarate concentrations lower than 1 mm. The tricarboxylic acid cycle intermediates released the enzyme from substrate inhibition by alpha-ketoglutarate. Double reciprocal rate-concentration plots displayed noncompetitive inhibition between succinate and NADP and a mixed noncompetitive and competitive inhibition between succinate and glutamate. The binding order of substrates in the reductive amination reaction was NADPH, alpha-ketoglutarate, and subsequently ammonia.