Unidirectional Inhibition of Glutamate Dehydrogenase by Metabolites

A variety of metabolites and chelating agents have been found to elicit a unique form of inhibition on an NAD-specific glutamate dehydrogenase from Blastocladiella emersonii. When measuring the initial rate of the reaction, the oxidative deamination of glutamate was completely inhibited by the metabolites citrate, isocitrate, fructose 1,6-diphosphate, and fumarate and by the nonmetabolites ATP and ethylenediaminetetraacetate. (ATP may be regarded as a metabolite in some reactions involving its transformation to ADP and AMP.) The rate of the reductive amination of α-ketoglutarate reaction was not affected by any of the inhibitors except ATP. The phenomenon of unidirectional inhibition in these initial velocity studies does not consider the microscopic reversibility of the reaction. Divalent cations, AMP, and ADP reversed this inhibition. AMP and ADP, in addition, served as allosteric effectors of this enzyme, activating the reaction in both directions of assay. ATP, on the other hand, participated as a negative allosteric effector. The inhibitory effects of the metabolites and EDTA were dependent on pH. All of the metabolic inhibitors are important intermediates which are involved in the production of energy in the tricarboxylic acid cycle and in glycolysis. A scheme that incorporates these findings into a regulatory network connecting glycolysis and the tricarboxylic acid cycle is formulated.