Inhibitions of beef liver D-glycerate dehydrogenase (EC 1. 1. 1. 29) by glycolytic intermediates and by nucleotides were studied kinetically. It was found that these metabolites inhibited the enzyme noncompetitively with the substrate. In order to clarify the relation between the substrate inhibition by hydroxypyruvate and the inhibition by these metabolites, a simple graphical method was applied to analyze the multiple inhibition kinetics of two noncompetitive inhibitors. The presence of a regulatory site was proved by this method and the binding of hydroxypyruvate or the metabolites to this site was responsible for the inhibition of the enzyme activity. It was also shown that the trinitrophenylation of this enzyme caused a remarkable change of the regulatory properties of this enzyme.

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