Dilute solutions of chicken heart lactic dehydrogenase were exposed to60 Co gamma irradiation. The dose effect curve varied in shape depending on the pyruvate concentration used in the assay of LDH activity. At a lower concentration of pyruvate, the LDH activity decreased exponentially with the dose. However, at a excess concentration of pyruvate, the residual LDH activity initially increased with increasing dose before a loss in activity was observed. The S-V plot of irradiated LDH showed that the substrate inhibition of chicken heart LDH at excess pyruvate concentration was considerably depressed by gamma irradiation. The mechanism of the decreased substrate inhibition was studied by examining the formation of an abortive ternary complex of the type pyruvate-NAD-LDH upon incubation of LDH with pyruvate and NAD. The result shows that irradiation decreases the formation of the abortive ternary complex, resulting in the release from substrate inhibition.

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