[Peculiarities of the regulation of adrenal oxoglutarate dehydrogenase complex by NADH and adenosine diphosphate].

It was demonstrated that NADH inhibits both the lipoamide dehydrogenase and the oxoglutarate dehydrogenase components of the bovine adrenal complex. NADH increases the region of 2-oxoglutarate concentrations, in which the signs of cooperative binding of the substrate are observed in the absence of phosphate ions. The rate of the oxoglutarate dehydrogenase complex-catalyzed reaction versus NADH curve in not hyperbolic. The coefficient q equivalent to the Hill coefficient exceeds 1 within the range of low concentrations of NADH. The value of q increases at a higher concentration of the substrate and in the presence of ADP. The plot of v0 versus ADP at low substrate concentrations gives a S-shaped curve. Hence the presence of positive homotropic cooperativity of the activator-binding sites can be postulated. The changes in the activity of the oxoglutarate dehydrogenase complex at various pH values in the presence and absence of NADH and ADP as well as the loss of sensitivity to ADP at pH 6.0 substantiate the allosteric type of action of the effectors. The effects of NADH and ADP on the oxoglutarate dehydrogenase component of the complex do not involve association-dissociation processes. ADP to some extent hampers NADH inhibition, but does not prevent the process even at high concentrations, which is indicative of isolation of binding sites of each one of the effectors.