Inactivation of alpha-ketoglutarate dehydrogenase during enzyme-catalyzed reaction.

Alpha-Ketoglutarate dehydrogenase is inactivated during the enzymatic reaction. This inactivation is revealed both in a model system with an artificial electron acceptor and in the overall reaction catalyzed by the alpha-ketoglutarate dehydrogenase complex. Neither the substrate depletion and the product accumulation nor the changing of the alpha-ketoglutarate dehydrogenase oligomeric structure induces the inactivation. There are two independent mechanisms of alpha-ketoglutarate dehydrogenase inactivation in the course of the enzymatic reaction which are consistent with the two stages of the process. The first one corresponds to an essential decrease in activity during several minutes from the beginning of the reaction. This process can be reversed, occurs only during catalysis and manifests itself in the same degree both in the model system and during the functioning of the alpha-ketoglutarate dehydrogenase complex. The second stage is slow, irreversible and more apparent in the model system; it coincides with the inactivation due to the alpha-ketoglutarate dehydrogenase preincubation with hexacyanoferrate alone. The data obtained provide evidence that irreversible inactivation of alpha-ketoglutarate dehydrogenase during the enzymatic reaction is caused by the oxidant.