[Modification of glutamate dehydrogenase by pyridoxal-5'-phosphate. Study of the cooperative type of inhibition by GTP].

The character of allosteric inhibition of glutamate dehydrogenase by GTP was studied. The derivative of the enzyme not capable of being polymerized was taken as a model. It was shown that: in the absence of NADH every protomer of this derivative can bind one molecule of GTP; in the presence of NADH the additional binding site for GTP was induced; the modification of the enzyme derivative by pyridoxal-5-phosphate in the presence of NADH and alpha-ketoglutarate blocked the NADH-induced GTP binding site and the disappearance of positive kinetic cooperativity induced by GTP was observed; to achieve the inhibitory action of GTP the binding of the effector to only one (NADH-induced) site was enough; the role of GTP binding to the NADH-induced site is to provide better affinity of the effector to the "inhibitory" centre; the positive kinetic cooperativity of inhibition of glutamate dehydrogenase by GTP depends probable on the cooperative character of interaction between the two molecules of GTP to each protomer of the enzyme.