Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

The effect was examined of two inhibitors, aporheine and thyroxine, on the kinetics of action of glutamate dehydrogenase in the presence of the coenzyme, NADH, in an inhibiting excess. from the results obtained it appears probable that these two competing ligands do not bind to the regulatory binding site of the enzyme. If the good substrate, glutamate, is replaced by a poor one, such as alanine, aporheine can behave under certain conditions as an activator; this phenomenon is most likely due to fact, like in other cases, that the association of alanine wit the enzyme-coenzyme complex is the slowest step of the reaction. The inhibitory action of aporheine in alkaline media is more complicated than its action at neutral and acidic pH. The sigmoid dependence of inhibition on aporheine concentration is best interpreted as a result of the induction of the second binding site of the enzyme subunit for aporheine after the binding of the first molecule of this ligand. The results obtained indicate the presence of a positive charge localized in the neighborhood of the binding site for the first aporheine molecule and also the important role played by some group of the enzyme with a pK-value about 8 during the induction of the second binding site with a relative high affinity for this ligand.