The Effect of Allosteric Modifiers on the Rate of Denaturation of Glutamate Dehydrogenase.

In this scheme monomer z catalyzes the glutamate dehydrogenase reaction (although this form of the enzyme also has alanine dehydrogenase activity) and aggregates to form higher molecular weight polymers as its concentration is increased. Monomer y has increased alanine dehydrogenase activity’ and little or no glutamate dehydrogenase activity, and does not aggregate readily at higher protein concentrations. Thus, alterations in monomer conformation (Equilibrium II) affect the distribution of all the species and result in changes in the catalytic properties of the protein. Previous studies on the effects of various reagents and conditions on the inactivation of glutamate dehydrogenase have appeared (11-13). In the present experiments it has been found that in solutions of low ionic strength the enzyme undergoes an apparently irreversible denaturation reaction, the rate of which has been followed by three independent methods: (a) exposure of the previously unreactive -SH groups, (b) a fall of about 40% in the intrinsic fluorescence of the protein, and (c) loss of both glutamate and alanine dehydrogenase activities. When measured under identical conditions, these parameters all followed the same first order kinetics. It has also been found that reagents which alter Equilibrium II in favor of form y accelerate this reaction. It is suggested, therefore, that at low ionic strength monomer y unfolds more readily than does monomer 2.