Kinetic studies to determine the mechanism of regulation of bovine liver glutamate dehydrogenase by nucleotide effectors

Abstract

A combination of kinetic and isotope effect studies in the presence and absence of effectors ADP and GTP was used to elucidate the mechanism of regulation of bovine liver glutamate dehydrogenase. ADP at low concentrations of glutamate competes with TPN for free enzyme. GTP exhibits a similar effect at high concentrations (.gtoreq. 100 .mu.M). When ADP binds at its allosteric site, it increases the off rates of both .alpha.-ketoglutarate and TPNH from their product complexes. This results in a decrease in Vmax/Km for both substrates, an increase in Vmax and an increase in the deuterium isotope effects for all 3 parameters so that they are all .apprx. 1.3. The rate of release of glutamate from E-TPNH-glutamate is also apparently enhanced since no substrate inhibition by glutamate is observed in the presence of ADP. The effect of GTP is in opposition to that of ADP in that GTP decreases the off rates for both TPN and glutamate from E-TPN-glutamate as well as the off rates for .alpha.-ketoglutarate and TPNH. This results in an increase in the Vmax/Km for both substrates, a decrease in the deuterium isotope effects for all 3 parameters to a value of 1. Substrate inhibition by glutamate is also eliminated by GTP probably by preventing any significant accumulation of E-TPNH to which glutamate binds an inhibitor.