Stability and Regulation of Phosphoribosyl Pyrophosphate Synthetase from Rat Liver

Phosphoribosyl pyrophosphate (PRPP) synthetase purified from rat liver loses activity when assayed at low protein concentrations, which results in a nonlinear relationship between velocity and enzyme concentration. A linear relationship is obtained at higher enzyme concentrations or in the presence of albumin, EDTA, or sulfhydryl compounds. Iodoacetamide and p-chloromercuribenzoate abolish the effect of these compounds, suggesting that stabilization of the enzyme results from the protection of an essential sulfhydryl group on the enzyme. Certain nucleotides and 2,3-diphosphoglycerate inhibit the unstabilized enzyme, but have no effect on the stabilized enzyme. When stabilized, the enzyme is inhibited by dATP, ADP, dADP, and AMP, and minor degrees of occur with several thiopurine compounds. Inhibition by ADP is competitive with respect to ATP, while AMP is noncompetitive with respect to both substrates. These findings are consistent with regulation of rat liver PRPP synthetase by cellular energy charge. The rat liver enzyme does not appear to respond to general metabolic pool inhibition as a regulatory parameter, as has been suggested for PRPP synthetase from other sources.