Purification and Properties of Phosphoribosyl Pyrophosphate Synthetase from Rat Liver

Phosphoribosyl pyrophosphate synthetase has been purified extensively from rat liver. Gel filtration studies indicate that the enzyme is a complex of large molecular weight. On electron microscopy, stacking of enzyme molecules into long, linear aggregates is observed. The subunit molecular weight is 40,500, as measured by disc gel electrophoresis in sodium dodecyl sulfate. MgATP appears to be the required form of ATP as substrate, and excess free Mg2+ stimulates the reaction at low MgATP concentrations. The enzyme is highly specific for ribose 5-phosphate, and substrate inhibition occurs at ribose 5-phosphate concentrations above 1.5 mm. Like phosphoribosyl pyrophosphate synthetases from other sources, the rat liver enzyme requires inorganic phosphate for catalytic activity, but is not inactivated by dialysis against phosphate-free buffer. In the presence of optimal levels of phosphate and free Mg2+, the apparent Km is 0.22 mm for MgATP and 0.29 mm for ribose 5-phosphate.