Properties of Quinolinate Phosphoribosyl transferases from Rat Liver and Kidney

Abstract

Quinolinate phosphoribosyltransferase (nicotinatenucleotide : pyrophosphate phosphoribosyltransferase (carboxylating), EC 2.4.2.19) was found to localize in cytosol in rat liver and kidney by sucrose density gradient centrifugation. An enzyme, degrading 5-phosphoribosyl-1-pyrophosphate, existed in rat liver and kidney homogenates. Therefore, the addition of 1 mM NaF into the assay medium was necessary to measure the actual enzyme activity when the homogenate was used as quinolinate phosphoribosyltransferase source. The optimum pH of rat liver and kidney quinolinate phosphoribosyltransferases were 7.0 and 6.0, respectively, in the presence of 1 mM 5-Phosphoribosyl-1-pyrophosphate. Rat liver enzyme activity was inhibited by 5-phosphoribosyl-1-pyrophosphate at pH 9.0 but not at pH 7.0. At lower concentrations of 5-phosphoribosyl-1-pyrophosphate (below 0.02 mM), the enzyme activity at pH 9.0 was higher than that at pH 7.0. Rat kidney enzyme have also the same property as rat liver enzyme. Phthalic acid inhibited both enzyme activities. Niacin nucleotides did not inhibit either enzyme activities.