Inhibition of Hog Liver Crystalline Quinolinate Phosphoribosyltransferase by Nucleotides,Quinolinate Analogues and Sulfhydryl Reagents

Effects of the precursors and intermediates of the NAD biosynthetic pathway, and of quinolinate analogues etc. on hog liver crystalline quinolinate phosphoribosyltransferase (an intermediary enzyme in the de novo NAD biosynthetic pathway) activity were investigated. The enzyme activity was inhibited by many kinds of nucleotides, phthalic acid and SH reagents. But amino acids, nicotinic acid and nicotinamide had practically no effect. The apparent inhibition by ATP removed by raising Mg²⁺ concentration. Phthalic acid was proved to be a competitive inhibitor to quinolinic acid. The Ki value for phthalic acid was calculated at 1.7 × 10^?4 m by a Dixon plot.