Abstract
Effects of various compounds on crystalline quinolinate phosphoribosyltransferase (EC 2.4.2.19) from hog kidney were investigated. Phthalic acid and Pyrazine-2, 3-dicarboxy-lic acid were both competitive inhibitors for quinolinic acid. The enzyme activity was inhibited by heavy metal divalent cations and trivalent cations. Nucleoside triphosphates also inhibited the enzyme activity and this inhibition was recovered by raising the MgCl2 concentration. Glycerol activated the enzyme activity at pH from 4.0 to 4.4, while it inhibited at above pH 5.0. Isoleucine content of the crystalline enzyme was extremely low (17 residues per mol of enzyme). The total SH groups determined by 5, 5'-dithio-bis-(2-nitrobenzoic acid) were 30 groups per mol of enzyme protein in the presence of guanidine hydrochloride. The enzyme activity was inhibited by 5, 5'-dithio-bis-(2-nitrobenzoic acid), p-chloromercuribenzoic acid, 2, 4, 6-trinitrobenzenesulfonic acid, β-naphthoquinone-4-sulfonic acid and diethylpyrocarbonate. The results suggest that SH, amino and imidazole groups exist in the active site of the enzyme. pKe value of the enzyme was 9.25.
