Studies on hog liver quinolinate phosphoribosyltransferase

Quinolinate phosphoribosyltransferase in a crystalline form is isolated from an animal for the first time successfully, and its physicochemical properties and enzymatic properties are examined. The 14CO2 produced by decarboxylation of a carboxyl group which bondes with a second carbon of quinolinate was measured quantitatively, using 14C labeled quinolinic acid. The isolated enzymatic protein was 231 g with 61% yield. The molecular weight of the enzyme was about 170,000, and the molecular weight of subunit was 34,000. This protein is supposed to have a possibility to constitute five same subunits. The most preferable pH of the enzyme reaction was 6.1 and the reaction required a metallic ion. The Km values for quinolinic acid and PRPP were 1.2 x 10-4M and 1.8 x 10-4M respectively. The enzyme reaction product was identified as β-niacin mononucleotide and was inhibited strongly with a metallic ion of 2 or 3 valency. The Mg+2 ion was captured by nucleotide such as ATP and showed apparent inhibition. Phthalic acid was found to be an antagonistical inhibitor for quinolinic acid. This enzyme was SH enzyme, and number of total SH group was 25.2 per one molecule of this enzyme by DTNB determination. Strong reactive SH groups were 10.2. Numbers of semicystine were 29 by detection with amino acid analysis. Activity of this enzyme is reduced remarkably with a chemical modifying agent. An important role was played for SH group, amino group, histidine residue and arginine residue, which bondes quinoline with the enzyme. (Iwakiri, K.)