Crystalline Quinolinate Phosphoribosyltransferase from Hog Liver: The Molecular Weight and Some Enzymic Properties

Abstract

The molecular weight of crystalline quinolinate phosphoribosyltransferase from hog liver was re-evaluated at 202,000 and 210,000 by the methods of sedimentation equilibrium and Sephacryl S–200 gel filtration, respectively. Since the molecular weight of its subunit was 34,200, this enzyme was considered to be composed of six identical subunits. The enzyme required Mg2+ for its activity but it was found that the crystalline enzyme did not contain any detectable amounts of Mg, Mn, Fe, Cu, Zn and Ca, by using an atomic absorption spectrophotometer. In the liver enzyme reaction, Mn2+ could fully replace Mg2+. The reaction mechanism of the enzyme was like ping-pong. The enzyme activity was inhibited by various carboxylic acids, but this inhibition was suppressed by increasing the Mg2+ ion concentration. The enzyme activity was inhibited by the addition of glycerol and the inhibition increased as the pH was raised.