Pyruvate carboxylase from chicken liver: Effects of sulfate and other anions on catalytic activity and structural parameters

Abstract

Sulfate ion acts as a competitive inhibitor of pyruvate carboxylase purified from chicken liver with respect to CoASAc. The K i for SO 4 2− is obtained as 1.0–1.3 m m at pH 7.8 and 5.4 m m at pH 7.1. The extent of cooperativity in the relationship between initial rate and SO 4 2− concentration is minimal at very low CoASAc concentration but increases as the activator concentration is raised. However, the presence of SO 4 2− does not alter the extent of cooperativity in the relationship between initial rate and CoASAc concentration. Inhibition by SO 4 2− is either uncompetitive (pyruvate) or noncompetitive (MgATP 2−, HCO 3 −, phosphate) with respect to the substrates of the pyruvate carboxylase reaction. Sulfate decreases the rate of inactivation observed when pyruvate carboxylase is incubated at 2 °C and increases the rate of inactivation caused by incubation of this biotin-enzyme with avidin. Dissociation constants calculated from these data are in reasonable agreement with the K i obtained from initial rate studies conducted at the same pH. Several other divalent and monovalent anions also act as competitive inhibitors with respect to CoASAc and cause similar alterations in the rates of inactivation resulting from incubation at 2 °C or in the presence of avidin. The data indicate that inhibition of this pyruvate carboxylase by anions such as SO 4 2− may be due to interaction of these ions at the CoASAc site.