Some properties of the malic enzyme of pigeon liver. 1. Conversion of malate into pyruvate

Abstract

Some aspects of the kinetics, and of the inhibition by carboxylic acids, of the conversion of L-malate into pyruvate have been investigated. Mn++ greatly increased the rate of malate decarboxylation. Mg++, Co++ and Ni++ also activated the enzyme, but the maximum rate obtained was less than with Mn++. Cu++, Zn++ and Ca++ inhibited the enzyme. The decarboxylation of malate was inhibited by high malate concentrations. Since the inhibition was reversed by increased Mn++ concentrations, it was probably due to chelation of Mn++ by the malate. All the dicarboxylic acids tested inhibited the reaction. The inhibitory power decreased in the follow order: oxaloacetic acid (which inhibited at concentrations less than that of the substrate), tartronic acid, mesoxalic acid, malonic acid, oxalic acid, mesotartaric acid, D-tartaric acid, L-aspartic acid, succinic acid, fumaric acid and maleic acid. For the last 4 acids concentrations 100 times that of the substrate were required for appreciable inhibition. D- and L-Lactate, pyruvate and DL-[alpha]-hydroxy-butyrate inhibited slightly at high concentrations. Propionate and DL-[beta]-hydroxybutyrate were not inhibitory at the same concentrations. Citrate and fluorocitrate also inhibited malate decarboxylation. For equal inhibition, the concentration of citrate required was over 10 times that of the fluorocitrate. The inhibition by di- and tri-carboxylic acids was partly reversed by addition of Mn++ (or Mg++) and partly by raising the substrate concentration. The data are in accordance with the view that the inhibitions are due partly to chelation of the bivalent cations required as cofactors and partly to competition with the malate for the enzyme-binding site.