Catalysis of Both Oxidation and Reduction of Glyoxylate by Pig Heart Lactate Dehydrogenase Isozyme 1

Abstract Pig heart l-lactate dehydrogenase isozyme 1 was purified by preparative polyacrylamide gel electrophoresis. The amino acid composition, specific activity, and pH optima with several substrates were determined. Catalysis of both the oxidation and the reduction of glyoxylate by this enzyme was studied. Reduction of glyoxylate to glycolate requires DPNH, and the rate is optimal at pH 6.9. Oxidation of glyoxylate to oxalate requires DPN+, and the pH optimum is 9.3. General rate equations for both reactions were determined. The two reactions appear to be catalyzed in a common protein site, and a compulsory order mechanism for the reactions is suggested by analogy with other lactate dehydrogenase-catalyzed reactions. Inhibitor constants with oxalate, oxamate, and several other glyoxylate analogues were determined for both reactions at pH 8.6. The pattern of inhibition by oxalate and oxamate is similar to that observed when pyruvate and lactate are used as substrates for the enzyme. The glyoxylate reactions appear to occur in the same site and probably involve the same amino acid functional groups as the pyruvate-lactate reactions.