Inhibition of malic enzyme by S-oxalylglutathione, a probable in vivo effector

Various oxalyl thiol esters (RSCOCOO-), especially S-oxalylglutathione (GS-Ox), were found to be very effective inhibitors of chicken liver malic enzyme. When the conditions are similar to those encountered physiologically high reduced nicotinamide adenine dinucleotide phosphate (NADPH) concentrations], inhibition is detectable with less than 1 microM concentrations of GS-Ox. The amount of inhibition is not reversed by excess glutathione, thus indicating that it is not due to oxalyl transfer to some enzymic thiol group with release of glutathione. Detailed kinetic studies show that the inhibition by GS-Ox can be treated as a simple reversible binding to the enzyme; the double reciprocal plot patterns indicate that the inhibition is linear noncompetitive (mixed type), vs. both L-malate in the oxidative decarboxylation reaction and pyruvate in the reverse reaction. At pH 7.4 and 25 degrees C in the presence of 100-200 microM NADPH, the Kis and Kii values for GS-Ox are 0.7 and 5 microM, respectively, and are the same for reactions run in either direction. The high specificity for GS-Ox is indicated by the observation that, under similar conditions, the Kis values for S-oxalyl coenzyme A and S-oxalyl-N-acetylcysteamine are 40 and 150 microM, respectively. Such high specificity indicates that the enzyme has evolved a specific binding site for the glutathione part of GS-Ox. The current results, when considered in conjunction with recent evidence that oxalyl thiol esters are present in animal tissues at concentrations up to 50 microM, imply that GS-Ox is an important in vivo regulator of malic enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)