Physiological studies on acid metabolism. 7. Malic enzyme from Kalanchoë crenata: effects of carbon dioxide concentration

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Abstract
Cell-free extracts of the crassulacean plant Kalanchoe crenata catalysed the reversible reductive carboxylation of pyruvate to yield malate. The malic enzyme was specific with respect to L-malate, Mn2+ and triphosphopyridine nucleotide. In the presence of 0.5 mM substrate, the pH optimum of the oxidative decarboxylation was 7.2. Km (malate) was 0.55 m-mole/1 (25[degree], pH 7.4.) The extracts also catalyzed the Mn2+-dependent decarboxylation of oxaloacetate with an optimum pH of 5.5. The characteristics of the enzyme are compared with those of malic enzyme from pigeon liver and wheat germ. CO2 accelerated the reductive carboxylation of pyruvate and retarded the oxidative decarboxylation of malate. Kinetically, the effects of CO2 on the initial rates of the oxidative decarboxylation of malate resembled a partially competitive inhibition. The results have been discussed in relation to the supposed role of malic enzyme in acid metabolism of crassulacean plants. An interpretation of the reaction kinetics has been suggested.