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CO₂ Metabolism in Corn Roots. III. Inhibition of P-enolpyruvate Carboxylase by L-malate ¹

Department of Life Sciences, University of California, Riverside, California 92502

Phosphoenolpyruvate carboxylase was purified from corn root tips about 80-fold by centrifugation, ammonium sulfate fractionation, and anion exchange and gel filtration chromatography. The resulting preparation was essentially free from malate dehydrogenase, isocitrate dehydrogenase, malate enzyme, NADH oxidase, and pyruvate kinase activity. Kinetic analysis indicated that L-malate was a noncompetitive inhibitor of P-enolpyruvate carboxylase with respect to P-enolpyruvate (K_I = 0.8 mM). D-Malate, aspartate, and glutamate inhibited to a lesser extent; succinate, fumarate, and pyruvate did not inhibit. Oxaloacetate was also a noncompetitive inhibitor of P-enolpyruvate carboxylase with an apparent K_I of 0.4 mM. A comparison of oxaloacetate and L-malate inhibition suggested that the mechanisms of inhibition were different. These data indicated that L-malate may regulate CO₂ fixation in corn root tips by a feedback or end product type of inhibition.