Malate Decarboxylation by Mitochondria of the Inducible Crassulacean Acid Metabolism Plant <italic>Mesembryanthemum crystallinum</italic>

Abstract

Mitochondria isolated from leaves of Mesembryanthemum crystallinum oxidized malate by both NAD malic enzyme and NAD malate dehydrogenase. Rates of malate oxidation were higher in mitochondria from plants grown at 400 mil NaCl in the rooting medium and performing Crassulacean acid metabolism (CAM) than in mitochondria from plants grown at 20 mM NaCl and exhibiting C3-photosynthetic CO2 fixation. The mitochondria isolated from plants both in the CAM and C3 modes were tightly coupled and gave high respiratory control. At optimum pH for malate oxidation (pH 7.0), pyruvate was the major product in mitochondria from CAM-M. crystallinum, whereas mitochondria from C3-M. crystallinum produced predominantly oxaloacetate. Both the extracted NAD malic enzyme in the presence of CoA and the oxidation of malate to pyruvate by the mitochondria from plants in the CAM mode had a pH optimum around 7.0 with activity declining markedly above this pH. The activity of NAD-malic enzyme, expressed on a cytochrome c oxidase activity basis, was much higher in mitochondria from the CAM mode than the C3 mode. The results indicate that mitochondria of this species are adapted to decarboxylate malate at high rates during CAM.