Malate decarboxylation in isolated mitochondria from the crassulacean acid metabolism plant Sedum praealtum

Abstract

Mitochondria isolated from the Crassulacean acid metabolism plant Sedum praealtum were demonstrated to decarboxylate added malate at basal rates of 30–50 μmol mg −1 original chlorophyll h −1. The basal rate could be stimulated markedly by the addition of ADP, oxaloacetic acid, an uncoupler of oxidative phosphorylation, or NAD, with maximum rates of 70–100 μmol mg −1 original chlorophyll h −1 observed. These observed rates were high enough to account for a large proportion of the estimated rate of malate decarboxylation in vivo. The major products of malate oxidation by the mitochondria in most cases were found to be pyruvate and CO 2, indicating that malate oxidation in these mitochondria proceeds mainly through NAD malic enzyme rather than NAD malate dehydrogenase. Under conditions employed little of the pyruvate formed was further oxidized, suggesting a fate other than oxidation (conversion to starch) for this pyruvate. Malate decarboxylation by mitochondria and by partially purified NAD malic enzyme was markedly inhibited by NaHCO 3. A possible physiological role is suggested for this inhibition as a feedback control on the enzyme.