SPECIFICITY OF THE ORGANIC-ACID ACTIVATION OF ALTERNATIVE OXIDASE IN PLANT MITOCHONDRIA

The claim that succinate and malate can directly stimulate the activit y of the alternative oxidase in plant mitochondria (A.M. Wagner, C.W.M . van den Bergen, H. Wincencjusz [1995] Plant Physiol 108: 1035-1042) was reinvestigated using sweet potato (Ipomoea batatas L.) mitochondri a. In whole mitochondria, succinate (in the presence of malonate) and both L- and D-malate stimulated respiration via alternative oxidase in a pH- (and NAD(+))-dependent manner. Solubilized malic enzyme catalyz ed the oxidation of both L- and D-malate, although the latter at only a low rate and only at acid pH. In submitochondrial particle preparati ons with negligible malic enzyme activity, neither L- nor D-malate sti mulated alternative oxidase activity. However, even in the presence of high malonate concentrations, some succinate oxidation was observed v ia the alternative oxidase, giving the impression of stimulation of th e oxidase. Neither L-malate nor succinate (in the presence of malonate ) changed the dependence of alternative oxidase activity on ubiquinone reduction state in submitochondrial particles. In contrast, a large c hange in this dependence was observed upon addition of pyruvate. Half- maximal stimulation of alternative oxidase by pyruvate occurred at les s than 5 mu M in submitochondrial particles, one-twentieth of that rep orted for whole mitochondria, suggesting that pyruvate acts on the ins ide of the mitochondrion. We suggest that malate and succinate do not directly stimulate alternative oxidase, and that reports to the contra ry reflect intramitochondrial generation of pyruvate via malic enzyme.