New insights into the regulation of plant succinate dehydrogenase - On therole of the protonmotive force

Regulation of succinate dehydrogenase was investigated using tightly couple d potato tuber mitochondria in a novel fashion by simultaneously measuring the oxygen uptake rate and the ubiquinone (Q) reduction level. We found tha t the activation level of the enzyme is unambiguously reflected by the kine tic dependence of the succinate oxidation rate upon the Q-redox poise. Kine tic results indicated that succinate dehydrogenase is activated by both ATP (K-1/2 similar to 3 mum) and ADP. The carboxyatractyloside insensitivity o f these stimulatory effects indicated that they occur at the cytoplasmic si de of the mitochondrial inner membrane. Importantly, our novel approach rev ealed that the enzyme is also activated by oligomycin (K-1/2 similar to 16 nm). Time-resolved kinetic measurements of succinate dehydrogenase activati on by succinate furthermore revealed that the activity of the enzyme is neg atively affected by potassium. The succinate-induced activation (+/-K+) is prevented by the presence of an uncoupler. Together these results demonstra te that in vitro activity of succinate dehydrogenase is modulated by the pr otonmotive force. We speculate that the widely recognized activation of the enzyme by adenine nucleotides in plants is mediated in this manner. A mech anism that could account for such regulation is suggested and ramifications for its in vivo relevance are discussed.