Novel insights in the regulation of the plant succinate dehydrogenase

We have investigated the regulation of succinate dehydrogenase (SDH) in potato tuber mitochondria by simultaneous measurement of the oxygen uptake rate and the ubiquinone-redox poise. SDH activity is stimulated by both ATP and ADP in a manner insensitive to carboxyatractyloside suggesting that activation occurs at the cytoplasmic side of the mitochondrial inner-membrane. It was found that ATP affects the SDH kinetics with respect to the Q-redox poise such that activity is considerably increased at any reduction level of the Q-pool. Using the Q-redox poise as a quantitative measure of activation, half-maximal activation by ATP was shown to occur at 2.7 µM. Interestingly, our experimental approach reveals that SDH is also activated by oligomycin (half-maximally at 16 nM). This oligomycin-induced activation results in increased SDH activities at any Q-redox poise, though the extent of stimulation is less than that caused by ATP. A comparable degree of activation was also observed when oligomycin was substituted with N,N'-dicyclohexycarbodiimide. De-energisation of the mitochondrial inner-membrane by an uncoupler results in a reversal of the oligomycin-mediated activation. Furthermore, the rate of activation by adenine nucleotides is decreased dramatically when mitochondria are pre-incubated in the presence of an uncoupler. These data indicate that, in vitro, SDH activity is regulated by the energy status of the mitochondrial inner-membrane. The possible physiological relevance of this regulatory mechanism will be discussed