Regulation of succinate dehydrogenase was investigated using tightly coupled potato tuber mitochondria in a novel fashion by simultaneously measuring the oxygen uptake rate and the ubiquinone (Q) reduction level. We found that the activation level of the enzyme is unambiguously reflected by the kinetic dependence of the succinate oxidation rate upon the Q-redox poise. Kinetic results indicated that succinate dehydrogenase is activated by both ATP (K ½ ϳ 3 M) and ADP. The carboxyatractyloside insensitivity of these stimulatory effects indicated that they occur at the cytoplasmic side of the mitochondrial inner membrane. Importantly, our novel approach revealed that the enzyme is also activated by oligomycin (K ½ ϳ 16 nM). Time-resolved kinetic measurements of succinate dehydrogenase activation by succinate furthermore revealed that the activity of the enzyme is negatively affected by potassium. The succinate-induced activation (؎K ؉ ) is prevented by the presence of an uncoupler. Together these results demonstrate that in vitro activity of succinate dehydrogenase is modulated by the protonmotive force. We speculate that the widely recognized activation of the enzyme by adenine nucleotides in plants is mediated in this manner. A mechanism that could account for such regulation is suggested and ramifications for its in vivo relevance are discussed.Succinate dehydrogenase is a membrane-bound component of the respiratory chain of aerobic organisms (1). It couples the reduction of ubiquinone (Q) 1 to the oxidation of succinate and is, as such, a Krebs cycle as well as a respiratory chain enzyme. This duality suggests a potentially important role in the control of energy metabolism, a reason why the enzyme's regulation has been the subject of previous extensive investigations in both mammalian (2, 3) and plant (4, 5) systems.The isolation of succinate dehydrogenase from mammalian sources yields a predominantly deactivated enzyme due to the presence of tightly bound oxaloacetate (6, 7). The isolated enzyme, however, can be activated by substrates and substrate analogues (8) as well as by many anions, acid pH (9, 10), reducing treatments, (11) and QH 2 (2, 12). Additionally, succinate dehydrogenase is activated by ATP (3) via a mechanism that, even to date, is poorly understood. In the presence of ADP or an uncoupler, the mammalian enzyme is rapidly deactivated (3). Gutman et al. (3) attributed physiological relevance to these observations reasoning that deactivation of succinate dehydrogenase would benefit the cell when the ATP:ADP ratio is low, because it would allow faster NADH oxidation and hence more efficient ATP synthesis. The subsequently formed ATP would then activate the enzyme to allow turnover of the Krebs cycle to proceed in an unhindered fashion (3).In plants, succinate dehydrogenase is activated by substrates, QH 2 , ATP, anions, and acid pH in a manner similar to that observed in mammalian systems (4, 5). The effect of ADP on the plant enzyme, however, has not been established conclusively. Oestreicher...
