Multiple control mechanisms for succinate dehydrogenase in mitochondria

Summary Succinate dehydrogenase (SD) in intact, respiring mitochondria undergoes activation and deactivation in response to the metabolic state of the mitochondria. Highest SD activity is observed in state 4 and lowest in state 2 or in the presence of uncouplers, while in state 3 the level of activation is intermediate and varies with the nature of the substrate. Transition from the active to inactive (unactivated) state of SD occurs in mitochondria with a lower energy of activation (10 Kcal/mole) than in soluble or membrane preparations (33 to 36 Kcal/mole). In addition to activation by succinate and CoQ 10 H 2 , the enzyme in mitochondria is uniquely activated by ATP (or a compound in equilibrium with it), a process which has not been previously observed in submitochondrial particles and is oligomycin-insensitive. In tightly coupled mitochondria reversible activation by all these agents may occur concurrently, but experimental conditions are described to study the action of each type of activator independently.