Activation of succinate dehydrogenase in heart-muscle preparations

Abstract

Keilin and Hartree (1947) heart-muscle preparations, diluted in phosphate buffer, showed increases in succinate-oxidase activity of up to more than fourfold on incubation at 37[degree]. This increase in activity was accompanied by an increase in succinate-phenazine-reductase activity and smaller increases in rate of the NADH2-fumarate and FMNH2-fumarate reactions, but the NADH2-oxidase activity was unaffected or inhibited. It was concluded that the activation of succinate-oxidase activity was caused by the activation of succinate dehydrogenase (Kearney, 1957). The kinetics of activation of succinate-oxidase activity of heart-muscle preparation, diluted in phosphate, were similar to those of soluble succinate dehydrogenase, as described by Kearney (1957). The activation of succinate-oxidase activity also took place in tris-acetate and borate buffers, but simultaneous inhibition tended to occur on prolonged incubation. This inhibition was greater in borate, especially at increasing dilutions of enzyme, but EDTA had a protective effect. The activation of succinate dehydrogenase in heart-muscle preparations diluted in tris-acetate was reversible and the equilibrium degree of activation depended on the temperature. When it was assumed that one form of the enzyme is catalytically inactive, calculations from the equilibrium data yielded consistent values for the changes in heat content and entropy for the conversion of the inactive into the active form. These values were [DELTA]H = 20 kcal./mole; [DELTA] S = 69.7 cal./mole/degree. The Km of succinate oxidase, determined in tris-acetate at 20[degree], was 0.15 mM for unactivated preparations, and 0.15-0.2 mM for activated preparations. Low succinate-oxidase activities observed in manometric experiments in borate buffer are the result of: a failure of activation of succinate dehydrogenase; a specific requirement of succinate oxidase for activators (Bonner, 1954). Phosphate and EDTA appear to act as protectors in the first case and as activators in the second.