Sulphate ion-induced slow transformation of succinate dehydrogenase

A new type of slow change of succinate dehydrogenase (EC 1.3.99), activity which is induced by sulphate ion is described. After preincubation of submitochondrial particles or soluble succinate dehydrogenase with sulphate both preparations catalyze succinate:phenazine methosulphate reductase reaction with a significant lag. When added to the assay medium sulphate ion induces biphasic time-dependent competitive inhibition of the enzyme. The sulphate-induced inhibition is apparently due to a rapid interaction of the anion with an active site of the enzyme which is followed by a slow pH-dependent (pKa = 7.2) transformation of the enzyme-inhibitor complex. pH profiles of the overall succinate dehydrogenase reaction and of equilibrium between fast and slow enzyme-sulphate complexes suggest that the same protolytic equilibrium step is involved in the formation of an active intermediate and an inactive enzyme-sulphate complex.