[Interaction of the sulfate ion with the active site of succinate dehydrogenase]

A new type of slow changes of the succinate dehydrogenase (EC 1.3.99.1) activity induced by the sulphate ion is described. After preincubation of submitochondrial particles or soluble succinate dehydrogenase with sulphate both preparations catalyze the phenazine methosulphate reductase reaction with a significant lag. When added to the assay medium, the sulphate ion induces biphasic time-dependent competitive inhibition of the enzyme. The sulphate-induced inhibition is due to a rapid interaction of the anion with the active site of the enzyme followed by a slow pH-dependent (pKa=7.2) transformation of the enzyme-inhibitor complex. The pH-profiles of the overall succinate dehydrogenase reaction and of equilibrium between the fast and slow enzyme-sulphate complexes suggest that the same protolytic step is involved in the formation of an active intermediate and inactive enzyme-sulphate complex.