Catalytic and molecular modifications of succinate dehydrogenase by monovalent inorganic anions

The enzymatic activity and the oxidation state of soluble, activated, substrate-reduced succinate dehydrogenase are modified by the presence of bromide. The anion inhibits the enzyme by two different mechanisms which depend on the ratio of bromide to succinate. At high ratios binding of two bromide ions is required and a competitive inhibition is observed: removal of succinate from the substrate binding site (Kd = 0.1 mM) leads to oxidation of the flavin. At lower ratios but with sufficient succinate to saturate a site with Kd = 1.52 mM, uncompetitive inhibition by a single bromide ion is observed. Mechanisms, as well as the possible physiological significance of the novel type of regulation of succinate dehydrogenase, are discussed.