On the possible interelations of the reactivity of soluble succinate dehydrogenase with ferricyanide, reconstitution activity, and the hipip iron sulfur center

Abstract

It was recently reported (Vinogradov et al., Biochem. Biophys. Res. Commun. 65 , 1264–1269, (1975)) that fresh preparations of succinate dehydrogenase, extracted anaerobically in presence of succinate, contain a reaction site for ferricyanide which had not been previously recognized. This site has a low K m for ferricyanide (∼200μM); it is very unstable to air and is not seen either in preparations extracted without succinate or in membrane-bound forms of the enzyme, presumably because in the latter the site is inaccessible to ferricyanide. This type of ferricyanide reduction is thus distinct from that conventionally measured using high concentrations of ferricyanide (K m ∼3mM). The lability of the “low K m site” for ferricyanide is reminiscent of the lability of reconstitution ability and the Hipip iron sulfur center of the soluble enzyme. This note presents evidence that the labile ferricyanide site and the reconstitution activity may both hinge on the integrity of the same component. It is shown that both activities decay at identical rates at three pH values on exposure of the enzyme to O 2 at 0°. The possibility is considered that the site involves the Hipip center. Concurrently with the disappearance of these activities, some 50–55% of the phenazine methosulfate reductase activity also disappears. The question whether this loss suggests different reaction sites for this dye in fresh and O 2 modified preparations is discussed in terms of current knowledge of the rate-limiting step in catalysis by succinate dehydrogenase.