Epr studies on the mechanism of action of succinate dehydrogenase in activated preparations

Summary Reductive titrations and rapid kinetic studies are reported on extensively or completely activated, particulate and solubilized succinate dehydrogenase (SD) preparations. There is one iron-sulfur (Fe−S) center of the ferredoxin type present per flavin which is reduced by succinate, but even in activated preparations at most 60% of these centers were reduced within the turnover time of the enzymes. Flavin semiquinone formation does not precede or significantly lag behind the reduction of the Fe−S centers. On reduction of the soluble enzymes an accumulation of semiquinone is observed. No qualitative difference between the behavior of preparations containing 4 Fe or 8 Fe per flavin was found. Succinate-ubiquinone reductase (Complex II) contains an Fe−S component with properties of high-potential Fe−S proteins (See Ruzicka and Beinert, Biochem. Biophys. Res. Communs. this issue). It occurs at a concentration close to that of the bound flavin and has been observed to be reduced by succinate at approximately the same rate as the ferredoxin type (g=1.94) component. With dithionite, reduction of additional Fe−S groups (0.2 to 0.5 per flavin) is observed but the significance of this is uncertain.