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Succinate-ubiquinone oxidoreductase (Complex II) is a membrane-bound enzyme that couples the oxidation of succinate to fumarate to the reduction of ubiquinone in the mitochondrial respiratory chain. It is composed of four subunits (1); two hydrophilic subunits, a flavoprotein containing covalently bound FAD (Mr = 70,000) and a smaller iron-sulfur protein (Mr. = 27,000), that together constitute the enzyme succinate dehydrogenase (2), and two small hydrophobic subunits which are associated with a b-type cytochrome (3). The two small peptides are required for anchoring the enzyme to the membrane and for the reduction of ubiquinone, but are not required for the catalytic oxidation of succinate in the presence of artificial electron acceptors (4,5).

Identification of the rate-limiting step in the catalytic cycle of succinate dehydrogenase and the related problem of defining the factors which determine its turnover number have for a long time occupied the interest of many investigators. During the past two years a collaboration among several laboratories has been initiated in order to use a variety of experimental approaches to these problems. The present report summarizes the progress which has been achieved.