Studies on Succinate Dehydrogenase

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Abstract
The cytoplasm of yeast contains several enzymes for the reduction of fumarate to succinate. One of these is excluded on Sephadex G‐200 and in this respect as in all of its known properties, resembles mitochondrial succinate dehydrogenase in the soluble form. Another type is distinguished by the apparent inability to oxidize succinate with any of the conventional electron acceptors. This enzyme has been subdivided into two types (I and II) which may be readily separated by chromatography on hydroxylapatite. Type I and II fumarate reductases differ from mitochondrial succinate dehydrogenase in containing acid‐extractable (rather than covalently bound) FAD, in their intracellular location, in being more stable and more resistant to inhibition by mercurials. The fumarate reductases also differ from the dehydrogenase in molecular weight, in lack of activation by substrate, and in having lower affinities for succinate and malonate and a higher one for fumarate. Conditions which are optimal for the development of mitochondrial succinate dehydrogenase (high O2 tension, low glucose concentration) repress the development of type I and II fumarate reductases and vice versa.“Petite” mutants, which lack succinate dehydrogenase, have a normal content of these fumarate reductases. Type I and II fumarate reductases, while similar in most respects, differ in molecular weight, kinetic constants and reactivity with electron donors.