C. on the Catalytic Function of Heart Flavoprotein

THE oxidation in animal tissues of metabolites such as lactate, malate, :hydroxybutyrate, triosephosphate etc., can be resolved into at least three consecutive reactions: (1) Metabolite +coenzyme I-* oxidized metabolite +reduced coenzyme I. (2) Reduced coenzyme I+ carrier -coenzyme I+ reduced carrier. (3) Reduced carrier +02-+ carrier. The first reaction is catalysed by a family of enzymes, each of which is specific for a particular metabolite. For example, the lactic enzyme catalyses the oxidation of lactate by coenzyme I; the malic enzyme the oxidation of malate, etc. There are three components in this reaction, viz. enzyme, metabolite and coenzyme I, and the sole chemical change is the catalysed transfer of hydrogen from the metabolite to the coenzyme. The second reaction is not spontaneous, but requires a special catalyst, referred to as coenzyme factor or diaphorase [Euler & Hellstr6m, 1938; Dewan & Green, 1938]. The mechanism whereby the coenzyme factor catalysed the oxidation of reduced coenzyme was obscure owing to lack of information concerning the chemical nature of the catalyst. Recently Straub [1939, 1, 2] isolated from heart muscle a highly purified flavoprotein compound with properties which at once suggested its identity with the coenzyme factor. This flavoprotein is rapidly reducible by dihydrocoenzyme I and its leucoor reduced form is rapidly oxidizable by suitable carriers. Reaction (2) therefore can be further resolved into two constituent processes: (2 a) Reduced coenzyme I+ flavoprotein -+ coenzyme I + leucoflavoprotein. (2 b) Leucoflavoprotein + carrier -* flavoprotein + reduced carrier. To complete our knowledge ofhow certain metabolites are oxidized by molecular oxygen via coenzyme I, it will be necessary to know what substance or substances in animal tissues can assume the role of carrier. In reconstructed systems artificial carriers such as methylene blue are employed. There is evidence that the cytochromes are the physiological equivalents of methylene blue, but the possibility remains that the hypothetical reaction between dihydrocoenzyme and the cytochromes is complex.