The role of the 4-carbon dicarboxylic acids in muscle respiration

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Abstract
Chemical estimations of fumaric, succinic, oxaloacetic and malic acids were carried out on samples of minced pigeon breast muscle incubated aerobically with added fumarate, in an attempt to confirm Szent Gyorgyi''s theory that fumaric acid acts as a catalyst in the respiration of muscle, being oxidized to oxaloacetic acid by the Warburg-Keilin system and reduced again to fumaric acid by the substrate dehydrogenases plus substrates. Using the concn. of fumaric acid suggested as necessary to maintain the catalytic cycle the oxygen uptake was in no case greater than that necessary for complete oxidation of some of the fumaric acid disappearing, which was not to be found as succinic, oxaloacetic, pyruvic or malic acids. It therefore appeared that the added fumarate was being used as substrate for respiration rather than as a catalyst and it was seen in exps. with washed tissue that added fumaric acid was oxidized in preference to added lactic acid in the presence of co-zymase.