A method of succinic acid estimation based on the action of a specific succinic dehydrogenase preparation from ox heart is described. The properties of this enzyme preparation have been studied in some detail. The only objection which can be made as to specificity is the presence of oc-glycerophosphate dehydrogenase. The activity of this enzyme is, however, more than ten times smaller than that of succinic dehydrogenase. A glutamic acid deaminase which is also present only attacks the non-natural isomeride. Succinic dehydrogenase is inhibited by oc-ketoglutaric acid, pyocyanine and phenosafranine.

The formation of succinic acid(cas:110-15-6) from pyruvic and oc-ketoglutaric acids is demonstrated anaerobically with brain slices and with minced brain. Aerobically, in experiments with minced brain poisoned with malonic acid, pyruvic, oc-ketoglutaric and, in some cases, acetic acids were found to yield succinic acid(cas:110-15-6). Under the same conditions the R.Q. of ketoglutaric oxidation is raised, indicating a specific inhibition of succinic dehydrogenase of roughly 50 %. Malonic acid does not lead to succinic acid accumulation with brain slices, and with mince only if used in certain concentrations. Small amounts of a volatile acid, probably acetic acid, are formed from pyruvic acid by minced brain. The significance of these observations and the possible mechanism of succinic acid formation are discussed.

I gratefully acknowledge the gifts of samples of hexosemonophosphate, phosphohexonic and 2-ketogluconic acids by Dr Dickens. Moreover, my sincere thanks are due to his constant help and advice.