Aminomalonate as an enzyme inhibitor.

When Rhodopseudomonas spheroides, which had been grown in a biotin-deficient medium, was transferred to a medium containing glycine, ocoxoglutarate and fumarate, and which normally promotes formation of much porphyrin, little or no porphyrin was formed (Lascelles, 1956); but, when glycine and oc-oxoglutarate were replaced by 8aminolaevulic acid, the biotin-deficient organisms were able to form porphyrin. The work of Lascelles was confirmed and extended by Gibson, Neuberger & Tait (1962), who showed that the addition of biotin to the medium enables biotin-deficient organisms to form porphyrin from glycine and CLoxoglutarate. The condensation ofthese compounds to form 8-aminolaevulate, catalysed by 8-aminolaevulate synthetase, was shown in a cell-free system to be inhibited by commercial preparations of avidin. These findings suggested that biotin might be concerned in the synthesis of 8-aminolaevulate, the first step in the formation of porphyrin. Two possibilities for the role of biotin were considered. One was that biotin was involved in the decarboxylation of oc-amino-,B-oxoadipate, at the time thought to be the immediate precursor of 8aminolaevulate. The other possibility was that one of the substrates for aminolaevulate synthetase was not glycine but aminomalonate, i.e. monocarboxylated glycine: this hypothesis was attractive on chemical grounds because the oc-carbon atom of aminomalonate is a more reactive nucleophilic centre than that of glycine. Aminomalonate was therefore synthesized and examined as a substrate for aminolaevulate synthetase. As reported briefly by Neuberger (1961), it was found to be not a substrate but a powerful inhibitor of the enzyme. This work is now reported in more detail, together with the effect of aminomalonate on other enzymes that have glycine as substrate or pyridoxal phosphate as coenzyme. A preliminary commnunication has been published (Gibson, Matthew, Neuberger & Tait, 1961).