DECARBOXYLATION OF [1-C-13]LEUCINE BY HYDROXYL RADICALS

The decarboxylation of [1-C-13]leucine by hydroxyl radicals was studie d by using gas chromatography-isotope ratio mass spectrometry (GC-IRMS ) to follow the production of (CO2)-C-13. A Fenton reaction between a (Fe2+)-porphyrin and hydrogen peroxide under aerobic conditions yielde d hydroxyl radicals. The decarboxylation rates (V-Leu) measured by GC- IRMS were dependent on [1-C-13]leucine, porphyrin and hydrogen peroxid e concentrations. The (CO2)-C-13 production was also dependent on bica rbonate or carbon dioxide added in the reaction medium. Bicarbonate fa cilitated (CO2)-C-13 production, whereas carbon dioxide decreased (CO2 )-C-13 production. Proton effects on some decarboxylation intermediate s could explain bicarbonate or carbon dioxide effects. No effect on th e decarboxylation rates was observed in the presence of the classical hydroxyl radicals scavengers dimethyl sulfoxide, mannitol, and uric ac id. By contrast, a competitive effect with a strong decrease of the de carboxylation rates was observed in the presence of various amino acid s: unlabeled leucine, valine, phenylalanine, cysteine, lysine, and his tidine. Two reaction products, methyl-4 oxo-2 pentanoate and methyl-3 butanoate were identified by gas chromatography-mass spectrometry in c omparison with standards. The present results suggest that [1-C-13]leu cine can participate to the coordination sphere of (Fe2+)-porphyrin, w ith a caged process of the hydroxyl radicals which cannot get out of t he coordination sphere. (C) 1998 Elsevier Science Inc.