Metabolism in rats in vivo of (2S)[3,3,3-2H3]isobutyrate. Identification of (2-pro-S)methyl group as the source of a proton in dehydrogenation.

(2S)[3,3,3-2H3]Isobutyrate, sodium salt, was synthesized and administered intraperitoneally to normal rats. Urinary organic acids were analyzed by gas chromatography-mass spectrometry. 3-Hydroxyisobutyric acid excreted by the rats, analyzed as both a methyl and an isopropyl ester, was found to be enriched almost exclusively with two deuteriums in the hydroxymethyl moiety of the molecule. This result, together with our previous observation that deuterium of [2-2H1]isobutyrate was lost before the formation of 3-hydroxyisobutyrate, indicates that isobutyric acid is dehydrogenated stereospecifically at the (2-pro-S)methyl and alpha-methine groups. The resulting methacrylate is then hydrated with hydrogen addition to the same side of the molecule as it was abstracted from to produce (S)(+)-3-hydroxyisobutyric acid.