Detection of a "nonaromatic" NIH shift during in vivo metabolism of the monoterpene carvone in humans.

High-resolution gas chromatography in combination with mass spectrometry and high-resolution mass spectrometry was used to determine the positions and extent of labeling in the metabolites of carvone, namely in alpha,4-dimethyl-5-oxo-3-cyclohexene-1-acetic acid (dihydrocarvonic acid), alpha-methylene-4-methyl-5-oxo-3-cyclohexene-1-acetic acid (carvonic acid), and 5-(1,2-dihydroxy-1-methylethyl)-2-methyl-2-cyclohexen-1-one (uroterpenolone), after human ingestion of 9,9-dideutero- and 9-(13)C-carvone. Carvonic acid was formed by oxidation at the methyl carbon of the isopropenyl group of carvone, whereas dihydrocarvonic acid was formed by oxidation at the methylene position, most probably via carvone epoxide. A "nonaromatic" NIH shift must occur during the subsequent reactions yielding dihydrocarvonic acid. Additionally, dehydrogenation of dihydrocarvonic acid and hydrogenation of carvonic acid were observed, resulting in minor amounts of both acids owning a carboxy group of opposite origin. Uroterpenolone was found to be exclusively formed by oxidation at the methylene carbon of the isopropenyl group of carvone, and thus, most probably by hydrolysis of carvone epoxide.