Isolation and characterization of natural allene oxides: unstable intermediates in the metabolism of lipid hydroperoxides.

Abstract

Allene oxides are unstable epoxides that have been as intermediates in the biotransformation of hydroperoxyicosatetraenoic acids and related hydroperoxidas to ketols and cyclopentenones. Direct proof of the structure of the putative allene oxide intermediates has been hampered by their extreme instability under the conditions of their biosynthesis (t1/2 .apprxeq. 15-30 sec at 0.degree.C and pH 7.4). We now report the isolation and structural elucidation of allene oxides prepared from the (13S)-hydroperoxides of linoleic and linolenic acids. The compounds were biosynthesized by using a very active enzyme preparation from flaxseed. After a 5-sec incubation at 0.degree.C, the allene oxide metabolites were extracted and purified as the methyl ester derivatives at -15.degree.C. The structures were established by UV, CD, NMR, and oxygen-18 labeling experiments. 12,13(S)-Oxido-9Z,11-octadecadienoic acid is derived from linoleic acid, and 12,13(S)-oxido-9Z,11,15Z-octadecatrienoic acid is from linolenic acid. Analysis of the breakdown products formed on exposure to water led to identification of hydrolysis and cyclization products previously characterized as enzymic derivatives of the (13S)-hydroperoxides in flaxseed. Our results give direct proof of the structure of the allene oxide intermediates and should facilitate further investigations of the metabolism of this class of epoxide to prostaglandins, clavulones, and other stable end products.