Hydroperoxides of alpha-ketols. Novel products of the plant lipoxygenase pathway.

Metabolism of [1-14C]linolenic acid, 13-hydroperoxy-8(Z),11(E),15(Z)-[1-14C]octadecatrienoic acid (13-HPOT) and 9-hydroperoxy-10(E),12(Z),15(Z)-[1-14C]octadecatrienoic acid (9-HPOT) was studied by enzyme preparations from flax, wheat and corn seeds, containing two enzymes of fatty acid metabolism, namely, lipoxygenase and hydroperoxide dehydrase. Along with the previously known products of the hydroperoxide dehydrase pathway, the radiolabel was incorporated into some more polar metabolites. These polar products 1 and 4, formed from 13-HPOT and 9-HPOT, respectively, were purified by reversed-phase and normal-phase HPLC, and investigated by ultraviolet spectroscopy, chemical-ionization and electron-impact mass spectrometry, and 1H-NMR. The data obtained suggest that metabolites 1 and 4 are 9-hydroperoxy-12-oxo-13-hydroxy-10(E),15(Z)-octadecadienoic acid and 9-hydroxy-10-oxo-13-hydroperoxy-11(E),15(Z)-octadecadienoic acid, respectively. 12-oxo-13-hydroxy-9(Z),15(Z)-[1-14C]octadecadienoic acid (12,13-alpha-ketol) and 9-hydroxy-10-oxo-12(Z),15(Z)-[1-14C]octadecadienoic acid (9,10-alpha-keto) are the direct precursors of metabolites 1 and 4, respectively. Metabolites 1 and 4 are formed from the corresponding HPOT precursors in two stages; (a) conversion of hydroperoxide into the alpha-ketol by hydroperoxide dehydrase and (b) the lipoxygenase oxidation of the alpha-ketol. Different lipoxygenases were found to oxidize alpha-ketols. Oxidation of the 3(Z)-buten-1-onyl moiety of alpha-ketols presents an unusual and previously unknown type of lipoxygenase reaction.