Hydroperoxide isomers and ketohydroxy product from oxidation of linoleic acid by eggplant lipoxygenase

Hydroperoxides produced by oxidation of linoleic acid with purified eggplant lipoxygenase were separated by TLC and analysed by IR spectroscopy. The methyl hydroxystearates from the enzymatically produced hydroperoxides were analysed by MS and GLC. Both analyses indicated that the eggplant enzyme converted linoleic acid almost exclusively (96%) into the 13-hydroperoxy isomer whereas the 9-hydroperoxy isomer was only a minor product (4%). HPLC of the methyl ester of the isolated hydroperoxides showed three components. Each component was collected, reduced to methyl hydroxystearate and characterized by GLC, MS and IR analysis. The components were identified as 13-hydroperoxy cis-trans isomer (92.8%), 13-hydroperoxy trans-trans isomer (2.6%) and 9-hydroperoxy cis-trans isomer (4.6%). A polar by-product present in the reaction mixture was identified by IR, ¹H NMR, and MS (of the toluene-p-sulphonyl derivative) as 13-hydroxy-12-oxo-octadec-cis-9-enoic acid.