MECHANISM OF LINOLEIC-ACID HYDROPEROXIDE REACTION WITH ALKALI

Treatment of (13S,9Z,11E)-13-hydroperoxy-9,11-octadecadienoic acid (13 S-HPODE) with strong alkali resulted in the formation of about 75% of the corresponding hydroxy acid, (13S,9Z,11E)-13-hydroxy-9,11-octadecad ienoic acid (13S-HODE), and the remaining 25% of products was a mixtur e of several oxidized fatty acids, the majority of which was formed fr om (9Z,11R,S,12S,R)-13-oxo-11,12-epoxy-9-octadecenoic acid by Favorski i rearrangement (Gardner, H.W., et al. (1993) Lipids 28, 487-495). In the present work, isotope experiments were completed in order to get f urther information about the initial steps of the alkali-promoted deco mposition of 13S-HPODE. 1. Reaction of [hydroperoxy-O-18(2)]13S-HPODE with 5 M KOH resulted in the formation of [hydroxy-O-18]13S-HODE and ( 9Z,11R,S,12S,R)-13-oxo-11,12-epoxy-9-octadecenoic acid; 2. treatment o f a mixture of [U-C-14]13S-HODE and [hydroperoxy-O-18(2)]13S-HPODE wit h KOH and analysis of the reaction product by radio-TLC showed that 13 S-HODE was stable under the reaction conditions and did not serve as p recursor of other products; 3. reaction of a mixture of [U-C-14]13-oxo -9,11-octadecadienoic acid (13-OODE) and [hydroperoxy-O-18(2)]13S-HPOD E with KOH resulted in the formation of [U-C-14-epoxy-O-18] (9Z,11R,S, 12S,R)-13-oxo-11,12-epoxy-9-octadecenoic acid; 4. treatment of a mixtu re of [hydroperoxy-O-18(2)]13S-HPODE and [carboxyl-O-18(1)]13S-HPODE w ith KOH afforded (9Z,11R,S,12S,R)-13-oxo-11,12-epoxy-9-octadecenoic ac id having an O-18-labeling pattern which was in agreement with its for mation by intermolecular epoxidation. It was concluded that (9Z,11R,S, 12S,R)-13-oxo-11,12-epoxy-9-octadecenoic acid is formed from 13S-HPODE by a sequence involving initial dehydration into the alpha,beta-unsat urated ketone, 13-OODE, followed by epoxidation of the Delta(11) doubl e bond of this compound by the peroxyl anion of a second molecule of 1 3S-HPODE. Rapid conversion of hydroperoxides by alkali appeared to req uire the presence of an alpha,beta-unsaturated ketone intermediate as an oxygen acceptor. This was supported by experiments with a saturated hydroperoxide, methyl 12-hydroperoxyoctadecanoate, which was found to be much more resistant to alkali-promoted conversion than 13S-HPODE.