On the mechanism of biosynthesis of divinyl ether oxylipins by enzyme from garlic bulbs

The microsomal fraction of homogenate of garlic (Allium sativum L.) bulbs contains a divinyl ether synthase which catalyzes conversion of (9Z,11E,13S)-13-hydroperoxy-9,11-octadecadienoic acid and (9Z,11E,13S,15Z)-13-hydroperoxy-9,11,15-octadecatrienoic acid into (9Z,11E,1'E)-12-(1'-hexenyloxy)-9,11-dodecadienoic acid (etheroleic acid) and (9Z,11E,1'E,3'Z)-12-(1',3'-hexadienyloxy)-9,11-dodecadienoic acid (etherolenic acid), respectively. Two isomers of etheroleic acid were isolated. As shown by NMR spectrometry, the double bond configurations of these compounds were (9E,11E,1'E) and (9Z,11Z,1'E). Experiments with linoleic acid (13R,S)-hydroperoxide demonstrated that the S enantiomer was a much better substrate for the divinyl ether synthase compared to the R enantiomer. Incubation of (9Z,11E,13S)-[O-18(2)]hydroperoxy-9,11-octadecadienoic acid led to the formation of etheroleic acid which retained O-18 in the ether oxygen. An intermediary role of an epoxyallylic cation in etheroleic acid biosynthesis is postulated.