Biosynthesis of vernoleate (cis-12-epoxyoctadeca-cis-9-enoate) in microsomal preparations from developing endosperm of Euphorbia lagascae

Epoxidated fatty acids are the major constituents of the triacylglycerols in a few plant species. We have investigated the biosynthesis of vernolic acid (cis-12-epoxyoctadeca-cis-9-enoic acid) in the seed oil of Euphorbia lagascae. Microsomes were isolated from developing endosperm. The membrane lipids were labeled in situ with 14C]oleate or 14C]linoleate, which mainly were recovered in phosphatidylcholine (PC), and the metabolization of the radioactive fatty acids was followed in incubations with or without NADPH. In the presence of NADPH, 14C]vernoleate was formed. After short incubations, most of the vernolic acid was found in PC, but with increasing incubation times, the free acid dominated. The synthesis of vernoleate was inhibited by carbon monoxide, but not by cyanide. The presence of anticytochrome b5 antibodies inhibited both the desaturation of 14C]oleate to 14C]linoleate and the epoxidation of 14C]linoleate to 14C]vernoleate. Free linoleic acid did not serve as substrate for epoxidation. The results indicate that, in the endosperm of E. lagascae, vernoleate is synthesized on PC from linoleate, and that the epoxidation is catalyzed by a cytochrome P450 and involves cytochrome b5.