Elucidation of the Biosynthesis of Eicosapentaenoic Acid (EPA) in the Microalga Porphyridium Cruentum

Abstract

I n the course of the study of the biosynthesis of the fatty acid eicosapentaenoic acid (EPA) in the microalga Porphyridium cruen-tum, cells were pulse-labeled with various radiolabeled fatty acid precursors. Our data show that the major end products of the biosynthesis are EPA-containing galactolipids of a eukaryotic and prokaryotic nature. The prokaryotic molecular species contain EPA and arachidonic acid at the sn-1 position and C, , fatty acids, mainly 16:0, at the sn-2 positions, whereas in the eukaryotic species both positions are occupied by EPA or arachidonic acid. However, we suggest that both the eukaryotic and prokaryotic molecular species are formed in two pathways, w6 and w3, which involve cytoplasmic and chloroplastic lipids. I n the w6 pathway, cytoplasmic 1 8 2-phosphatidylcholine (PC) is converted t o 20:4w6-PC by a sequence that includes a A6 desaturase, an elongation step, and a A5 desatu-rase. In the minor w3 pathway, 182-PC is presumably desaturated t o 18:3w3, which is sequentially converted by the enzymatic sequence of the w6 pathway t o 20:5w3-PC. The products of both pathways are exported, as their diacylglycerol moieties, t o the chloroplast t o be galactosylated into their respective monogalacto-syldiacylglycerol molecular species. The 20:4w6 i n both eukaryotic and prokaryotic monogalactosyldiacylglycerol can be further de-saturated to EPA by a chloroplastic A I 7 (w3) desaturase. The biosynthetic pathways leading to PUFA in leaves of higher plants were recently reviewed (Browse and Somer-ville, 1991). In the prokaryotic pathway, 18:l linked to a galactolipid is stepwise desaturated to 18:3w3. In the eu-karyotic pathway, 18:l produced in the chloroplast is exported to the cytoplasm, and after linking to PC it is similarly desaturated to 18:2 and 18:3w3. The 182-containing DAG moieties of PC are reimported to the chlo-roplast, galactosylated, and further desaturated to form the 18:303 / 18:303 eukaryotic molecular species of galactolip-ids. Prokaryotic species typically contain C,, and C,, fatty