Polyunsaturated Fatty Acid Synthesis by a Mixture of Chloroplasts and Microsomes from Spinach Leaves: Evidence for Two Distinct Pathways of the Biosynthesis of Trienoic Acids
In a mixture of chloroplasts and microsomes from spinach leaves, all the leaf lipids were synthesized from (1-14C)-acetate. In this system, all the lipids contained labelled oleate, linoleate and linolenate but labelled linolenate was mainly concentrated into diacylgalactosylglycerol (MGDG). A small but significant labelling was found in the linolenate of the diacyldigalactosylglycerol (DGDG). On the other hand, labelled hexadecamonoenoic acid (C16:1), hexadecadienoic acid (C16:2) and hexadecatrienoic acid (C16:3) were only found into MGDG. In such a reconstituted system, at the end of the incubation period, labelled MGDG was almost exclusively recovered into the chloroplast while the labelled phosphatidylcholine (PC) was found highly concentrated in the microsomes In the MGDG of the chloroplast, C16:1, C16:2 and C16:3 were found at the C2 position of the glycerol while oleic acid (C18:1), linoleic acid (C18:2) and a-linolenic acid (18:3) esterified specifically the position 1 of the glycerol. No C18 acids were found in position 2. In the PC of the microsomes, C18:1, C18:2 and C18:3 were found at the Cl and C2 positions of the glycerol while palmitic acid esterified exclusively the Cl of the glycerol. The biosynthetic pathway of trienoic fatty acids in leaves of higher plants is discussed.