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. Inthis system, all the lipids contained labelled oleate, linoleateand linolenate but labelled linolenate was mainly concentratedinto diacylgalactosylglycerol (MGDG). A small but significantlabelling was found in the linolenate of the diacyldigalactosylglycerol(DGDG). On the other hand, labelled hexadecamonoenoic acid (C^16:1),hexadecadienoic acid (C^16:2) and hexadecatrienoic acid (C^16:3)were only found into MGDG. In such a reconstituted system, atthe end of the incubation period, labelled MGDG was almost exclusivelyrecovered into the chloroplast while the labelled phosphatidylcholine(PC) was found highly concentrated in the microsomes In the MGDG of the chloroplast, C^16:1, C^16:2 and C^16:3 werefound at the C2 position of the glycerol while oleic acid (C^18:1),linoleic acid (C^18:2) and a-linolenic acid (^18:3) esterifiedspecifically the position 1 of the glycerol. No C¹⁸ acids werefound in position 2. In the PC of the microsomes, C^18:1, C^18:2and C^18:3 were found at the Cl and C2 positions of the glycerolwhile palmitic acid esterified exclusively the Cl of the glycerol. The biosynthetic pathway of trienoic fatty acids in leaves ofhigher plants is discussed. (Received July 19, 1982; Accepted October 18, 1982)