The role of phosphatidylcholine in fatty acid exchange and desaturation inBrassica napus L. leaves

The role of phosphatidylcholine (PC) in fatty acid exchange and desaturatio n was examined and compared with that of monogalactosyldiacylglycerol (MGDG ) in Brassica napus leaves using C-14-labelling in vivo. Data are presented which indicate that in the chloroplast newly formed saturated (palmitic ac id, 16:0) and monounsaturated (oleic acid, 18:1) fatty acid is incorporated into MGDG and desaturated ill situ. In the non-plastidic compartments, how ever, newly formed fatty acid is exchanged with polyunsaturated fatty acid in PC, the probable major site of subsequent desaturation. The unsaturated fatty acid is released to the acyl-CoA pool, which is then used to synthesi ze diacylglycerol (DAG) containing a high level of unsaturated fatty acid. This highly unsaturated DAG may be the source for the biosynthesis of other cellular glycerolipids. The generally accepted pathway in which PC is synt hesized from molecular species of DAG containing 16:0 and 18:1 followed by desaturation of the 18. 1 to linoleic (18.2) and linolenic (18: 3) acids is questioned.