Young leaves from three plants which accumulate hexadecatrienoic acid rather specifically in the sn-2-position of monogalactosyl diglyceride (MGD) were incubated with radioactive CO 2 or acetate to investigate the origin of this specificity. Labelled glycerolipids were extracted and analyzed for time-dependent changes of radioactivity in their fatty acids. The investigation of labelled MGD and digalactosyl diglyceride (DGD) included determination of radioactivity in hydrolysis products, separation of molecular species by argentation chromatography and analysis of the positional distribution of fatty acids. The results agree with previous observations on the accumulation of radioactive oleic acid in phosphatidyl choline (PC) and formally with the possibility of a PC-coupled desaturation to linoleic acid. They do not support the proposed function of PC as donator of polyenoic acids. Instead the radioactivity patterns may be interpreted as pointing to a relation between fatty acid desaturation and many if not all glycerolipids, although a different interpretation is also possible. Fatty acid patterns in lipids and their labelling indicate the existence of several pools for 1) MGD, from which only that without C 16 -unsaturated fatty acids is accessible for galactosylation to DGD ; 2) palmitic acid, from which one part is accessible to desaturation via C 16:0 and C 16:2 to C 16:3 . Since these acids are found labelled in the sn-2-position of MGD, the specific positioning may be related to this separation of C 16:0 pools. Desaturation of C 16:0 seems to be the major source of C 16:3 ; 3) linolenic acid, from which those parts present in the sn-2-position of galactolipids or in PC are characterized by a strikingly slow labelling.
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SIEBERTZ et al. (1977) studied this question.