APPARENT ROLE OF PHOSPHATIDYLCHOLINE IN THE METABOLISM OF PETROSELINIC ACID IN DEVELOPING UMBELLIFERAE ENDOSPERM

Studies were conducted to characterize the metabolism of the unusual f atty acid petroselinic acid (18:1cis Delta(6)) in developing endosperm of the Umbelliferae species coriander (Coriandrum sativum L.) and car rot (Daucus carota L.). Analyses of fatty acid compositions of glycero lipids of these tissues revealed a dissimilar distribution of petrosel inic acid in triacylglycerols (TAG) and the major polar lipids phospha tidylcholine (PC) and phosphatidylethanolamine (PE). Petroselinic acid comprised 70 to 75 mol% of the fatty acids of TAG but only 9 to 20 mo l% of the fatty acids of PC and PE. Although such data appeared to sug gest that petroselinic acid is at least partially excluded from polar lipids, results of [1-C-14]acetate radiolabeling experiments gave a mu ch different picture of the metabolism of this fatty acid. In time-cou rse labeling of carrot endosperm, [1-C-14]acetate was rapidly incorpor ated into PC in high levels. Through 30 min, radiolabel was most conce ntrated in PC, and of this, 80 to 85% was in the form of petroselinic acid. One explanation for the large disparity in amounts of petroselin ic acid in PC as determined by fatty acid mass analyses and C-14 radio labeling is that turnover of these lipids or the fatty acids of these lipids results in relatively low accumulation of petroselinic acid mas s. Consistent with this, the kinetics of [1-C-14]acetate timecourse la beling of carrot endosperm and ''pulse-chase'' labeling of coriander e ndosperm suggested a possible flux of fatty acids from PC into TAG. In time-course experiments, radiolabel initially entered PC at the highe st rates but accumulated in TAG at later time points. Similarly, in pu lse-chase studies, losses in absolute amounts of radioactivity from PC were accompanied by significant increases of radiolabel in TAC. In ad dition, stereospecific analyses of unlabeled and [1-C-14]acetate-label ed PC of coriander endosperm indicated that petroselinic acid can be r eadily incorporated into both the sn-1 and sn-2 positions of this lipi d. Because petroselinic acid is neither synthesized nor further modifi ed on polar lipids, the apparent metabolism of this fatty acid through PC (and possibly through other polar lipids) may define a function of PC in TAG assembly apart from its involvement in fatty acid modificat ion reactions.