Phospholipid molecular species from isolated bovine rod outer segments incorporate exogenous fatty acids at different...

The incorporation of radiolabeled palmitic (16:0), oleic (18:1), and docosahexaenoic (22:6) acids into different molecular species of membrane phospholipids was investigated in isolated bovine rod outer segments (ROS). Phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) were isolated, and their diacylglyceroacetate and diacylglycerobenzoate derivatives were prepared, separated by HPLC, quantified, and assayed for radioactivity. Maximal incorporation of fatty acids occurred within 15-30 min. The rate of incorporation of the fatty acids into PC was three to six times higher than it was into PS or PE. The rate of incorporation of 22:6 into the molecular species, 22:6-22:6, of PC was ten to 15 times higher than into that of PE or PS, and it was three to four times higher than the incorporation rates of 22:6 into the other 22:6-containing molecular species. Similarly, incorporation of 18:1 into 18:1-22:6 was ten to 30 times more rapid in PC than in PE and PS, but in both PE and PS, 18:1 was incorporated into 18:1-22:6 at a rate of 20 to 25 times higher than the incorporation into the other molecular species analysed. For PC, incorporation of 16:0 was most rapid into 16:0-16:0, but for PE and PS it was most rapid into 16:0-20:4; for all cases, incorporation of 16:0 into these molecular species was four to six times more rapid than into the other 16:0-containing molecular species. These results are further evidence for the presence within a membranous organelle, the ROS, of an active acylation-deacylation system that is selective with regard to phospholipid, molecular species of phospholipid, and fatty acid.