Selective incorporation of polyunsaturated fatty acids into phosphatidylcholine by rat liver microsomes

The various polyunsaturated fatty acids found in cellular lipids are transferred from their coenzyme A thiol esters to phospholipid acceptors with greatly different maximal velocities. The very low apparent Km values for the thiol esters in acylating 1-acylglycerol-3-phosphocholine could not be directly measured, but their values could be estimated relative to that for arachidonate. The competitive effectiveness of various polyunsaturated acyl-CoAs was estimated by measuring the equivalent concentrations which allow incorporations equal to arachidonyl-CoA. These values help predict the way in which various polyunsaturated acyl-CoAs may be selectively esterified to membrane lipids by the 1-acylglyceol 3-phosphocholine (1-acyl-GPC) acyltransferase system of liver microsomes. The acyl-CoA esters of saturated acids, as well as those for 22:1, 22:2, and 22:3, had negligible ability to compete for the active sites of the 1-acyl-GPC acyltransferase system. The acyl-CoA esters of arachidonate (20:4n-6), eicosatrienoate (20:3n-6), eicosapentaenoate (20:5n-3), and both isomers of linolenate (18:3n-6 and n-3) were handled preferentially by the 1-acyl-GPC acyltransferases. The system from liver has a high selectivity for unsaturated acids but does not appear to discriminate among the polyunsaturated acids of the n-6 and n-3 series that serve as precursors of prostaglandins and leukotrienes.