Phospholipid biosynthesis in human platelets. Formation of phosphatidylcholine from 1-acyl lysophosphatidylcholine by acyl-CoA:1-acyl-sn-glycero-3-phosphocholine acyltransferase.

Arachidonic acid and other fatty acids are taken up by human platelets from plasma and incorporated into membrane phospholipids. However, little is known about the mechanism and specificity of the various steps of fatty acid insertion into phospholipid. Previous findings from this laboratory have shown that the incorporation of radioactive C20-unsaturated fatty acids (arachidonic, 8,11,14-eicosatrienoic, and 5,8,11,14,17-eicosapentaenoic) into the phospholipids of "resting"p platelets is more rapid than that of the radioactive C16- and C18-saturated and unsaturated fatty acids. We now provide evidence that human platelet microsomes contain acyl-CoA:1-acyl-sn-glycero-3-phosphocholine acyltransferase. The enzyme preparation has a pH optimum of 7.0. Activity is insensitive to 1 mM EDTA and is inhibited 37% by 1 mM Ca2+ and 20% by 1 mM Mg2+. Maximal activity is observed at 100 microM 1-acyl lysophosphatidylcholine at several concentrations of fatty acyl-CoA esters. Apparent Km values from 1.05 to 5.70 microM were obtained for saturated and unsaturated fatty acyl group donors in the presence of 100 microM 1-acyl lysophosphatidylcholine as fatty acyl group acceptor. Comparison of the apparent Vmax values showed that unsaturated CoA esters were transferred more rapidly to 1-acyl lysophosphatidylcholine than saturated CoA esters. Oleate, linoleate, and arachidonate, the major unsaturated fatty acids in platelet phosphatidylcholine, were transferred at similar rates. 8,11,14-eicosatrienoate was transferred about two times faster than these three fatty acyl groups. The data indicate that the incorporation of unsaturated fatty acids into phosphatidylcholine by human platelets occurs via reacylation of 1-acyl lysophosphatidylcholine.