Effect of specific phospholipid molecular species incorporated in human platelet membranes on thromboxane A2/prostaglandin H2 receptors.

The incorporation of albumin-bound docosahexaenoic acid (22:6n-3), but not linoleic acid (18:2n-6), into cellular phospholipids inhibits platelet aggregation induced by the thromboxane analogue U46619. [3H]U46619 specific binding to thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors, as well as specific binding of the antagonist [3H]SQ29548 to these sites were also decreased in these modified cells (P. G., Swann et al. 1990. J. Biol. Chem. 265: 21692-21697). More than 80% of the 22:6n-3 incorporated in these cells was esterified in the various endogenous phospholipid classes and the remaining was found in neutral lipids and in the unesterified fatty acid pool. In this study, we determined whether the effects observed could be attributed to the esterification of 22:6n-3 in phospholipids and whether the 22:6n-3 biological activity might depend on its esterification in specific phospholipid classes. Therefore, pure phosphatidylcholine (PC) and phosphatidylethanolamine (PE) molecular species were transferred to platelet membranes, using lipid transfer proteins. PC and PE containing palmitate (16:0) and 22:6n-3 or 16:0 and 18:2n-6 at position sn-1 and sn-2, respectively, were incorporated into membranes only at the expense of the corresponding endogenous phospholipid class, by an apparent exchange process. When such modified membranes were tested for specific binding of U46619 and SQ29548, a significant decrease of the receptor site affinity was only observed in membranes highly enriched with 1-palmitoyl-2-docosahexaenoyl-glycerophosphocholine (16:0/22:6-GPC). Fluidity parameters measured by electron spin resonance of 5- and 16-nitroxy-stearic acids were not significantly different in membranes enriched with 16:0/22:6-GPC relative to those enriched with 16:0/18:2n-6-GPC, arguing against a generalized perturbation of the membrane due to 22:6n-3 incorporation. We conclude that molecular species of PC with 22:6n-3 at the sn-2 position can affect TXA2/PGH2 receptors. The selectivity of the inhibitory effect of PC containing 22:6n-3 is discussed.