EFFECT OF SPECIFIC PHOSPHOLIPID MOLECULAR-SPECIES INCORPORATED IN HUMAN PLATELET MEMBRANES ON THROMBOXANE A(2) PROSTAGLANDIN H-2 RECEPTORS

The incorporation of albumin-bound docosahexaenoic acid (22:6n-3), but not linoleic acid (18:2n-6), into cellular phospholipids inhibits pla telet aggregation induced by the thromboxane analogue U46619. [H-3]U46 619 specific binding to thromboxane A(2)/prostaglandin H-2 (TXA(2)/PGH (2)) receptors, as well as specific binding of the antagonist [H-3]SQ2 9548 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% o f the 22:6n-3 incorporated in these cells was esterified in the variou s endogenous phospholipid classes and the remaining was found in neutr al lipids and in the unesterified fatty acid pool. In this study, we d etermined whether the-effects observed could be attributed to the este rification of 22:6n-3 in phospholipids and whether the 22:6n-3 biologi cal activity might depend on its esterification in specific phospholip id classes. Therefore, pure phosphatidylcholine (PC) and phosphatidyle thanolamine (PE) molecular species were transferred to platelet membra nes, 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, respect ively, were incorporated into membranes only at the expense of the cor responding endogenous phospholipid class, by an apparent exchange proc ess. When such modified membranes were tested for specific binding of U46619 and SQ295488, a significant decrease of the receptor site affin ity was only observed in membranes highly enriched with -palmitoyl-2-d ocosahexaenoyl-glycerophosphocholine (16:0/22:6-GPC). Fluidity paramet ers measured by electron spin resonance of 5- and 16-nitroxy-stearic a cids 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 aga inst a generalized perturbation of the membrane due to 22:6n-3 incorpo ration. We conclude that molecular species of PC with 22:6n-3 at the s n-2 position can affect TXA(2)/PGH(2) receptors. The selectivity of th e inhibitory effect of PC containing 22:6n-3 is discussed.