Incorporation and turnover of eicosapentaenoic and docosahexaenoic acids in human blood platelets in vitro

Abstract

Mass changes in the incorporation of linoleic (C(18:2) eicosapentaenoic (C(20:5)) and docosahexaenoic (C(22:6)) acids in human blood platelet phospholipids were induced by incubating the cells and these fatty acids complexed to albumin. The remodelling of [14C]C(18:2), [14C]C(20:5) and [14C]C(22:6) in classes, subclasses and molecular species of platelet phospholipids was studied in resting and thrombin-stimulated cells. More than 85% of the incorporation was located in phospholipids, representing 5-fold and 2.5-fold increases in the phospholipid C(20:5) and C(22:6) endogenous content respectively. Thrombin stimulation induced a 30%, degradation of 1-acyl-2-C(20:5)-glycerophosphocholine (GPC) and 1-acyl-2-C(22:6)-GPC, but did not induce significant release of C(18:2) from 1-acyl-2-C(18:2)-GPC. There was no change in the [(l4)C]fatty acid composition of 1-alkyl-2-acyl-GPC. Thrombin-dependent increases in 1-alkenyl-2-C(20:5)-glycerophosphoethanolamine (GPE) and 1-alkenyl-2-C(22:6)-GPE of 2.1-fold and 2.5-fold respectively accounted for the rise in GPE radioactivity and partly compensated for the loss of these fatty acids from 1,2-diacyl-GPC: transfer to 1-alkenyl-2-acyl-GPE was 0.4 and 1.5 nmol/109 platelets for C(20:6) and C(22:6) respectively. [14C]C(20:5) and [14C]C(22:6) were incorporated into six different species of 1,2-diacyl-GPC, with acylation in the major endogenous forms (C(18:1) + C(16:0) and C15:0) species) representing 76% and 66% respectively of the total radioactivity present in 1,2-diacyl-GPC. Stimulation by thrombin induced significant release of these fatty acids from the main molecular species of 1,2-diacyl-GPC, but significantly stimulated the synthesis of alkenyl forms of GPE containing C(18:1)/C(22:6) + C(16:O)/C(22:6), C(18:0)/C(22:6) and C(18:0)/C(20:6), C(18:O)/C(18:2), the major endogenous C(18:2) molecular species, represented only 10.5% of the incorporation; none of the [14C]C(16:2) molecular species was a substrate for transfer towards 1-alkenyl-2-acyl-GPE. It is concluded that when C(20:5) and C(22:6) but not C(18:2), are acylated in 1,2-diacyl-GPC, they participate in thrombin-dependent phospholipid remodelling, and might compete with the turnover and release of arachidonic acid from platelet phospholipids and the subsequent activation of the cells.