Apparent specificity of the thrombin-stimulated deacylation of endothelial glycerolipids for polyunsaturated fatty acids with a delta-5 desaturation.

Human umbilical vein endothelial cells readily incorporate exogenous polyunsaturated fatty acids. Subsequent stimulation with thrombin results in the release of both arachidonate and eicosapentaenoate from cellular phospholipids. The present study has investigated the utilization of 8,11,14-[14C]eicosatrienoate, the precursor of prostaglandin E1. Analysis of released 14C-fatty acids by radio-gas chromatography indicated that thrombin stimulated the release of 6-10% of the [14C]arachidonate synthesized by desaturation of the [14C]eicosatrienoate, but did not stimulate release of [14C]eicosatrienoate per se (less than 1%). As determined by digestion of cellular lipid extracts with pancreatic phospholipase A2, both 8,11,14-[14C]eicosatrienoate and [14C]arachidonate were esterified primarily in the 2-position. Similarly, separation of phospholipid classes by two-dimensional thin-layer chromatography did not indicate any major differences in the distribution of the incorporated 14C-fatty acids. Experiments with additional 14C-fatty acids indicated that 5,8,11-eicosatrienoate is released in response to thrombin but that 8,11,14,17-eicosatetraenoate is not. These results suggest that the delta-5 double bond is required for the thrombin-stimulated release of free fatty acids from endothelial phospholipids and their subsequent availability as substrates for eicosanoid synthesis.