In this study, the 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine content of human platelets was determined. The distribution of arachidonate among the 1,2-diacyl, 1-O-alkyl-2-acyl, and 1-O-alk-l'-enyl-2-acyl classes of choline- and ethanolamine-containing phosphoglycerides was also assessed. The major platelet phospholipids were choline-containing phosphoglycerides (38%), ethanolamine-containing phosphoglycerides (25%) and sphingomyelin (18%), with smaller amounts of phosphatidylserine (11%) and phosphatidylinositol (4%). In addition to the diacyl class, the choline-linked fraction was found to contain both 1-O-alkyl-2-acyl (10%) and 1-O-alk-l'-enyl-2-acyl (9%) species. The ethanolamine-linked fraction, on the other hand, had an elevated level of the 1-O-alk-1'-enyl-2-acyl (60%) species and a small amount of the 1-O-alkyl-2-acyl component (4%). The major fatty acyl residues found in all classes of the choline and ethanolamine phospholipids were 16:0, 18:0, 18:1 (delta 9), 18:2(n-6) and 20:4(n-6). The 1-O-alkyl and 1-O-alk-1'-enyl fraction of the ethanolamine-linked phospholipids also contained substantial amounts of 22:5(n-3) and 22:6(n-3) acyl chains. Arachidonate comprised 44% of the acyl residues in the sn-2 position of 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine. Corresponding values for the diacyl and 1-O-alk-1'-enyl-2-acyl species were 23% and 25%, respectively, based on all 20:4(n-6) being linked to the sn-2 position of all classes.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet-activating factor (PAF, 1- O -alkyl-2-acetyl- sn -glycero-3-phosphocholine) is a potent phospholipid mediator of numerous inflammatory and thrombotic responses. The purpose of this study was to determine if PAF synthesis is elevated in damaged coronary arteries after a sustained period of cyclic flow variation (CFV), a phenomenon caused by alternating periods of thrombosis and reperfusion at sites of endothelial injury. Cyclic flow was established and maintained in the left anterior descending coronary arteries (LADs) of 10 dogs. After 8 hours of CFV, the section of damaged LAD containing the thrombus and control sections of the circumflex artery, carotid artery, and saphenous vein was excised, and the total lipids were extracted. The PAF was then purified by silica column chromatography and high-performance liquid chromatography and assayed by both a rabbit platelet bioassay and a PAF radioimmunoassay. With the platelet bioassay, PAF levels of 8.9±4.0 (range, 4.8 to 15.5) pg/mg wet wt were found in the damaged LADs from the 10 dogs. This PAF bioactivity was completely inhibited by a PAF receptor antagonist. When the radioimmunoassay was used, slightly higher PAF levels of 16.3±12.9 (range, 4.5 to 41.8) pg/mg wet wt were observed in the LADs. Overall, these PAF levels were 3- to 64-fold higher than in the control vessels when either assay method was used. Although increases in PAF were observed in the damaged LADs, measurements of PAF in blood samples taken from the LAD and the aorta (control) failed to demonstrate any site-specific increase of PAF in the blood. In related experiments, PAF was also measured in 23 endarterectomy samples taken from the coronary arteries of 16 patients with severe atherosclerosis. The PAF levels in these samples were highly variable (2.9±2.2 [range, 0.3 to 8.5] pg/mg wet wt) and showed no correlation with tissue mass, suggesting that PAF is affected by factors other than the simple presence of atherosclerotic tissue in the vessel. These findings provide direct evidence that PAF is synthesized locally at the site of endothelial injury during thrombosis and that PAF accumulates in the atherosclerotic plaque of some patients with advanced coronary artery disease.

This study was undertaken to determine if rabbit neutrophils contain sufficient ether-linked precursor for the synthesis of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet activating factor) by a deacylation-reacylation pathway. The phospholipids from rabbit peritoneal polymorphonuclear neutrophils were purified and quantitated, and the choline-containing and ethanolamine-containing phosphoglycerides were analyzed for ether lipid content. Choline-containing phosphoglycerides (37%), ethanolamine-containing phosphoglycerides (30%), and sphingomyelin (28%) were the predominant phospholipid classes, with smaller amounts of phosphatidylserine (5%) and phosphatidylinositol (less than 1%). The choline-linked fraction contained high amounts of 1-O-alkyl-2-acyl-(46%) and 1,2-diacyl-sn-glycero-3-phosphocholine (54%), with a trace of the 1-O-alk-1'-enyl-2-acyl species. The ethanolamine-linked fraction contained high amounts of 1-O-alk-1'-enyl-2-acyl-(63%) and 1,2-diacyl-sn-glycero-3-phosphoethanolamine (34%), and a low quantity of the 1-O-alkyl-2-acyl species (3%). The predominant 1-O-alkyl ether chains found in the sn-1 position of the choline-linked fraction were 16:0 (35%), 18:0 (14%), 18:1 (26%), 20:0 (16%), and 22:0 (9%). The major 1-O-alk-1'-enyl ether chains found in the sn-1 position of the ethanolamine-linked fraction were 14:0 (13%), 16:0 (44%), 18:0 (27%), 18:1 (12%) and 18:2 (3%). The major acyl groups in the sn-1 position of 1,2-diacyl-sn-glycero-3-phosphocholine and 1,2-diacyl-sn-glycero-3-phosphoethanolamine were 16:0, 18:0 and 18:1. The most abundant acyl group in the sn-2 position of all classes of choline- and ethanolamine-linked phosphoglycerides was 18:2. Although this work does not define the biosynthetic pathway for platelet activating factor, it does show that there is ample precursor present to support its synthesis by a deacylation-reacylation pathway.

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