Inflammatory platelet-activating factor-like phospholipids in oxidized lowdensity lipoproteins are fragmented alkyl phosphatidylcholines

Oxidation of human low density lipoprotein (LDL) generates proinflammatory mediators and underlies early events in atherogenesis. We identified mediat ors in oxidized LDL that induced an inflammatory reaction in vivo, and acti vated polymorphonuclear leukocytes and cells ectopically expressing human p latelet-activating factor (PAF) receptors, Oxidation of a synthetic phospha tidylcholine showed that an sn-l ether bond confers an 800-fold increase in potency. This suggests that rare ether-linked phospholipids in LDL are the likely source of PAF-like activity in oxidized LDL, Accordingly, treatment of oxidized LDL with phospholipase A, greatly reduced phospholipid mass, b ut did not decrease its PAF-like activity. Tandem mass spectrometry identif ied traces of PAF, and more abundant levels of 1-O-hexadecyl-2-(butanoyl or butenoyl)-sn-glycero-3-phosphocholines (C-4-PAF analogs) in oxidized LDL t hat comigrated with PAF-like activity. Synthesis showed that either C-4-PAF was just 10-fold less potent than PAF as a PAF receptor ligand and agonist . Quantitation by gas chromatography-mass spectrometry of pentafluorobenzoy l derivatives shows the C-4-PAF analogs were 100-fold more abundant in oxid ized LDL than PAF, Oxidation of synthetic alkyl arachidonoyl phosphatidylch oline generated these C-4-PAFs in abundance. These results show that quite minor constituents of the LDL phosphatidylcholine pool are the exclusive pr ecursors for PAF-like bioactivity in oxidized LDL.