COPPER-CATALYZED OXIDATION MEDIATES PAF FORMATION IN HUMAN LDL SUBSPECIES - PROTECTIVE ROLE OF PAF-ACETYLHYDROLASE IN DENSE LDL

Free radical-mediated oxidation of cholesterol-rich LDL plays a key ro le in atherogenesis and involves the formation of oxidized phospholipi ds with proinflammatory biological activity. We evaluated the producti on of platelet-activating factor (PAF), a potent inflammatory mediator , in human LDL subspecies on copper-initiated oxidation (4 mu mol/L Cu Cl3, 80 mu g/mL for 3 hours at 37 degrees C). PAF formation was determ ined by biological assay of HPLC-purified lipid extracts of copper-oxi dized lipoproteins; chemical identity was confirmed by gas chromatogra phic and mass spectrometric analyses. PAF, characterized as the C16:0 molecular species, was preferentially produced in intermediate LDL (d= 1.029 to 1.039 g/mL) (8.6+/-5.7 pmol PAF/3 h per mg LDL protein) and l ight LDL (d=1.019 to 1.029 g/mL), but was absent from dense LDL partic les (d=1.050 to 1.063 g/mL). As PAF:acetylhydrolase inactivates PAF an d oxidized forms of phosphatidylcholine, we evaluated the relationship of lipoprotein-associated PAF:acetylhydrolase to PAF formation. We co nfirmed that PAF:acetylhydrolase activity was elevated in native, dens e LDL (41.5+/-9.5 nmol/min per mg protein) but low in LDL subspecies o f light and intermediate density (d 1.020 to 1.039 g/mL) (3.5+/-1.6 nm ol/min per mg protein) [Tselepis et al, Arterioscler Thromb Vasc Biol. 1995;15:1764-1773]. On copper-mediated oxidation for 3 hours at 37 de grees C, dense LDL particles conserved 20+/-14% of their initial enzym atic activity; in contrast, PAF:acetylhydrolase activity was abolished in light and intermediate LDL subspecies. Clearly, the elevated PAF: acetylhydrolase activity of dense LDL efficiently diminishes the poten tial inflammatory role of endogenously; formed PAF; nonetheless, forma tion of proatherogenic lysophospholipids results. in contrast. LDL par ticles of the light and intermediate subclasses can accumulate PAF on oxidative modification.