Lipoprotein-associated phospholipase A2, platelet-activating factor acetylhydrolase, generates two bioactive products during the oxidation of low-density lipoprotein: use of a novel inhibitor

摘要

pA novel and potent azetidinone inhibitor of the lipoprotein-associated phospholipase Asub2/sub (Lp-PLAsub2/sub), i.e. platelet-activating factor acetylhydrolase, is described for the first time. This inhibitor, SB-222657 (iK/isubi/sub = 40±3 nM, ik/isubobs/sub/[I] = 6.6×10sup5/sup Msup-1/sup·ssup-1/sup), is inactive against paraoxonase, is a poor inhibitor of lecithin:cholesterol acyltransferase and has been used to investigate the role of Lp-PLAsub2/sub in the oxidative modification of lipoproteins. Although pretreatment with SB-222657 did not affect the kinetics of low-density lipoprotein (LDL) oxidation by Cusup2+/sup or an azo free-radical generator as determined by assay of lipid hydroperoxides (LOOHs), conjugated dienes and thiobarbituric acid-reacting substances, in both cases it inhibited the elevation in lysophosphatidylcholine content. Moreover, the significantly increased monocyte chemoattractant activity found in a non-esterified fatty acid fraction from LDL oxidized by Cusup2+/sup was also prevented by pretreatment with SB-222657, with an ICsub50/sub value of 5.0±0.4 nM. The less potent diastereoisomer of SB-222657, SB-223777 (iK/isubi/sub = 6.3±0.5 iμ/iM, ik/isubobs/sub/[I] = 1.6×10sup4/sup Msup-1/sup·ssup-1/sup), was found to be significantly less active in both assays. Thus, in addition to generating lysophosphatidylcholine, a known biologically active lipid, these results demonstrate that Lp-PLAsub2/sub is capable of generating oxidized non-esterified fatty acid moieties that are also bioactive. These findings are consistent with our proposal that Lp-PLAsub2/sub has a predominantly pro-inflammatory role in atherogenesis. Finally, similar studies have demonstrated that a different situation exists during the oxidation of high-density lipoprotein, with enzyme(s) other than Lp-PLAsub2/sub apparently being responsible for generating lysophosphatidylcholine./p