Hydrolysis and transesterification of platelet-activating factor by lecithin-cholesterol acyltransferase.

Purified lecithin-cholesterol acyltransferase (LCAT, EC 2.3.1.43) from human plasma was found to hydrolyze platelet-activating factor (PAF) to lyso-PAF and acetate. In addition, it catalyzed the transfer of the acetate group from PAF to lysophosphatidylcholine, forming lyso-PAF and a 1-acyl analog of PAF. In contrast to the cholesterol-esterification reaction carried out by the enzyme, the hydrolysis and transacetylation of PAF by LCAT did not require an apoprotein activator and were not inhibited by sulfhydryl inhibitors but were inhibited by serum albumin. When added to a proteoliposome substrate of LCAT or to whole plasma, PAF inhibited cholesterol esterification by LCAT competitively. PAF acetylhydrolase (EC 3.1.1.47), purified from human plasma, also catalyzed the transfer of acetate from PAF to lysophosphatidylcholine. However, the LCAT-catalyzed reactions of PAF were not due to contamination with PAF acetylhydrolase, since the ratio of acetyl transfer to acetyl hydrolysis was 3 times greater for LCAT, when compared with PAF acetylhydrolase under identical conditions. Furthermore, recombinant human LCAT secreted by baby hamster kidney cells also catalyzed the hydrolysis and transacetylation of PAF. These results demonstrate that LCAT can inactivate PAF in plasma by transacetylation and suggest that it may have a role in the metabolism of PAF, and possibly of oxidized phospholipids, in plasma.

从人血浆中纯化得到的卵磷脂胆固醇脂酰转移酶（lecithin-cholesterol acyltransferase, LCAT, EC 2.3.1.43）可将血小板活化因子（platelet-activating factor, PAF）水解为溶血PAF（lyso-PAF）与乙酸。此外，该酶还可催化乙酸基团从PAF转移至溶血磷脂酰胆碱，生成溶血PAF与PAF的1-酰基类似物。与该酶介导的胆固醇酯化反应不同，LCAT对PAF的水解与转乙酰化反应无需载脂蛋白激活剂（apoprotein activator），且不受巯基抑制剂（sulfhydryl inhibitors）抑制，但可被血清白蛋白（serum albumin）阻断。当将PAF加入LCAT的蛋白脂质体（proteoliposome）底物体系或全血浆中时，PAF会竞争性抑制LCAT介导的胆固醇酯化反应。从人血浆中纯化得到的PAF乙酰水解酶（PAF acetylhydrolase, EC 3.1.1.47）同样可催化乙酸从PAF转移至溶血磷脂酰胆碱。然而，LCAT催化的PAF反应并非源于PAF乙酰水解酶的污染：在相同反应条件下，LCAT的乙酰转移与乙酰水解的比值是PAF乙酰水解酶的3倍。此外，幼仓鼠肾细胞（baby hamster kidney cells）分泌的重组人LCAT同样可催化PAF的水解与转乙酰化反应。上述结果表明，LCAT可通过转乙酰化作用使血浆中的PAF失活，提示该酶可能参与血浆中PAF乃至氧化磷脂（oxidized phospholipids）的代谢过程。