Stremler K E, Stafforini D M, Prescott S M, Zimmerman G A, McIntyre T M
Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah, Salt Lake City 84112.
J Biol Chem. 1989 Apr 5;264(10):5331-4.
Platelet-activating factor (PAF) is a glycerophospholipid that has diverse potent biological actions. A plasma enzyme catalyzes the hydrolysis of the sn-2 acetoyl group of PAF and thereby abolishes its bioactivity. This PAF acetylhydrolase is specific for phospholipids, such as PAF, with a short acyl group at the sn-2 position. The majority of it (60-70%) is associated with low density lipoprotein (LDL), and the remainder is with high density lipoprotein (HDL). LDL also has a phospholipase A2 activity that is specific for oxidized polyunsaturated fatty acids, which may be important in determining how LDL is recognized by cellular receptors. We previously have purified and characterized the PAF acetylhydrolase from human plasma. We now have found that the purified PAF acetylhydrolase catalyzes the hydrolysis of the oxidized fragments of arachidonic acid from the sn-2 position of phosphatidylcholine. One of the preferred substrates appeared by mass spectrometry to have 5-oxovalerate at the sn-2 position. We synthesized 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine and found that the PAF acetylhydrolase had the same apparent Km for it (11.3 microM) as for PAF (12.5 microM), with Vmax values of 100 and 167 mumol/h/mg of protein, respectively. We also conclude that the PAF acetylhydrolase is the sole activity in LDL that degrades oxidized phospholipids since we found co-localization of the activity against both substrates to LDL and HDL, and precipitation of enzyme activity with an antibody to the PAF acetylhydrolase. Thus, the PAF acetylhydrolase in human plasma degrades oxidized phospholipids, which may be involved in the modification of apolipoprotein B100 and other pathological processes.
血小板活化因子（PAF）是一种具有多种强大生物学作用的甘油磷脂。一种血浆酶催化PAF的sn-2乙酰基水解，从而消除其生物活性。这种PAF乙酰水解酶对sn-2位带有短酰基的磷脂（如PAF）具有特异性。其大部分（60 - 70%）与低密度脂蛋白（LDL）相关，其余部分与高密度脂蛋白（HDL）相关。LDL还具有一种对氧化多不饱和脂肪酸具有特异性的磷脂酶A2活性，这在决定LDL如何被细胞受体识别方面可能很重要。我们之前已从人血浆中纯化并鉴定了PAF乙酰水解酶。我们现在发现，纯化的PAF乙酰水解酶催化从磷脂酰胆碱的sn-2位水解花生四烯酸的氧化片段。通过质谱分析，其中一种优选的底物在sn-2位似乎具有5-氧代戊酸。我们合成了1-棕榈酰-2-(5-氧代戊酰基)-sn-甘油-3-磷酸胆碱，发现PAF乙酰水解酶对其的表观Km（11.3 microM）与对PAF（12.5 microM）相同，Vmax值分别为100和167 mumol/h/mg蛋白质。我们还得出结论，PAF乙酰水解酶是LDL中降解氧化磷脂的唯一活性，因为我们发现针对这两种底物的活性与LDL和HDL共定位，并且用抗PAF乙酰水解酶的抗体沉淀酶活性。因此，人血浆中的PAF乙酰水解酶降解氧化磷脂，这可能参与载脂蛋白B100的修饰和其他病理过程。
