An oxidized derivative of phosphatidylcholine is a substrate for the platelet-activating factor acetylhydrolase from human plasma

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.