Abstract
Although the major function of lecithin-cholesterol acyltransferase (LCAT) is cholesterol esterification, our previous studies showed that it can also hydrolyze platelet-activating factor (PAF). Because of the structural similarities between PAF and the truncated phosphatidylcholines (polar PCs) generated during lipoprotein oxidation, we investigated the possibility that LCAT may also hydrolyze polar PCs to lyso-PC during the oxidation of plasma. PAF acetylhydrolase (PAF-AH), which is known to hydrolyze polar PCs in human plasma, was completely inhibited by 0.2 mM p-aminoethyl benzenesulfonyl fluoride (Pefabloc), a new serine esterase inhibitor, which had no effect on LCAT at this concentration. On the other hand, 1 mM diisopropylfluorophosphate (DFP) completely inhibited LCAT but had no effect on PAF-AH. Polar PC accumulation during the oxidation of plasma increased by 44% in the presence of 0.2 mM Pefabloc and by 30% in the presence of 1 mM DFP. The formation of lyso-PC was concomitantly inhibited by both of the inhibitors. The combination of the two inhibitors resulted in the maximum accumulation of polar PCs, suggesting that both PAF-AH and LCAT are involved in their breakdown. Oxidation of chicken plasma, which has no PAF-AH activity, also resulted in the formation of lyso-PC from the hydrolysis of polar PC, which was inhibited by DFP. Polar PCs, either isolated from oxidized plasma or by oxidation of labeled synthetic PCs, were hydrolyzed by purified LCAT, which had no detectable PAF-AH activity. These results demonstrate a novel function for LCAT in the detoxification of polar PCs generated during lipoprotein oxidation, especially when the PAF-AH is absent or inactivated.
Highlights
Lecithin-cholesterol acyltransferase (LCAT)1 is a plasma enzyme that circulates mostly in association with the high density lipoproteins (HDL) and is responsible for the synthesis of most of the cholesteryl esters present in human plasma [1, 2]
Polar PCs, either isolated from oxidized plasma or by oxidation of labeled synthetic PCs, were hydrolyzed by purified lecithin-cholesterol acyltransferase (LCAT), which had no detectable platelet-activating factor (PAF)-AH activity. These results demonstrate a novel function for LCAT in the detoxification of polar PCs generated during lipoprotein oxidation, especially when the PAF acetylhydrolase (PAF-AH) is absent or inactivated
Selective Inhibition of PAF-AH and LCAT Activities—To determine the relative roles of LCAT and PAF-AH in the hydrolysis of oxidized phospholipids we first sought to identify an inhibitor that can inhibit PAF-AH, the enzyme known to carry out this hydrolysis in human plasma [16]
Summary
Our recent investigations on the effect of lipoprotein oxidation on LCAT showed that while its cholesterol esterification activity is destroyed rapidly, its ability to transfer the oxidized acyl group of PC to exogenous lyso-PC is retained or even stimulated [28]. This reaction, which was named lysolecithin acyltransferase II (LAT II), is not dependent upon the presence of an apoprotein activator, and it was insensitive to sulfhydryl blocking agents. The present study provides experimental evidence that LCAT can hydrolyze the oxidized PCs generated during lipoprotein oxidation and that it may account for a significant percentage of the phospholipase A activity found in the plasma following lipoprotein oxidation
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