Substrate specificity of plasma lysolecithin acyltransferase and the molecular species of lecithin formed by the reaction.

Human plasma lecithin-cholesterol acyltransferase also converts lysolecithin to lecithin in the presence of low density lipoproteins. To understand the physiological importance of this lysolecithin acyltransferase reaction, we investigated the molecular species of lysolecithin available for acylation in normal plasma and the lecithins which are formed by the acylation of each of these lysolecithins. Palmitate- and stearate-containing lysolecithins were formed by the lecithin-cholesterol acyltransferase reaction, whereas oleate- and linoleate-containing lysolecithins were formed by the action of post-heparin lipase(s). All the natural lysolecithins were esterified at comparable rates by the isolated enzyme. Lyso platelet-activating factor was esterified about 70% as efficiently as the lysolecithins, while lysophosphatidylethanolamine was esterified at about 30% the rate observed with lysolecithin. The 2-acyl isomers of lysolecithin were acylated to the same extent as the 1-acyl isomers, although considerable isomerization of the former took place during the incubation. There were no net changes in the concentrations of lecithin and lysolecithin after 6 h of incubation with the enzyme, although over 10% of the labeled lysolecithin was converted to lecithin, indicating that the endogenous lecithin serves as the acyl donor in the reaction. When the molecular species of lecithin formed were analyzed by high performance liquid chromatography, the same pattern of fatty acid incorporation was observed with all the lysolecithins used. The bulk of the radioactivity was incorporated into molecular species formed by the acylation with linoleic, oleic, and palmitic acids, in decreasing order. However, in each case, the lecithins formed by acylation with palmitic acid had the highest specific radioactivity, followed by those acylated with linoleic and oleic acids. From these results it is postulated that the enzyme alters the molecular species composition of lecithin in plasma without increasing the net amount of total lecithins.