Utilization of Neutral Glycerides and Phosphatidylethanolamine by the Phospholipase A1 of the Plasma Membranes of Rat Liver

The phospholipase A1 solubilized by heparin from the plasma membranes of rat liver can catalyze the hydrolysis of monoglyceride, diglyceride, and monoacylglycerophosphorylethanolamine in addition to phosphatidylethanolamine. Furthermore, the enzyme catalyzes the transacylation of the acyl group from position 1 of these glycerides to an acceptor monoglyceride to form diglyceride. Triton X-100 which disperses the lipids reduces transacylation and favors the utilization of water as the acyl acceptor (hydrolysis). Since free fatty acid is not incorporated, the transacylation appears to be direct rather than the combination of hydrolysis and reacylation. The influence of Ca2+ on transacylation and hydrolysis was dependent upon the mixtures of lipids used as substrates. Co-elution of the hydrolytic and transacylation activities from a column of Sephadex G-200 and heat inactivation of all activities indicate that a single enzyme catalyzes both reactions. It is tentatively suggested this enzyme be more appropriately named monoglyceride acyltransferase since monoglyceride is the preferred substrate and transacylation is the predominating reaction.