Utilization of serum lipoprotein lipids by the monoacylglycerol acyltransferase

The plasma membrane of rat liver contains an enzyme which is stimulated by heparin and hydrolyzes liposomes composed of phosphoglycerides as well as mono-and diacylglycerol (Waite, M. and Sisson, P (1973) J.Biol, Chem. 248, 7985-7902). Further, in liposomes this enzyme catalyzes a transacylation in which the acyl group is removed from position 1 of an acyl glyceride donor, and combined with the hydroxy group of a variety of acyl acceptors. To investigate the possible role of this enzyme in lipoprotein metabolism, we have incorporated specific labeled glycerides into lipoproteins. The high density, low and very low lipoproteins were then separated by molecular sieving and characterized by their physical and chemical properties. Using the labeled substrates as model lipoproteins, we found the following: 1)The enzyme is capable of hydrolyzing monoacylglycerol, diacylglycerophosphoethanolamine and diacylglycerophosphocholine; monoacylglycerol however is the preferred substrate in all three lipoprotein fractions. 2)Relative to the activity found on liposomes, the transacylation activity is low. 3)The specific radioactivity of the substrates in fraction B (low density lipoprotein) did not change during the reaction, which indicates that the labeled lipid is not a pool separate from the endogenous lipid of the lipoprotein.