Role of low density lipoprotein in the activation of plasma lysolecithin acyltransferase activity: Effect of chemical and enzymatic modifications of the lipoprotein on enzyme activity

Abstract

The effect of various chemical and enzymatic modifications of low density lipoprotein (LDL) on its ability to activate the isolated human plasma lysolecithin acyltransferase (LAT) was studied. Removal of all lipids from LDL resulted in the complete loss of LAT activation. Removal of only neutral lipids by extraction with heptane retained up to 50% of the original activity, which was not increased further by reconstitution of the LDL with the extracted lipids. Hydrolysis of the diacylphosphoglycerides of the LDL with phospholipases resulted in complete loss of LAT activation which was partially restored by the addition of egg lecithin. Hydrolysis of more than 4% of LDL protein by trypsin led to a linear decrease in activity with complete loss of activity occurring when about 25% of the LDL protein is hydrolyzed. Modification of the arginine groups of LDL reversibly inhibited the activation of LAT. Modification of lysine residues of LDL by acetylation, acetoacetylation or succinylation also abolished its ability to activate lysolecithin acylation. Cationization of the lipoprotein destroyed its activity, but the reduction and alkylation only decreased the activity slightly. Complete removal of sialic acid groups by treatment of LDL with neuraminidase reduced the activation of LAT by about 20%. From these results it is concluded that the phospholipids of LDL and the charged groups of LDL apoprotein B are essential for its activation of the acylation of lysolecithin by the lysolecithin acyltransferase of human plasma.