Study of the lecithin: cholesterol acyltransferase reaction with liposome and high density lipoprotein substrates

The activity of highly purified preparations of human plasma lecithin: cholesterol acyltransferase were stabilized by precipitating the enzyme with ammonium sulfate and using the dilutions of the particulate lecithin: cholesterol acyltransferase suspension for enzyme assays. Ammonium sulfate concentrations in the assay mix up to 0.1 M had no significant effect on lecithin: cholesterol acyltransferase activity. The basic enzymatic properties of lecithin: cholesterol acyltransferase were investigated using liposomes and high density lipoprotein (HDL) substrates. pH optima for both substrates was approximately 8.0. The temperature dependence of lecithin: cholesterol acyltransferase activity resulted in non-linear Arrhenius plots with both substrates. The activity vs. temperature (degrees C) curves showed slight inflections at 30 degrees C, which may have been due to the relatively rapid inactivation of the enzyme above this temperature. HDL3 was found to be a better substrate than HDL or HDL2. HDL3 was also considerably better than egg phosphatidylcholine/cholesterol liposomes as an lecithin: cholesterol acyltransferase substrate. Addition of HDL2 to a reaction mix of enzyme and HDL3 indicated that HDL2 acts as an inhibitor of cholesterol esterification in this system.