Initial rate of cholesterol esterification associated with high density lipoproteins in human plasma

The enzyme 1ecithin:cholesterol acyl transferase has been measured both in total plasma and in the fraction of plasma from which very low and low density lipoproteins have been removed by ultracentrifugation. The correlation between the activity of the enzyme and the free cholesterol concentration was positive in whole plasma and negative in apoB-deficient plasma. On the other hand, the positive correlation between plasma triglycerides and cholesterol esterification was not changed by the removal of apoBcontaining lipoproteins. Subjects with the highest levels of high density lipoprotein cholesterol were found to have the lowest enzyme activity, but this correlation was disclosed only in apoB-deficient plasma. This inverse relationship was abolished when the enzyme activity was measured in the absence of all lipoproteins with a density less than 1.125 g/ml. Cholesterol esterification, when determined after removal of lipoproteins with a density less than 1.063 g/ml, was negatively correlated with the in vivo plasma concentration of lipoproteins in the density range 1.0631.125 g/ml. The same results were obtained in vitro by addition of increasing amounts of this class of high density lipoproteins either in total plasma or in the ultracentrifuged fractions of plasma. This provides further evidence that the lighter density class of high density lipoproteins inhibits the enzyme reaction under physiological conditions. Pinon, J-C., A-M. Bridoux, and M-H. Laudat. Initial rate of cholesterol esterification associated with high density lipoproteins in human plasma. J. Lipid Res. 1980. 21: 406-414. Supplementary key words 1ecithin:cholesterol acyl transferase * apoB-deficient plasma triglycerides high density lipoprotein cholesterol The enzyme 1ecithin:cholesterol acyl transferase (LCAT, EC.2.3.1.43) acts mainly in the plasma and catalyzes the transfer of fatty acids from lecithin to cholesterol with the formation of cholesteryl esters and lysolecithin. The LCAT activity is the major source of cholesteryl esters in human plasma. Despite the fact that high density lipoproteins (HDL, 1.063 < d < 1.2 1 g/ml) and very high density lipoproteins (VHDL, 1.21 < d < 1.25 g/ml) rather than other plasma lipoproteins, have been recognized as the lipoprotein substrate for LCAT (1 -3), there are no reports regarding the relationship in vivo between plasma LCAT activity and HDL concentration. It has been reported that plasma triglycerides are positively correlated with LCAT activity (4-6) and negatively related with HDL-cholesterol(7). However, a significant correlation between LCAT activity and HDL-cholesterol has not been established so far (8-10). One subfraction of HDL (HDL2, d 1.0631.125 g/ml) appears to be the major contributor to variations occurring in HDL or HDL-cholesterol levels and was consequently proposed as the critical HDL component responsible for the reported inverse correlation between HDL-cholesterol and coronary heart disease (1 1). This HDL fraction was also shown to inhibit the LCAT reaction in vitro (12, 13). Experiments with partially purified LCAT enzyme have shown that HDL, (d 1.1251.2 1 g/ml), but not HDL2, is the preferential substrate for the reaction (3). In vitro studies, using purified LCAT and synthetic lipid mixtures, have demonstrated that the major protein of HDL (apo A-I) is an activator of the reaction (14), and that quantitative relationships exist among enzyme, substrate, and activator (15). Other studies, using the residual protein fraction of d > 1.21 g/ml as a source of enzyme, have revealed that with sonicated dispersions of lecithin-cholesterol mixtures, the rate of cholesterol esterification was primarily governed by the molar ratio of the lecithin to cholesterol and by the amounts of HDL, added to the incubation medium (16). However, information concerning the dependence in vitro of plasma LCAT activity on HDL2 or HDL, concentration has not been reported to date. The present research was thus focussed on the in vivo and in vitro dependence of cholesterol esterification activity on high density lipoprotein levels. The first goal was to determine whether the initial rate of LCAT activity, when measured both in whole plasma and in apoB-deficient plasma, would correlate with the in vivo concentration of HDL2. The second part of this work was to examine the influence of Abbreviations: LCAT, lecithin: cholesterol acyltransferase; HDL, high density lipoprotein; TG, triglycerides. 406 Journal of Lipid Research Volume 21, 1980 by gest, on N ovem er 6, 2017 w w w .j.org D ow nladed fom isolated HDL, upon the LCAT reaction rate in total plasma and in the fraction of plasma containing either one or the two subclasses of HDL. MATERIAL AND METHODS