Regulation of plasma lecithin:cholesterol acyltransferase in man. III. Role of high density lipoprotein cholesteryl esters in the activating effect of a high-fat test meal.

Plasma lecithin:cholesterol acyltransferase (LCAT) activity is increased during the clearance phase of alimentary lipemia induced by a high-fat test meal in normal subjects. Ultracentrifugal fractionation of high density lipoproteins (HDL) into HDL(2), HDL(3), and very high density (VHD) subfractions followed by analyses of lipid and protein components has been accomplished at intervals during alimentary lipemia to seek associations with enzyme changes. HDL(2) lipids and protein increased substantially, characterized primarily by enrichment with lecithin. HDL(3), which contain the main LCAT substrates, revealed increased triglycerides and generally reduced cholesteryl esters which were reciprocally correlated, demonstrating a phenomenon previously observed in vitro by others. Both changes correlated with LCAT activation, but partial correlation analysis indicated that ester content is primarily related to triglycerides rather than LCAT activity. The VHD cholesteryl esters and lysolecithin were also reduced. Plasma incubation experiments with inactivated LCAT showed that alimentary lipemic very low density lipoproteins (VLDL) could reduce levels of cholesteryl esters in HDL by a nonenzymatic mechanism. In vitro substitution of lipemic VLDL for postabsorptive VLDL resulted in enhanced reduction of cholesteryl esters in HDL(3) and VDH, but not in HDL(2), during incubation. Nevertheless, augmentation of LCAT activity did not result, indicating that cholesteryl ester removal from substrate lipoproteins is an unlikely explanation for activation. Since VHD and HDL(3), which contain the most active LCAT substrates, were also most clearly involved in transfers of esters to VLDL and low density lipoproteins, the suggestion that LCAT product lipoproteins are preferentially involved in nonenzymatic transfer and exchange is made. The main determinant of ester transfer, however, appears to be the level of VLDL, both in vitro and in vivo. Rose, H. G., and J. Juliano. Regulation of plasma lecithin: cholesteryl acyltransferase in man. III. Role of high density lipoprotein cholesteryl esters in the activating effect of a high-fat test meal.