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.