Capacity of lipoproteins to act as substrates for lecithin:cholesterol acyltransferase. Enhancement by pre-incubation with an artificial triacylglycerol emulsion.

Previous studies have shown that high concentrations of very low density lipoproteins (VLDL) stimulate the formation of cholesteryl esters in human plasma, possibly by acting as recipients of cholesteryl esters transferred from high density lipoproteins (HDL). To gain further insight into this phenomenon, experiments were performed to determine whether the triacylglycerol emulsion Intralipid, which acts as an artificial recipient of HDL cholesteryl esters, has an effect on cholesterol esterification similar to that of VLDL. Intralipid, in contrast to VLDL, is devoid of apolipoproteins which may stimulate cholesterol esterification. Human plasma, which had previously been depleted of VLDL, was pre-incubated in the presence or absence of Intralipid. After pre-incubation, the Intralipid was removed and rates of cholesterol esterification were measured in subsequent incubations of the Intralipid-depleted fractions. The presence of Intralipid during the pre-incubation had a marked stimulatory effect on cholesterol esterification, comparable to that of VLDL in earlier studies. The pre-incubation with Intralipid also markedly reduced the cholesteryl ester content, and increased the triacylglycerol content, of the HDL. These changes in composition coincided with two changes in the elution profile of HDL after density-gradient ultracentrifugation which were (i) a reduction in the density of particles in the HDL3 subfraction, which virtually disappeared as an identifiable peak, and (ii) the emergence of a discrete population of very dense lipoproteins consisting primarily of protein and phospholipid. Since all these changes were related to redistributions of lipids, the results highlight the importance of lipid transfers in the regulation of plasma cholesterol esterification.