Transfer and equilibration of LCAT-generated cholesteryl esters in human lipoproteins

Non-enzymatic net transport of cholesteryl esters (CE) from HDL to VLDL in exchange for triglycerides has been reported in vitro. Likewise, in vivo observations have been adduced favoring the same process in human subjects and implicating LCAT product lipoproteins as carriers. Exchange of CE among lipoproteins is thought not to occur. In experiments in which LCAT-generated radioactive CE are formed from cholesterol dispersion in plasma, exchange to near equilibrium is observed. To better define this process, a labeling procedure employing lecithin: 3H-unesterified cholesterol single bilayer vesicles was devised. The order of lipoprotein labeling with isotopic CE is HDL greater than VLDL greater than LDL. After 19 hours of incubation, esters reached equilibrium. Control experiments with vesicles treated by purified LCAT showed that this result could not be explained by distribution of discrete ester-rich vesicle products or their binding to lipoproteins. Subfractionation of labeled lipoproteins on agarose columns confirmed completeness of equilibration, except for the HDL subfraction of smallest size, which is incompletely equilibrated. These results indicate that LCAT-generated CE are capable of equilibration among human lipoproteins.