Effect of unesterified cholesterol on the compartmentation of a fluorescent cholesteryl ester in a lipoprotein-like lipid microemulsion.

The access of enzymes and lipid transfer proteins to neutral lipids located predominantly in the core compartment of lipoproteins may be determined to some degree by the solubility of the neutral lipids in the surface monolayer of phospholipid. This report concerns the hypothesis that unesterfied cholesterol can affect the partition of a cholesteryl ester between the surface monolayer of a lipid emulsion and the internal core compartment, thus controlling the degree to which the cholesteryl ester is presented at the emulsion surface. For microemulsions composed of dimyristoyl phosphatidylcholine and cholesteryl oleate, the addition of unesterified cholesterol results in an increase in the particle size from about 170 nm diameter to 210 nm diameter at 13.5 mol% unesterified cholesterol. Fluorescent quenching methods were devised to determine the apparent partition of a fluorescent cholesteryl ester (cholesteryl anthracene-9-carboxylate) between surface and core compartments. The addition of unesterified cholesterol resulted in the movement of the fluorescent cholesteryl ester from the surface monolayer to the core compartment. The apparent partition coefficient, defined as the ratio of the concentration of probe in the monolayer to that in the core, decreased from 1.03 in the absence of unesterfied cholesterol to 0.54 at 28 mol% unesterified cholesterol in the emulsion. In this process, the fluorescent cholesteryl ester becomes less accessible to a quencher (5-doxyl stearate) located in the surface monolayer. The decrease in the surface curvature resulting from incorporation of unesterified cholesterol into the particle does not influence this quenching process. We conclude that the presence of unesterified cholesterol in the emulsion causes the fluorescent cholesteryl ester to become less soluble in the surface monolayer.