Novel surface phase containing cholesteryl esters. 2. Nonequivalence of cholesteryl arachidonate and those with 18-carbon, cis-unsaturated acyl groups

Abstract

Surface pressure-area isotherms for binary mixtures of cholesteryl octanoate, elaidate, stearate, oleate, linoleate, linolenate, and arachiodonate in mixtures with dioleoyllecithin, triolein, oleic acid and oleoyl alcohol were measured at 24.degree. C. Analysis of the pressure and area characteristics as a function of composition showed that double-layer surface phase formation is primarily dependent on the structure of the acyl moiety of the cholesteryl ester. Cholesteryl esters with saturated or trans-unsaturated acyl chains apparently do not form double-layer surface phases. The esters of oleate, linoleate and linolenate formed double-layer and monolayer phases and their properties in these phases were similar. In contrast to other cis-unsaturated esters, cholesteryl arachidonate formed a mixed monolayer phase with miscibility in all proportions and did not form a double-layer phase. The polar lipid monolayer separating bulk cholesteryl ester from the aqueous milieu solubilizes finite amounts of cholesteryl esters and can contribute to the organization of lipid adjacent to the monolayer. That such organization is observed with the predominant cholesteryl ester species of blood and aorta suggests a role for double-layer structure in regulating the transport and metabolism of cholesteryl esters in lipoproteins, arterial lipid deposits, and adrenal cortex. The absence of double-layer formation and high monolayer solubility of cholesteryl arachidonate suggest that it should be more abundant than other cholesteryl esters in bilayers and in monolayers surrounding bulk lipid phases.