Solubilization and localization of cholesteryl oleate in egg phosphatidylcholine vesicles. A carbon 13 NMR study

Co-sonicated mixtures of egg phosphatidylcholine and small amounts (less than or equal to 4%, w/w) of [carbonyl-13C]cholesteryl oleate have been studied by 13C NMR spectroscopy at 50.3 MHz. The carbonyl chemical shift from cholesteryl oleate solubilized in vesicles was 1 ppm downfield from the carbonyl chemical shift of cholesteryl oleate present in a separate oil phase. The maximum solubility of the steroid in vesicles determined by chemical analysis of purified vesicles was 1.6 weight % (approximately 2 mol %), in close agreement with the maximum solubility estimated from NMR speak intensity measurements (1.4 weight %). The downfield shift indicates hydrogen bonding of solvent (H2O) molecules with the cholesteryl oleate carbonyl group, suggesting that vesicle-solubilized cholesteryl oleate molecules are located in the phospholipid bilayer with the carbonyl group close to the aqueous interface and the sterol ring and fatty acyl chain approximately parallel to the fatty acyl chains of the phospholipid. Such a folded conformation and localization of the carbonyl group at the aqueous interface may facilitate interactions of sterol esters with cholesterol ester transfer proteins and cholesterol esterases.