Solubilization of triolein and cholesteryl oleate in egg phosphatidylcholine vesicles

The incorporation of cholesteryl oleate and triolein into phospholipid vesicles was studied in cosonicated mixtures of 94 weight % egg phosphatidylcholine and 6 weight % neutral lipid (0-6% triolein and 6-0% cholesteryl oleate). 13C NMR spectroscopy was used to quantitate both neutral lipids in vesicles containing 90% isotopically substituted [carbonyl-13C]cholesteryl oleate and [carbonyl-13C]triolein. Vesicles were also prepared with radiolabeled cholesteryl oleate and triolein and the composition of ultracentrifugal subfractions determined by chemical and radioisotopic methods. For a given starting composition, the incorporation of neutral lipids into vesicles was similar for vesicles prepared and analyzed by the two methodologies. The maximum solubility in vesicles prepared at 55 degrees C with a single neutral lipid was 3.1 weight % triolein (2.8 mol %) and 2.3 weight % cholesteryl oleate (2.8 mol %). In sonication mixtures with both triolein and cholesteryl oleate, the incorporation of each lipid into vesicles was proportional to the starting concentration; the total incorporation of neutral lipid was less than or equal to 4.0% (weight or mole per cent). The solubility limits were intermediate between the theoretical cases of complete additivity and complete competition. The [13C]carbonyl chemical shifts showed that the carbonyl groups of the vesicle-solubilized neutral lipids were close to the vesicle surface and that excess triolein and cholesteryl oleate partitioned into an oil phase containing both triolein and cholesteryl oleate.