Influence of Cholesterol on Physical Properties of Egg Phosphatidylcholine Liposomes

Physical properties of unilamellar vesicles (ULVs) and multilamellar vesicles (MLVs) prepared from egg phosphatidylcholine have been investigated as a function of cholesterol content. Molecular dimensions of ULVs and MLVs with or without cholesterol have been measured by dynamic laser light scattering method. Inclusion of cholesterol in the liposomal bilayer structure significantly affected the mean diameter of vesicles suspended in buffer. The mean molecular dimension of MLVs was found to increase by 28% whereas that of ULVs decreased by 52% at the highest concentration of cholesterol (50 mole%)). Studies on stability of unilamellar vesicles containing cholesterol were carrried out by monitoring the release of encapsulated carboxyfluorescein (CF) after suspension in phosphate buffered saline (PBS) and human blood plasma at 37°C. Vesicles with cholesterol displayed greater stability than controls in buffer as well as in human plasma. Moreover, the mean size of these vesicles significantly decreased after suspension in human plasma compared with phosphate buffered saline (PBS) which was dependent on the concentration of choleserol. However, vesicles without cholesterol showed larger decrease of size in plasma than in buffer. Furthermore, cholesterol-free vesicles were found to be more stable in buffer than in plasma. These results suggest that the cholesterol content of liposomes affects their molecular size and stability in biological fluid environment which may govern the fate of vesicles in body circulation.