Some Properties of Liposomes of Different Sizes

microscope of cells fixed in 5 % (v/v) glutaraldehyde after 20min incubation with glycerol mono-oleate. Fused cells were discernible by their size; the number of cells involved in fusion was calculated as a percentage of the total cell number in 10-15 fields for each incubation. Although considerable individual variation occurred, statistically significant increases in cell cholesterol concentration and cholesterol/phospholipid molar ratios were found in the cells from patients with liver diseases (Table 1). The erythrocytes of some patients also exhibited increased concentrations of phosphatidylcholine and phosphatidylserine, but not of other phospholipid fractions, and there was no difference in the mean values for total phospholipid in each group. Glycerol mono-oleate-induced cell fusion was more extensive in the erythrocytes of the patients with liver diseases. Taking data from all subjects studied (patients and control subjects), there was a positive correlation between percentage polykaryocytosis and cholesterol content or cholesterol/phospholipid ratio, but not with the total concentration of phospholipid. A correlation was observed between cell fusion and the cellular concentration of phosphatidylcholine or phosphatidylserine, but not with other phospholipids. From these observations it appears that increases in membrane lipids facilitate cell fusion. A mechanism for fusion has been proposed in which the intercalated membrane particles are excluded from the regions of membrane involved in fusion (Ahkong et al., 1975). Enrichment of bovine erythrocytes with cholesterol decreases the density of intramembranous particles (Deuticke & Ruska, 1976) and this has also been demonstrated in cells from patients with severe cholestasis (Verkleij et al., 1976). Furthermore, in erythrocytes from these patients, fusion of lipoprotein LP-X coincides with an increase in smooth areas in the fracture faces of the membrane. We therefore suggest that increases in membrane lipid concentrations, especially cholesterol, may facilitate phase separations of protein-free areas in the lipid bilayer, permitting fusion to occur more readily.