Lipid-sterol interactions in liposomes and membranes.

Sterols are main constituents of many membranes. Cholesterol is the most abundant sterol in membranes of higher organisms. Many studies have been undertaken to try to understand the function of cholesterol, in which the interaction between cholesterol and various phosphatidylcholines were investigated in monolayers and liposomes (Phillips, 1972). The physicochemical state of the lipids determines the lipid-sterol interaction. When the lipids are in the liquid-crystalline statecholesterol decreases the chain mobility and decreases the mean moleculararea(condensing effect), whereas when the lipids are in the gel state cholesterol increases the chain mobility (liquefying effect). In order to extrapolate the results obtained with model systems to the biological membrane, similar studies on biological membranes are necessary. Such studies are difficult because the lipid composition in most natural membranes is carefully controlled. Among mammalian cells only erythrocytes provide possibilities of varying the sterol content and composition to a limited extent via an exchange procedure (Bruckdorfer et al., 1969). In some micro-organisms belonging to the order of the Myoplasmatales, larger changes in the sterol content can beachieved. The cholesterol content of Mycoides mycoides (Rottem et al., 1973a,b) could be varied from 2 to 25% (w/w) of total lipids by using different growth media. Acholeplasma laidlawiicells do not require and cannot synthesize sterols. However, sterols can be incorporated in the membrane up to about 8 % (w/w) of the total lipids by growing the cells in the presence of sterols. The lipid/ protein ratio, the phospholipid/glycolipid ratio and the fatty acid composition of the membranes are not significantly affected by the incorporation of cholesterol (de KruyfT et al., 1972). These properties make these organisms very suitable for studying the role of sterols in biological membranes.