Role of sterols in eukaryote membranes

Sterols are present in the cell and internal membranes of most eukaryotic organisms [125,219] but the actual role of membrane sterol is not clear [220]. The membranes of some organisms may function with or without sterols (Acho-eleplasrm or Pythium spp.) or in the complete absence of sterols (most prokaryotes) but for the majority of eukaryote cells the presence of sterol is essential to proper membrane function. 

It has been mentioned previously that sterols were readily incorporated into lecithin or mixed phospholipid monolayers [194—196]. The area of sterol-containing films was smaller than that calculated for the separate components, indicating some kind of interaction between the two molecules [221,222]. Sterols have also been shown to reduce the phase transitions of pure lecithin dispersions [214]. It has been suggested that cholesterol modifies the fluidity of the hydrocarbon chains of the phospholipid molecules by disrupting the crystalline chain lattice of the gel phase and by inhibiting the flexing of the chains in the dispersed liquid-crystalline phase [221—226]. 

Cholesterol (13) has little ionic character and is shorter than membrane phospholipids [227]. In a mixed lipid layer, cholesterol would be expected to be accommodated towards the centre of the hydrocarbon region of the layer. 

Although the composition and structure of mammalian (mainly erythrocyte and myelin) membranes has received extensive study, little attention has been paid to the membranes of yeasts, fungi and protozoa. Extraction of lipids from these organisms suggest similarities in overall structure, although cholesterol, which has an almost universal distribution in mammalian cells was replaced by other sterols, e.g. ergosterol (14) in yeasts and fungi [126]. The presence of 

Several models for the structure of polyene—sterol pores in lipid bilayers have been presented [154,249-252]. They generally involve hydrophobic interaction between cholesterol and the unsaturated region of the polyene antibiotic, with possible hydrogen bond formation between the hydroxyl group of the sterol and the carbonyl group of the polyene ring. 

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