Sterols, in membranes

The most common sterol in membranes is cholesterol (Chapter 14), which resides mainly in the plasma membranes of mammalian cells but can also be found in lesser quantities in mitochondria, Golgi complexes, and nuclear membranes. Cholesterol intercalates among the phospholipids of the membrane, with its hydroxyl group at the aqueous interface and the remainder of the molecule within the leaflet. Its effect on the fluidity of membranes is discussed subsequently. 

The main occurrence of free sterols Is In the cytoplasmic membrane, where they Interact with other lipids and proteins. Two modes of action for sterols In membranes are proposed. One Is the so called bulk menfl>rane function, l.e., the Interaction with phospholipids and the spatial separation of these charged molecules [13]. The other role Is a cofactor function for the Incorporation of unsaturated fatty acids Into lipids [14]. 

The membrane is composed of lipid and phospholipid and contains proteins and sterols. The absolute requirement for unsaturated fats and sterols in membranes accounts for much of the oxygen requirement of brewing yeast strains (see Chapter 18). The nature of the unsaturated fats in the cell membrane affects its properties, e.g. in relation to ethanol tolerance [30]. 

Dahl J S, Dahl C E, Bloch K 1980 Sterols in membranes Growth characteristics and membrane properties of Mycoplasma capricolum cultured on cholesterol and lanosterol. Biochemistry 19 1467-1472... 

Amphotericin B, is a polyene antibiotic, used in the therapy of systemic fungal infections. Its mode of action exploits differences in membrane composition between the pathogen and the human host. Ergosterol, the predominant sterol of fungi, plants, and some protozoan parasites, interacts with Amphotericin B, resulting in an increased ion permeability of the membrane. Humans contain cholesterol, which has a low affinity for amphotericin B. 

Cholesterol is found in many biological membrane and is the main sterol of animal organisms. It is eqnimolar with phospholipids in membranes of liver cell, erythrocytes, and myelin, whereas in human stratum comeum it lies in the outermost layer of the epidermis... 

Sterols are compounds that have a steroid structure (Figure 12.5) and contain a hydroxyl group, which is capable of forming an ester. They are widespread in nature and commonly exist as mixtures of the free sterol and esters with fatty acids. Cholesterol (Figure 12.5) is by far the commonest sterol in animals, a high concentration being found in brain, nervous tissue and membranes. Plants... 

Lysolecithins act by dissolving cholesterol and cause massive losses of the sterol from membranes (19). Lysolecithins have been shown to cause the formation of openings 300-400 A in diameter in erythrocyte plasma membranes (20). Unlike saponins, lysolecithin membrane openings are permanent. 

MecHanism of Action A fungistatic antifungal that binds to sterols in the fungal cell membrane. Therapeutic Effect Increases fungal cell-membrane permeability, allowing loss of potassium and other cellular components. 

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