Inclusion of cholesterol

Crook K, Stevenson BJ, Dubouchet M, Porteous DJ. Inclusion of cholesterol in DOTAP transfection complexes increases the delivery of DNA to cells in vitro in the presence of serum. Gene Ther 1998 5(1) 137-143. 

Cholesterol has also been employed as a colipid due to its ability to modify bilayer fluidity. Inclusion of cholesterol results in the formation of more stable but less efficient in vitro complexes than those containing DOPE. In contrast, addition of cholesterol results in very active complexes for in vivo administration [123— 127]. 

Schreier et al. [85] examined the effects of liposome encapsulation on the pharmacokinetics in sheep of amikacin, a water-soluble aminoglycoside. The dmg was formulated in 200 nm liposomes and administered by means of intratracheal instillation. The liposome formulations were soy PC/phosphatidyl glycerol (PG) (7 3 molar ratio) and soy PC/PG/CH (4 3 3). They found that both liposome formulations reduced plasma Cmax and prolonged the plasma half-life of the amikacin compared with the dmg administered as a solution, once again indicating that liposomes were controlling dmg delivery in the lungs. The inclusion of cholesterol in liposomes more than tripled the plasma half-life for the dmg compared with the liposomes without cholesterol. Cholesterol reduces the fluidity and permeability of liposomes in their liquid crystalline phase. 

Vibrational spectroscopy shows that inclusion of cholesterol in phospholipid bilayers tends to decrease the fluidity of the hydrophobic region above the main transition point Tm and to increase it below Tm. The presence of cholesterol in DPPC or DMPC muti-layered vesicles does not affect the transition point but simply broadens the transition by decreasing the CH2-stretching wavenumber in the liquid crystalline phase and by increasing it in the gel-like phase (Lippert and Peticolas, 1971 Spiker and Levin, 1976 Casal and Mantsch, 1984). There is also evidence that lipid-cholesterol interaction increases the amount of bound water in the headgroups (Levin et al., 1985). 

The broad definition of lipids allows the inclusion of cholesterol, a molecule which contains many hydrocarbon groups and is not too soluble in water. Cholesterol contains a skeleton sterol ring (fig. 4.1), and is the precursor of other sterol-pontaining molecules, including the steroid hormones and bile acids (H-11). Cholesterol is ingested in the diet but may also be synthesized. All the carbons of cholesterol come from acetyl CoA. Acetyl CoA units... 

As known, cholesterol is one of the main structural elements of biomembranes. It is obtained that the DC content contributes in the inclusion of cholesterol in the lipid bilayer. It is seen from data, which is presented in the Fig. 3. However, the coefficient of the linear regression of the direct correlation between the cholesterol share in the total lipid composition and... 

The inclusion of cholesterol disturbs the crystalline structure of the gel phase, and the phospholipid chains are more mobile than in its absence. This prevents the crystallization of the hydrocarbon chains into the rigid crystalline gel phase. In the more fluid liquid crystalline phase, the rigid cholesterol molecules restrict the movement of the hydrocarbon chains. In consequence, the addition of cholesterol to lipid bilayers or lamellar mesophases gradually diminishes the gel-liquid crystal transition temperature and the enthalpy and broadens the DSC transition peak [72,73]. No transition can be detected by DSC at 50% cholesterol [73,74] (curve/of Fig. 7), which is the maximum concentration of cholesterol that can be incorporated before phase separation. However, laser Raman spectroscopic studies show that a noncooperative transition occurs over a very wide temperature range [75]. 

Because of the long equilibration times involved in mixtures of lipids and the problems that arise in pure lipids, to our knowledge to date only two simulations have been performed on lipid mixtures. These concern the important interactions between cholesterol and saturated lipids. Robinson et al. studied the effect of inclusion of cholesterol into a dimyristoylphosphatidyl choline bilayer. It was concluded that cholesterol caused an increase in the trans fraction of the lipid tail dihedrals adjacent to cholesterol and that the tail of the cholesterol lipids was flexible. Gabdoulline et al. also performed simulations on the same system, specifically looking at dipole potentials of the pure and mixed system. It was concluded in this study that the inclusion of cholesterol decreased the dipole potential of a bilayer, but it is not clear in this study that a liquid-crystalline state was reached. 

Sterols are steroid-based alcohols, the most commonly employed of which is cholesterol (Fig. 28.6). Cholesterol when used in equimolar proportions with a phospholipid is known to condense the packing of the phospholipids in bilayers above Tq thereby reducing their permeability to encapsulated drags. However, it has been shown that the inclusion of cholesterol in the liposome significantly decreases the solubilisation of hydrophobic drugs in the inlayers. 

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