Lipid-Cholesterol Complexes

Cholesterols are one of the major components constituting a biological membrane. The insertion of cholesterols into a lipid membrane dramatically changes the molecular fluidity and mechanical strength of the membrane. However, the molecular-scale mechanisms for such influence of cholesterols have not been well understood. This is partly because of the lack of a method to visualize the molecular-scale structure of the lipid-cholesterol complexes formed in the membrane. 

The distance between the adjacent molecular rows is 0.71 nm. This is much longer than the molecular spacing (0.46 nm). The 

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Figure 18.10 (a) FM-AFM image of a DPPC-cholesterol (50-50 mol%) mixed bilayer formed on mica in PBS solution, (b) The molecular-scale model of the lipid-cholesterol complex proposed based on the FM-AFM image shown in (a). ... 

On the basis of these discussions, the molecular-scale model for the lipid-cholesterol complex is proposed, as shown in Fig. 18.10b. Before that time, several models for the lipid-cholesterol complexes were proposed on the basis of the results obtained by spectroscopic methods and molecular dynamics simulations. However, it had been difficult to unambiguously determin the molecular-scale structure. The result obtained in this experiment showed that FM-AFM can be a powerful means to determine the molecular-scale structure of biomolecular complexes. 

In addition to the structure of the molecular surfaces, the distribution of water " and ions interacting with the surface is also visualized by FM-AFM. Finally, the molecular-scale arrangement of the unknown biomolecular complex (i.e., the lipid-cholesterol complex) was determined by FM-AFM imaging. 

Unesterified fatty acids are carried in plasma by albumin (chapter 18). The plasma also transports more complex lipids (cholesterol, triacylglycerols) among the various tissues as components of lipoproteins (spherical particles composed of lipids and proteins). Because cholesterol and triacylglyc-erol are insoluble in an aqueous medium such as the plasma, these lipoproteins (which are soluble in plasma) have evolved for the purpose of transporting complex lipids among tissues. In this section we are concerned with the structure and metabolism of these lipoproteins. 

Free fatty acids, derived primarily from adipocyte triglycerides, are transported as a physical complex with plasma albumin. Triglycerides and cholesteryl esters are transported in the core of plasma lipoproteins [134], Deliconstantinos observed the physical state of the Na+/K+-ATPase lipid microenvironment as it changed from a liquid-crystalline form to a gel phase [135], The studies concerning the albumin-cholesterol complex, its behavior, and its role in the structure of biomembranes provided important new clues as to the role of this fascinating molecule in normal and pathological states [135]. 

Vibrational spectroscopy also shows interactions of polyene antibiotic ion channels nystatin and amphotericin B with phospholipid bilayers (Bunow and Lewin, 1977a Iqbal and Weidekamm, 1979 Van de Ven et al., 1984). In particular, Fourier Transform Raman spectroscopy demonstrates that at high temperature, the amphotericin A complex of DPPC/cholesterol is more ordered, whereas the amphotericin B complex is as ordered as the pure lipid/cholesterol system. In the low temperature phase and in the presence of the sterol-antibiotic complex, the bilayers were suggested to be in the interdigitated state (Levin and Neil Lewis, 1990). 

AndreollTE. On the anatomy of amphotericin B-cholesterol pores in lipid bilayer membranes. Kidney Int 1973 4 337-45. DeKruijiff B, Demel RA. Polyene antibiotic-sterol interactions in membranes of Acholeplesma laidlawii cellsand lecithin liposomes. III. Molecular structure of the polyene antibiotic-cholesterol complexes. Biochem Biophys Acta 1974 339 57-70. HoIzRW.Theeffectsofthe polyene antibiotics nystatin and amphotericin Bon thin lipid membranes. Ann N Y Acad Sell 974 235 469-79. 

If the early endosomal release is not possible, another way to keep DNA intact in lysosome is to protect the DNA from lysosomal degradation. Cationic liposomes formulated with cholesterol believed to offer a useful role in keeping DNA intact (121,156,157). Straub-inger et al. (158) have demonstrated that the lysosomal enzymes work at lower pH, i.e., pH < 6. It was also shown that cholesterol-containing liposomes, which possess greater stability and lower ion-permeability compared with DOPE-containing liposomes, provide an improved stability to the lipid-DNA complex in the cytosol (158-160). It is easily conceivable that if the endosomal content passes onto lysosome before being released from endosomes, the lipid/DNA complex could remain secured in the lysosome. 

Cationic lipids can destabilize a cellular membrane because of its intrinsic detergent property. Therefore, destabilization of endosomal and/or lysosomal membrane may be a contribution from the cationic lipids itself In the same context, it was shown that the cationic lipid/DOPE or cationic lipid/cholesterol liposome formulation exhibit surface anisotropies in terms of increased liposomal surface pH (161,162). The surface pH of the liposomal formulations exhibits at least two pH units higher than the pH of the solution at which they are made. Therefore, a liposomal solution made at physiological pH may in reality exhibit a surface pH > 9, which is detrimental for both the stability of endosome and activity of lysosomal enzymes. Endosomal disruptions were also done with fusogenic peptides, which promote pH-dependent fusion of small liposomes when associated with lipid bilayer. When these peptides were co-delivered with lipid/DNA complex, they imparted formidable endosomal disruption by changing its usual random coil conformation into amphipathic a-helix conformation at lower pH, resulting in consequent cytoplasmic delivery of DNA (163). 

Receptor-mediated endocytosis plays a key role in cholesterol metabolism (p. 745). Some cholesterol in the blood is in the form of a lipid—protein complex called low-density lipoprotein (LDL). Low-density lipoprotein... 

CETP is a plasma protein of unknown origin that transfers CE from one lipoprotein or artificially prepared bilayer to another (Fig. 2A). In addition to CE, the protein may also transfer other lipids such as TG or PC. It has been referred to as lipid transfer complex (ETC) [68,69], esterified cholesterol transfer/exchange protein (ECTEP) [70], CE transfer protein (CETP) [71], and lipid transfer fraction (LTP-1) [72]. The original observations regarding plasma CE transfer activity were made nearly 20 years ago [73], but even today the literature is confusing and incomplete and no generally accepted recent review is available. 

A variety of lipid-protein complexes are used in the body to transport relatively water-insoluble lipids, such as triglycerides and cholesterol, in circulating blood. These complexes are commonly called lipoproteins they contain both proteins and lipids in varying concentrations. The density of these lipoproteins depends on the relative amounts of protein, because lipids are less dense than protein. Low density lipoproteins, or LDLs, have a relatively higher ratio of lipid to protein. LDLs are used to transport cholesterol and triglycerides from the liver to the tissues. In contrast, high density... 

Lipids are nonpolar molecules and are relatively insoluble in aqueous solutions. At low concentrations, cholesterol and cholesterol esters, as well as other lipids, may form microscopic droplets called chylomicrons (lipid-protein complexes) that are somewhat stable in solution. At high concentrations, the lipids would form larger droplets and clog blood vessels, so they must be transported as complexes of lipid and protein called lipoproteins. Lipoproteins are complexes of lipid and precursor protein molecules called apolipoproteins. 

Following their liberation from the food matrix, the compounds are dissolved into gastrointestinal fluids, complex media containing, e.g., bile salts, ions, lipids, cholesterol, as well as enzymes derived from secretions, shed enterocytes, and/or intestinal flora. A range of endogenous factors related to health status and dietary status determine the composition of this medium in which the food compound is presented to the mucosal surface [6]. 

The release of active compounds from liposomes is directly related to liposomal bilayer stability. Therefore, a controlled-release profile from CyD-containing liposomes is difficult to obtain because of interaction between inclusion complexes and the hpidic vesicle (cholesterol and phospholipids) [103]. Indeed, lipid/CyD complex formation can occur within the hposomes, thus destabilizing the liposomal bilayer [85]. Liposome destabilization is also closely related to the stability of the encapsulated complex the higher the affinity between drug and CyD, the slower the drug release [84]. 

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