Cholesterol, and membranes

At the physiological level it is well established that vital dyes such as nile blue, neutral red and methylene blue retard larval development under normal lighting conditions (12L/12D with source unspecified) (25 27). Female but not male pupal weights are also reduced. Unfortunately experiments were conducted without dark controls so that it is difficult to evaluate the role of photosensitization in these effects. As house flies and fire ants succumb to photosensitization, they lose motor control and become more excitable (28). This suggested a neurotoxic effect and investigation of fire ant acetylcholinesterase vitro revealed that this enzyme was sensitive to photo-oxidation. vivo results, however, revealed no effect on the enzyme which suggests another mode of action. Epoxldatlon of cholesterol and membrane lysis may be alternative primary sites. If this were the case ecdysone metabolism of insects would probably also be effected. 

Cholesterol and membrane proteins, including structural ones such as glycophorin and myelin basic protein and functional ones such as -ATPase, bacteriorhodopsin, and cytochrome c, are important components of biological membranes. Cholesterol-lipid and a number of protein-lipid interactions have therefore been extensively investigated by vibrational spectroscopy. Interactions of hormones and toxins with phospholipid bilayers were also investigated. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

Cholesterol is a principal component of animal cell plasma membranes, and much smaller amounts of cholesterol are found in the membranes of intracellular organelles. The relatively rigid fused ring system of cholesterol and the weakly polar alcohol group at the C-3 position have important consequences for the properties of plasma membranes. Cholesterol is also a component of lipoprotein complexes in the blood, and it is one of the constituents oiplaques that form on arterial walls in atherosclerosis. 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

Lipid rafts are specific subdomains of the plasma membrane that are enriched in cholesterol and sphin-golipids many signaling molecules are apparently concentrated in these subdomains. 

Jessup W, Gelissen IC, Gaus K, Kritharides L (2006) Roles of ATP binding cassette transporters Al and Gl, scavenger receptor BI and membrane lipid domains in cholesterol export from macrophages. Curr Opin Lipidol 17(3) 247-57... 

The polyene macrolide filipin was isolated in 1955 from the cell culture filtrates of Sterptomyces filipinensis, and was later shown to be a mixture of four components [36]. Although too toxic for therapeutic use, the filipin complex has found widespread use as a histochemical stain for cholesterol and has even been used to quantitate cholesterol in cell membranes [37]. The flat structure of filipin III, the major component of the filipin complex, was assigned from a series of degradation studies [38]. Rychnovsky completed the structure determination by elucidating the relative and absolute stereochemistry [39]. The total synthesis plan for filipin III relied heavily on the cyanohydrin acetonide methodology discussed above. 

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

Compound lipids (phospholipids, sphingolipids, glycolipids, and cholesterol and its esters) that make part of the biomembrane are subject to a less active renew-al as compared with triacylglycerides. Their renewal is associated either with the restoration of an impaired portion of the membrane, or with the replacement of a defective molecule by a new one. 

Because membranes components participate in nearly every cell activity their structures are also dynamic and far from the equilibrium states that are most readily understood in biophysical terms. Newly synthesized bilayer lipids are initially associated with endoplasmic reticulum (Ch.3) whereas phospholipids initially insert into the cytoplasmic leaflet while cholesterol and sphingolipids insert into the luminal endoplasmic reticulum (ER) leaflet. Glycosylation of ceramides occurs as they transit the Golgi compartments, forming cerebrosides and gangliosides in the luminal leaflet. Thus, unlike model systems, the leaflets of ER membranes are asymmetric by virtue of their mode of biosynthesis. 

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