Membranes cholesterol containing

In milk fat, cholesterol is associated with Hpoproteins in the milk fat globule. It is also a component of animal membranes and controls rigidity and permeabihty of the membranes. Cholesterol has interesting surface properties and can occur in Hquid crystalline forms. Plants contain sterols such as P-sitosterol [83-46-5] (4b) or stigmasterol [83-48-7] (4c). Their functions in plant metaboHsm are not yet well understood. Analysis of sterols has proven useful for detection of adulteration of edible fats (9). 

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. 

Without an artificial sink, the membrane retentions are very high, with many basic probe molecules showing R > 80%. With the imposed sink, many of the retentions dropped by as much as 50%. Furthermore, just 0.5% wt/vol cholesterol in dodecane (in addition to the sink) caused increased retention to drop by at least a further 10-30%. It was not possible to form stable cholesterol-containing lipid models under sink conditions with Avanti s egg lecithin acceptor buffer solutions turned significantly turbid in the untenable model 13.1. 

Bolard J, et al. One-sided action of amphotericin B on cholesterol-containing membranes is determined by its self-association in the medium. Biochemistry 1991 30 5707. 

The fluidity of membranes primarily depends on their lipid composition and on temperature. At a specific transition temperature, membranes pass from a semicrystalline state to a more fluid state. The double bonds in the alkyl chains of unsaturated acyl residues in the membrane lipids disturb the semicrystalline state. The higher the proportion of unsaturated lipids present, therefore, the lower the transition temperature. The cholesterol content also influences membrane fluidity. While cholesterol increases the fluidity of semicrystalline, closely-packed membranes, it stabilizes fluid membranes that contain a high proportion of unsaturated lipids. 

Cholesterol is found almost exclusively in eukaryotic cells. Animal membranes contain substantially more cholesterol than plant membranes, in which cholesterol is usually replaced by other sterols. There is no cholesterol at all in prokaryotes (with a few exceptions). The inner mitochondrial membrane of eukaryotes is also low in cholesterol, while it is the only membrane that contains large amounts of cardiolipin. These facts both support the endosymbiotic theory of the development of mitochondria (see p. 210). 

In many cases, these polymer chains take on a rod-like (calamitic LCPs) or even disc-like (discotic LCPs) conformation, but this does not affect the overall structural classification scheme. There are many organic compounds, though not polymeric in nature, that exhibit liquid crystallinity and play important roles in biological processes. For example, arteriosclerosis is possibly caused by the formation of a cholesterol containing liquid crystal in the arteries of the heart. Similarly, cell wall membranes are generally considered to have liquid crystalline properties. As interesting as these examples of liquid crystallinity in small, organic compounds are, we must limit the current discussion to polymers only. 

TE of the DOTAP/DOPC/Chol-DNA complexes strongly deviates from the universal bell-shaped curve observed for binary systems. The TE of cholesterol-containing complexes increases more rapidly with increasing cholesterol content than the increase in membrane charge density predicts for 0 < 4>chol < 0.4. No further TE increase is seen for 4>chol > 0.4 (where the membrane is saturated with cholesterol [Pg.201]

The lipid bilayer is such that the polar heads (often phosphatidylcholine or phosphati-dylethanolamine) of the phospholipids are juxtaposed on the external and internal surfaces of the membrane, causing the ends of the hydrophobic (i.e., long-chained alkyl) portions of the phospholipids to extend inside the membrane. Also contained within the lipid bilayer are cholesterol and other sterols. 

The sterol cholesterol (Fig. 2b) is a major constituent of animal plasma membranes but is absent from prokaryotes. The fused ring system of cholesterol means that it is more rigid than other membrane lipids. As well as being an important component of membranes, cholesterol is the metabolic precursor of the steroid hormones (see Topic K5). Plants contain little cholesterol but have instead a number of other sterols, mainly stigmasterol and P-sitosterol which differ from cholesterol only in their aliphatic side chains. 

The detailed structure of membranes is not uniform in terran life (Figure 2.12). Membrane bilayers in bacteria arise from supramolecular organization of phospholipids, with hydrophobic fatty acids attached to a variety of polar head groups by ester linkages. Eukaryotic membranes also contain phospholipids, but they are distinctive in the inclusion of sterols (such as cholesterol), another class of hydrophobic molecules. 

Because cholesterol contains an -OH group, it is amphipathic. It controls membrane fluidity in mammals by inhibiting the ordering of fatty acid side chains, but it is absent from bacterial plasma membranes. 

A few proteins exist that sequester PIP2 in a cholesterol-dependent manner. One of these proteins is the N-terminal myristoylated peptide of NAP-22 (33, 34). Combined confocal microscopy and AFM show that this peptide forms new cholesterol-rich domains within the liquid-disordered domain to which it attracts PIP2 (31). In addition, a peptide segment of caveolin promotes the formation of membrane domains containing both cholesterol and PIP2 (35). 

The released unesterified cholesterol can then be usedfor membrane biosynthesis. Alternatively, it can be reesterified for storage inside the cell. In fact, free cholesterol activates acyl CoA cholesterol acyltransferase (ACAT), the enzyme catalyzing this reaction. Reesterified cholesterol contains mainly oleate and palmitoleate, which are monounsaturated fatty acids, in contrast with the cholesterol esters in LDL, which are rich in linoleate, a polyunsaturated fatty acid (see Table 24.1). It is imperative that the cholesterol be reesterified. High concentrations of unesterified cholesterol disrupt the integrity of cell membranes. 

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