Other Sterols

Cholesterol is an essential component of mammalian membranes and is a precursor for all steroid hormones. However, cholesterol can accumulate in certain tissues 

Sterols caimot be analyzed by ESI-MS without derivatization as they are not readily ionized [115]. Sandhoff et al. have used chemical sulfatation to achieve high-sensitivity detection of cholesterol [116], Cholesterol has also been deriva-tized with dimethylglycine, MDMABS [117], or ferrocenecarbamate [118], Notably, derivatization can be avoided by using APCI or APPI, which have been applied for the analysis of cholesterol and other sterols [119-122] or oxidized cholesterol [123]. 

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C. A trilerpenoid or irimethylsierol, first found in the non-saponifiable material of wool wax. Lanosterol (4,4,14ot-trimethyl-5a-choiesta-8,24-dien-3 -ol) is the precursor in animals and fungi of other sterols such as... 

Isoprenoids are intermediates and products of the biosynthetic pathway that starts with mevalonate and ends with cholesterol and other sterols. 

Sterols are minor constituents of most fats. Those of animal origin contain cholesterol and traces of other sterols, whereas plants contain phytosterols, of which p-sitosterol is the most common. Sterols occur in the free form or, after esterification to fatty acids through the 3-OH group, as steryl esters. The presence of sterols in archaeological residues can be a useful indicator of a plant or animal origin or an indicator of both if cholesterol and phytosterols are detected in the same sample. That noted, cholesterol is a potential contaminant of all archaeological samples subjected to handling. 

In many tissues cholesterol and other sterols exist as a mixture of the free alchohol and its long chain fatty acid ester (esterified at position 3 of the steroid nucleus). The determination of the cholesterol content of a sample may involve the measurement of either of these two fractions individually or the total cholesterol. It is possible to precipitate free cholesterol by adding an equal volume of digitonin (1 gl-1 in 95% ethanol), a naturally occurring glu-coside, to form a complex that is insoluble in most solvents, including water. 

The colour reaction of cholesterol and cholesterol esters with acetic anhydride and concentrated sulphuric acid provides the basis of the method attributed to Liebermann and Burchard. This reaction in not entirely specific for cholesterol or its esters because other sterols will also react. In its original form the reagent consists of acetic anhydride, concentrated sulphuric acid and glacial acetic acid and the intensity of the green colour is affected by the proportions of the reagents and the amount of water present. It is possible to achieve an increase in sensitivity if the reagent contains ferric ions. Various modifications of reagent composition have been used and some methods are fluorimetric. 

Hepatic CYP39A1 mRNA not induced by cholesterol or bile acids, unlike other sterol 7alpha-hydroxylases protein found in hepatic microsomal fraction (Li-Hawkins et al, 2000). 

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). 

Cholesterol (Appendix 3C) is the principal sterol in milk (>95% of total sterols) the level ( 0.3%, w/w, of total lipids) is low compared with many other foods. Most of the cholesterol is in the free form, with less than 10% as cholesteryl esters. Several other sterols, including steroid hormones, occur at trace levels. 

Cholesterol is the sterol characteristic of animal cells plants, fungi, and protists make other, closely related sterols instead. They use the same synthetic pathway as far as squalene 2,3-epoxide, at which point the pathways diverge slightly, yielding other sterols, such as stigmasterol in many plants and ergosterol in fungi (Fig. 21-37). 

In addition to the enzymes that are embedded in the membranes of the ER, conversion of lanosterol to cholesterol depends upon soluble cytoplasmic carrier proteins.174 See also Box 21-A. Other sterols formed in... 

The body contains sulfate esters of cholesterol and other sterols,245 sometimes in quite high concentrations relative to those of unesterified steroids. These esters are presumably soluble transport forms. They... 

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. 

Among various sterols, sitosterol was predominant in nutmeg. Other sterols reported... 

Three other GC analyses now used in authentication, largely for olive and other oils which should not be refined or solvent extracted, are the determination of waxes, aliphatic alcohols, triterpene alcohols (uvaol and erythrodiol), and stigmastadiene and other sterol-dehydration products (EEC, 1991). These analyses are used at present not to detect adulteration with other oils, but with solvent-extracted or refined oils. However, it is possible that, with solvent-extracted oils, wax, aliphatic alcohol and terpene alcohol compositions, could prove useful in differentiating or detecting different oils. 

It is claimed (Youk et al., 1999) that olive, sunflower and peanut oils contain mainly esterified sterols, while soyabean and sesame oils contain mainly free sterols. This does not appear to have been utilized previously, but could be useful with mixtures of the two classes. Although it is possible to de-sterolize oils, and remove characteristic sterols, this usually forms other sterols that can be detected (Biedermann and Grob, 1996 Lanuzza and Micali, 1997 Mariani and Venturini, 1997). If it is suspected that this has occurred, then the presence of other suspect components should be investigated. 

The energy of activation for sitosterol dehydration was calculated as 191.0 9.0 kJ/mol, but this is reduced to 70.8-80.6 kJ/mol in the presence of bleaching earths (Gordon and Firman, 2001). The rate of dehydration of other sterols is expected to be similar to that of sitosterol. 

Sterols are minor components of milk lipids, which make up just 0.3% of the total fat (Table 1.1). The principal component is cholesterol, which accounts for over 95% of the total about 10% of the cholesterol is present in the esterified form. Small amounts of other sterols, namely campesterol, stigmasterol and (3-sitosterol, have also been identified in milk fat (Mincione et al., 1977). 

The major sterol in both human and bovine milk is cholesterol. Trace amounts of other sterols are present also (e.g., lanosterol in bovine milk and desmosterol and some phytosterols in human milk). The amount of cholesterol present in human milk is 10—15 pg/100ml, and although supplemented in some commercial formulae, it is present in amounts 5-10 times less than in human milk. Since the role of dietary cholesterol is still not fully defined, an intake similar to that obtained through breast-feeding is generally recommended (Uauy, 1990 Suskind and Lewinter-Suskind, 1993 Tsang et al., 1993). 

The precursors of vitamins D2 and D3 are ergosterol and 7-dehydrocholesterol, respectively. These precursors or provitamins can be converted into the respective D vitamins by irradiation with ultraviolet light. In addition to the two major provitamins, there are several other sterols that can acquire vitamin D activity when irradiated. The provitamins can be converted to vitamin D in the human skin by exposure to sunlight. Because very few foods are good sources of vitamin D, humans have a greater likelihood of vitamin D deficiency than of any other vitamin deficiency. Enrichment of some foods with vitamin D has significantly helped to eradicate rickets, which is a vitamin D deficiency disease. Margarine and milk are the foods commonly used as carrier for added vitamin D. 

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