From Other 5 sterols

Ergosterol is the predominant sterol in fungi. It is easy to determine ergosterol because it can be separated from other sterols endogenous to the solid substrate by high-performance liquid chromatography and can be quantified simply by spectrophotometry. 

Vitamin Dg is known to undergo thermal cyclisation at temperatures above 200° yielding pyrocalciferol and f opyrocalciferol. Quantitative separation of vitamin Dg from other sterols by gas chromatography has been achieved (using 0-75 per cent w opentyl glycol succinate on 100 to 140-mesh Chromosorb P with 6 feet by 4 mm columns at 220° and /3-ionisation detection). Two peaks are recorded but their areas are relatively constant... 

According to the type of block Y, the correlation can be used for different problems. The correlation between different chemical variables is used to investigate whether some quantities can be evaluated from others, so that the measurement of the first quantities can be avoided (major and minor chemical components, traces), or whether the relationships between the groups of chemical variables can be reduced (e.g., acid and sterolic fraction). 

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 above shows that rapeseed oil can easily be detected, or eliminated, as a contaminant by sterol analysis. It is also, at least in Europe, the oil most likely to be used to dilute another oil. Although low levels (as a percentage of the total sterols) have been reported in some other oils (Desbordes etal., 1993), the presence of brassicasterol in an oil is good evidence of contamination in any oil from a non-Brassica species. It is likely that the traces reported as present in some other oils arise from contamination of the sample with rapeseed oil, or from some other Brassica species, or from traces of some similarly behaving non-sterol not fully separated from the sterol fraction during the work-up of the sample (Desbordes et al., 1983). 

Even though cholesterol (substituted by other sterols in plants) is chemically classified as a lipid, the molecule is better grouped it into a special category, which is distinct from membrane proteins and (phospho- and sphingo-) lipids, when discussing its role in biomembrane stmcture and function. Cholesterol constitutes between 25% and 40% of the total lipid plus cholesterol fraction of most typical cell membranes. These numbers translate into a 33-66% fraction of the noncholesterol lipids, or considering its smaller size, about a 10-20%... 

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... 

Provitamin D2. Ergosterol is isolated exclusively from plant sources. The commercial product is ca 90—100% pure and often contains up to 5 wt % of 5,6-dihydroergosterol. Usually, the isolation of provitamin D2 from natural sources iavolves the isolation of the total sterol content, followed by the separation of the provitamin from the other sterols. The isolation of the sterol fraction iavolves extraction of the total fat component, its saponification, and then reextraction of the unsaponifiable portion with an ether. The sterols are ia the unsaponiftable portion. Another method is the saponification of the total material, followed by isolation of the nonsap onifiable fraction. Separation of the sterols from the unsap onifiable fraction is done by crystallization from a suitable solvent, eg, acetone or alcohol. Ethylene dichloride, alone or mixed with methanol, has been used commercially for recrystallization. In the case of yeasts, it is particularly difficult to remove the ergosterol by simple extraction, thereby obtainiag only ca 25% recovery. Industrially, therefore, the ergosterol is obtaiaed by preliminary digestion with hot alkaUes or with amiaes (28—33). Variations of the isolation procedure have been developed. Eor example, after saponification, the fatty acids may be precipitated as calcium salts, which tend to absorb the sterols. The latter are then recovered from the dried precipitate by solvent extraction. 

Other polar lipids such as sphingolipids and amphiphiles could be introduced in liposomal bilayers. Cholesterol or other sterols isolated from natural sources (i.e. plants P-sitosteroJ), as well as several lipid conjugated polymers, may participate in the liposome bilayers. 

Large amounts of C28 and C29 sterols occur in asteroids and other echinoderms but it seems that they cannot be synthesized from C27 sterols by transmethylation as is the case in plants (but not in other animals). This was confirmed with the starfish Laiaster leechii, which could convert MVA into A -cholesterol but could not alkylate this at C-24 or introduce a A -bond. Similarly the starfish Asterias rubens can only synthesize C27 sterols de novo, but here there is evidencethat a A -bond can be introduced into both cholesterol and A -cholesterol. This starfish can also synthesize A -sterols and there is some indication that cholesterol itself may be synthesized even though it was previously concluded that the family could not perform the final steps in this biosynthesis. This species also rapidly metabolized dietary steroids and such A -sterols could be converted into A -products, but it was considered unlikely that the organism had the ability to dealkylate C28 and C29 sterols. 

Cholesterol, on reaction with dibenzoyl peroxide followed by trichloroacetic acid, gave a blue colour attributed to cations with conjugated unsaturated structures. Cholesta-2,4,6-triene and 3-(cholest-5-en-3/8-yl)cholesta-2,4,6-triene were isolated from the product mixture. Some other sterols gave diverse colours.The coloured solutions formed from cholesterol or oestrone with antimony trichloride give e.s.r. spectra suggesting the presence of radical-cations. Cholesta-3,5-diene and 3,3 -bicholesta-2,4-diene were isolated from the reaction of cholesterol, and the hexaene (131) from oestrone. Dimeric and trimeric products have been... 

Dietary lipids are composed mainly of triglycerides with only small amounts of phospholipids, cholesterol, and other sterols. Chemically, triglycerides are the triacylglycerols or a glycerol molecule esterified with three FAs. Saturated FAs have no double bonds in their carbon chain examples include palmitic (16 0) and stearic acid (18 0). Unsaturated FAs have one (monoun-saturated) or more (polyunsaturated) double bonds in the carbon chain. Depending on the position of the first double bond from the methyl end, these FAs are divided into n-9, n-6, or n-3 (also called co-9, co-6, or co-3) series, with the first double bonds being between carbon 9 and 10, carbon 6 and 7, and carbon 3 and 4, respectively. Common sources of different dietary FAs are shown in Table I. Animal fats are a rich source of saturated... 

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