Esters of cholesterol

The organism would probably grow quite well. The structure shown is simply analogous to the acetyl ester of cholesterol except that the side chain has been removed. The organism would, presumeably, degrade the sterol nucleus by its normal route. 

This coating, which accounts for the high shine Chemically it consists of esters of cholesterol... 

The larger lipoproteins are spherical micelles containing a core of triacylglycerols and esters of cholesterol surrounded by a 2- to 3-nm-thick layer... 

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 best known low molar mass substances forming cholesteric phases are esters of cholesterol, which are responsible for the name cholesteric phase. Therefore it was straightforward to attach these derivatives to a polymer in order to obtain cholesteric polymers. These experiments, however, failed because in all cases not cholesteric but isotropic, smectic or crystalline polymers resulted 22) (refer also to Shibaev and Plate, this issue). 

The various esters of cholesterol, retinoyl esters, esters of vitamin D and E, probably depend solely on the octi vity of carboxy tester lipase for their hydrolysis. 

Remington suggested that I examine some work done at Central Research Department on liquid crystals. Two scientists, Walter Mahler and M. Panar, had discovered that certain half-acid esters of cholesterol gave rise to solid compositions, which possessed the iridescent qualities of conventional cholesteric esters... 

GC-MS has been responsible for the identification of a variety of unusual lipids associated with diseased conditions. Hydroxyocta-decadienoic esters of cholesterol for example, have been isolated from aortal atheroma placques [277] and branched chain and odd numbered fatty acids identified in the glycerolipids of brain, spinal cord and sciatic nerve [278] from a patient with methylmalonic aciduria. The latter compounds are thought to arise by the replacement of malonyl CoA with methyl malonyl CoA, and acetyl CoA with propionyl CoA at certain stages of fatty acid synthesis. In these and other examples, the lipids need to be hydrolysed to permit the identification of the constituent fatty acids. As the class of lipids is usually known from the separation procedure used, the nature of the fatty acids may allow the characterisation of the complete molecule. However, volatilisation of the intact lipid into the mass spectrometer when possible would be preferable, particularly when it is present in a mixture and separation of the components is first made by GC. 

A second category of liquid crystals is the type produced when certain substances, notably the esters of cholesterol, are heated. These systems are referred to as thermotropic liquid crystals and, although not formed by surfactants, their properties will be described here for purposes of comparison. The formation of a cloudy liquid when cholesteryl benzoate is heated to temperatures between 145 and 179°C was first noted in 1888 by the Austrian botanist Reinitzer. The name liquid crystal was applied to this cloudy intermediate phase because of the presence of areas with crystal-like molecular stmcture within this solution. 

The ability of insect sulfotransferases to catalyze the sulfation of plant and insect steroids may simply reflect the broad substrate specificity of the enzymes. On the other hand, it may be indicative of a more important physiological function of the enzymes. Sulfate esters of cholesterol, campesterol and e-sitosterol have been identified in the meconium of tobacco hornworm (M. sexta) pupae ( ), and these steroids are known precursors of a-and e-ecdysone and other molting hormones in this species ( ). Further, there is evidence that houseflies (M. domestica) and diapausing pupae of M. sexta convert 22,25 bisdeoxyecdysone, a-ecdysone and 20-hydroxyecdysone into sulfate and glucoside conjugates ( ). 

This class of lipid conjugates Is the most nonpolar yet Identified, a characteristic which Is often useful In pursuing the Identification of unknown metabolites. The first cholesterol ester of a xenobiotic was reported In 1976 for a saturated methoprene metabolite which contributed 15% of the total C-resldue In the liver of a chicken given a single oral dose of methoprene at 64 mg/kg (17). The tai-cyc lop ropy 1 fatty acids derived from cycloprate also form esters of cholesterol. Three such esters contributed 5% of the total residual radiocarbon In rat carcasses four days after a single oral dose of cycloprate at 21 mg/kg ( ). 

Small Molecule Adducts. Inhalational phosgene exposure has been reported to react with other components in the lungs. In 1933, treatment of lung pulp with phosgene was found to form the chlorocarbonic ester of cholesterol (Kling, 1933). Kling also indicated that there was a destruction of the hydrophilic character of the fats in the cells due the loss of free sterols and suggested this was the cause of acute pulmonary edema. 

The various esters of cholesterol, retinoyl esters, esters of vitamin D and E, probably depend solely on the activity of carboxylester lipase for their hydrolysis. They are, before their uptake by the enterocytes, transformed into the corresponding alcohols to form mixed micelles with bile salts. Carboxylester lipase catalyzes not only the cleavage of esters but also their formation. The most studied example is the reversible esterification of cholesterol, which is favored by low bile salt concentrations and pH [42]. Additionally, the laige specificity of carboxylester lipase probably functions as a first-line detoxification mechanism for a broad variety of orally ingested xenobiotics. 

Subbaram, M. R., Separation of saturated and unsaturated fatty acid esters of cholesterol by gas-liquid chromatography. J. Chromatog. 16, 79-80 (1964). 

The distribution of UC and CE in 22 rat tissues is shown in Table 1. It will be seen that CE comprise only about one-eighth of the total body cholesterol, and that long-chain fatty acid esters of cholesterol predominate over CS. It is clear also that long-chain fatty acid esters of cholesterol are distributed very unevenly among different tissues. They are essentially absent from erythrocytes and are present in only extremely low amounts in tissues of the nervous system. On the other hand, they are present in relatively high amounts in the adrenal, ovaries and blood plasma, and the liver, too, can be relatively rich in CE. 

Table 2 shows that UC and long-chain fatty acid esters of cholesterol are also distributed unevenly among the membranes and organelles of individual cells. (No information is available about the intracellular distribution of CS or cholesterol... 

Studies of the physical properties of UC, reviewed in Chapter 6 of this volume, have contributed much to our understanding of the role of this Upid in membranes and lipoprotein surfaces. The shape and polarity of UC promote its association with the phosphoUpids of membranes and Upoproteins, and this association has important effects on membrane fluidity and permeability. The physical properties of long-chain fatty acid esters of cholesterol, on the other hand, differ strikingly from those of UC, and cause these esters to be largely excluded from phospholipid bilayers and monolayers and to aggregate instead in oil droplets. 

Long-chain fatty acid esters of cholesterol are much bulkier than is UC, and lack a polar hydroxyl group. They do not become hydrated in the presence of water, and are essentiaUy water insoluble. When mixed with water they form a floating oil phase, unlike UC, which forms a film [11], They mix with phosphoUpids only in very... 

Nearly all animal cells contain an enzyme that can catalyze the formation of long-chain fatty acid esters of cholesterol. This enzyme, ACAT, is associated with the endoplasmic reticulum [15,18] and appears to be an integral membrane protein [16,19]. It catalyzes the following reaction ... 

Within the body there are 3 and possibly 4 separate enzymes that can hydrolyze long-chain fatty acid esters of cholesterol. These CEH catalyze the reversible reaction shown below ... 

Esters of cholesterol cannot be directly attacked by the enzyme [60,61]. Rat liver microsomes catalyse 7a-hydroxylation of cholestanol at a rate comparable to that of cholesterol [62]. Since 7a-hydroxylation of cholestanol is stimulated after treatment with cholestyramine, it appears likely that the same enzyme is involved as in hydroxylation of cholesterol. Shght changes in the side chain lead to marked loss of activity [63,64]. Loss of a terminal methyl group reduces the rate to about 50% and addition of an ethyl group at C-24 (sitosterol) leads to almost complete loss of the activity. From a study with a great number of structurally closely related steroids, Aringer concluded that cholesterol 7a-hydroxylase requires a rather flat steroid (A", A or 5a) and an equatorial or quasi-equatorial hydroxyl group at C-3 [65]. 

The liquid-crystalline mesophases of cholesteryl benzoate and p-phenylbenzoate provide useful stationary phases for the gas chromatography of steroids, giving separations largely on the basis of molecular shape. These steroidal materials have distinct advantages over the more conventional stationary phases for certain applications, including separations of the isomeric androstane-3,17-diols or pregnane-3,20-diols, and of cholest-4-ene from the 5-ene. The dicho-lesteryl esters of some dicarboxylic acids are reported to exhibit liquid-crystalline properties. The temperature-dependence of the Kerr effect has been measured for a series of liquid-crystalline esters of cholesterol. 

Cholesterol Esters Fatty acid esters of cholesterol which constitute about two-thirds of the cholesterol in the plasma. The accumulation of cholesterol esters in the arterial intima is a characteristic feature of atherosclerosis, [nih]... 

In 1888, Austrian botanist Friedrich Reinitzer (1857-1927) was synthesizing esters of cholesterol when he discovered a very curious phenomenon. Investigating the melting point of cholesterol benzoate, he discovered that at 293°F (145°C) it did not really melt but turned milky ... 

An optical method which is thought to reveal the presence of cholesterol esters and perhaps hormone esters in lipid droplets involves the study of frozen sections of formalin-fixed tissue under the polarizing microscope. Birefringent crystals occur in the lipid droplets. Earlier (p. 175), it was pointed out that Aschoff (1909) and others demonstrated that fatty acid esters of cholesterol are birefringent. Abundance of crystals in the lipid droplets of the various steroid-producing organs was reported by many early histologists. 

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