Autoxidation of cholesterol

It has been suggested that vitamin D3 (378) is metabolized into a more polar substance before stimulating calcium transport to the intestine. The principal metabolite from the blood, produced by the liver, has been found to be 25-hydroxycholecalciferol (379), whereas the trihydroxy-derivative (380) is the principal metabolite from the intestine. Autoxidation of cholesterol via hydroperoxide intermediates afforded a variety of hydroxylated cholesterol derivatives and products of side-chain degradation. ... 

Figure 9. The products of free radical autoxidation of cholesterol (1), 25-hydroxycholesterol ...

The rates of autoxidation of cholesterol have been determined in aqueous dispersions and in monomolecular films. The main products, 7-oxo- and 7-hydroxy-cholesterols, are formed rapidly at 85 °C in an aerated dispersion stabilized by sodium stearate (over 60 % oxidized in 8 h) or when a surface film is exposed to air at room temperature. No oxidation was observed, however, when a dispersion was aerated at 25 Cholesterol 26-hydroperoxide has... 

Oxysterols are oxidized forms of cholesterol they are formed enzymatically in the first steps of cholesterol metabolism, and they may also be formed as a result of autoxidation of cholesterol [50]. The high abundance of cholesterol in biological systems means that oxysterols can easily be formed nonenzymatically during sample handling and workup unless care is taken. Oxysterols are biologically active molecules they have been shown in vitro to activate nuclear receptors, for example, liver X receptors (LXRs) [51], to interact with INSIG protein and thereby to repress cholesterol synthesis [52], and, like bile acids, to interact with G-protein coupled receptors. Oxysterols also represent transport forms of cholesterol [53] (Table 2.3). 

Bascoul, N., Domergue, M., Olle, M. et al (1986) Autoxidation of cholesterol in tallows heated under deep frying conditions evolution of oxysterols by GLC and TLC-FID. Lipids, 21, 383-7. 

Autoxidation of cholesterol, which is accelerated manyfold by 18 2 and 18 3 fatty acid... 

About 4% of the egg lipids consists of sterols. The main corrqtonent is cholesterol (96%), ca. 15% of which is esterified with fatty acids. The cholesterol content is 2.5%, based on the egg yolk solids. Disregarding mammalian brain, this level exceeds hy far that in all other foods (cf. 3.8.2.2.1) and, therefore, serves as an indicator of the addition of eggs. Cholestanol, 7-cholestenol, campesterol, P-sitosterol, 24-methylene cholesterol and lanosterol are other components of the sterol fraction. The quality of egg products is endangered hy autoxidation of cholesterol (cf. 3.8.2.2.1). 

Purification of cholesterol through the dibromide completely eliminates cholestanol, 7-dehydrocholesterol, and latho-sterol (A7-cholestenol). The first crop of material from methanol-ether is also free from cerebrosterol (24-hydroxycholesterol) and 25-hydroxycholesterol, a product of autoxidation present in cholesterol that has been stored in the crystalline state for a few years with access to air. When material of highest purity is desired, only first-crop dibromide should be employed, since de-bromination of second-crop material gives sterol melting at 146-147° and giving a positive Beilstein test. 

Figure 7 Oxidation of cholesterol regiochemistry of the oxidation with singlet oxygen and of autoxidation (50).

Oxysterols are defined as oxygenated derivatives of cholest-5-en-3(3-ol (cholesterol) (Figure 18.1) or precursors of CHOL that may be formed directly by autoxidation or by the action of a specific monooxygenase, or that may be secondary to enzymatic or nonenzymatic lipid peroxidation (Guardiola et al., 1996 Schroepfer, 2000 Bjorkhem and Diczfalusy, 2002). These OS may be formed in the human body by endogenous free-radical attack on CHOL or by enzymatic processes, mainly in the biosynthesis of bile acids and steroid hormones. In addition, OS may be formed exogenously by autoxidation of CHOL in foods. The nomenclature and abbreviations of OS are presented in Table 18.1. It should be emphasized at this point that the OS that occur in... 

Rose-Sallin, C., Huggett, A.C., Bosset, J.O., Tabacchi, R., Fay, L.B. 1995. Quantitation of cholesterol oxidation products in milk powder using [2Hy] cholesterol to monitor cholesterol autoxidation. J. Agric. Food Chem. 43, 935-941. 

A review of the properties and uses of cholesterol includes references to such diverse information as complex formation, inclusion in cosmetics, liquid crystals, solubilities, and ice nucleation 265 Carcinogenicity associated with 5,6a-epoxy-5a-cholestan-3/3-ol ( cholesterol a-oxide ) may account for earlier indications that cholesterol samples can contain carcinogens hydroperoxides, including the 7-hydroperoxycholesterols which result from autoxidation, have been shown to epox-idize cholesterol in low yields.266... 

ABSTRACT This article describes physiological properties of two classes of natural products, namely cholesterol oxides and dehydroepiandrosterone (DHEA). The cholesterol oxides, also called oxysterols, are autoxidation products of cholesterol. Dehydroepiandrosterone and its sulfate DHEAS are formed mainly in the adrenals. The biological activities of these compounds are utilized to construct the two following hypotheses. 

Even less well understood than MA is the possible occurrence of cholesterol oxidation products in foods and their health significance. It seems clear that some of the products of cholesterol autoxidation are atherogenic. Much more research will be required to establish or refute their proposed carcinogenic properties. Methods for the determination of cholesterol oxidation products in foods and studies to establish levels of occurrence, if any, are most urgently needed. Further studies on antioxidants and procedures for the inhibition of oxidation are also needed. It is difficult to overstate the potential importance to the animal products industry of studies on quantification of cholesterol oxides in food products. Concern about this area of research is becoming widespread (47,48). 

Figure 2.19. Products of cholesterol autoxidation ( O ) and singlet oxygenation ( O ).

In the solid or crystalline phase, cholesterol side chain oxidation occurs at the tertiary carbon-25 position and yields the 25-hydroperoxycholesterol, which is readily degraded thermally into the corresponding 25-hydroxy-cholesterol. Two dihydroperoxides (3j8-hydroxy-207 - and 20iS-cholest-5-ene-20,25-dihydroperoxides) have also been isolated in autoxidized bulk cholesterol. The monohydroperoxides of cholesterol decompose into complex... 

Havrilla, C.M., Hachey, D.L. and Porter, N.A. Coordination (Ag+) ionspray-mass spectrometry of peroxidation products of cholesterol linoleate and cholesterol arachidonate High-performance liquid chromatography-mass spectrometry analysis of peroxide products from polyunsaturated lipid autoxidation. J. Am. Chem. Soc. 122, 8042-8055 (2000). 

Karu, K., Turton, J., Wang, Y. and Griffiths, W.J. (2011) Nano-liquid chromatography-tandem mass spectrometry analysis of oxysterols in brain Monitoring of cholesterol autoxidation. Chem. Phys. Lipids 164,411-424. 

Cholesterol of high quality and of recent production was employed. Cholesterol undergoes slow autoxidation in the solid state, and samples that have been in storage for a few years give lower yields of dibromide. The checkers used u.s.p. material, m.p. 149-150°, as supplied by Wilson Company, Chicago, Illinois. 

Figure 18.2. Cholesterol autoxidation initiated by peroxy (LOO ) or alkoxyl (LO ) radicals arising from peroxidation of polyunsaturated fatty acids (LH). Compounds are as follows (1) CHOL (2) 7-CHOL (3) CHOO (4) 7a-OOH (5) 7fLOOH (6) CHO (7) 7a-OH (8) 7(3-OH (9) 7-keto (10) 7-keto-3,5-dien. For abbreviations, see Table 18.1.

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