26-Hydroxy cholesterol

This isoform was found as the result of a mouse deficient in the oxysterol 7a-hydroxylase gene (CYP7B1 in humans), which nonetheless did not accumulate 24-hydroxy cholesterol from the CYP46pathway, suggesting the existence of another 7ot-hydroxylase (Li-Hawkins et al., 2000). However, very httle evidence exists for its expression in human brain. Nishimura et al. quantified CYP39A1 mRNA by RT-RT-PCR and reported some 100-fold less than they reported for CYP51 mRNA (Nishimura et al., 2003). There is no other evidence for the expression of this isoform in the brain as of this writing and this is beheved to be the first report of its kind. The isoform has been shown to be sexually dimorphic in the liver (Li-Hawkins et al., 2000). 

The C-24 configurations of the isomeric 24-hydroxy-cholesterols have been firmly established5 by chemical correlation with the known 24R,25- and 24S,25-epoxyderivatives, and by the modified Horeau method.6 With the configurations known, differences in 13C n.m.r. spectra and in polarity were used to establish the configurations of the la,24-dihydroxy-derivatives of vitamin D3.5 Cerebrosterol , isolated from brain tissue, is 24S-hydroxycholesterol,6a but the natural 24,25-dihydroxy-vitamin D3 has the (24R) configuration. The C-24 isomeric 24,25-dihydroxy-cholesterols have also been separated and characterized.7... 

CYP27B1, mitochondrial vitamin D3 1-a-hydroxylase activates vitamin Ds CYP27C1, unknown function CYP39, 7-hydroxylase of 24-hydroxy cholesterol with unknown function CYP46, cholesterol 24-hydroxylase with unknown function CYP51, lanosterol 14a-demethylase, for converting lanosterol into cholesterol, inhibited by ketoconazole. 

The classic reaction of P450 7A1 is cholesterol 7a-hydroxylation [37], and esterified cholesterol is not a substrate [1771], The enzyme also catalyzes the 7a-hydroxylation of 24-hydroxy-cholesterol, with preferenee for the (S)-isomer [1772]. 7a-Hydroxylation (with recombinant human P450 7A1) was observed with 20(5)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol [1773]. The relevance of the activity towards 25(5)-hydroxycholesterol is unknown compared to P450 39A1 [1774],... 

Koizumi, N., M. Morisaki, and N. Ikekawa Absolute Configurations of 24-Hydroxy-cholesterol and Related Compounds. Tetrahedron Lett. 1975, 2203. 

FIGURE 25.41 Cholic acid, a bile salt, is synthesized from cholesterol via 7o -hydroxy-cholesterol. Conjugation with taurine or glycine produces taurocholic acid and glycocholic acid, respectively. Taurocholate and glycocholate are freely water-soluble and are highly effective detergents. 

Figure 1.1 illustrates a condensed version of the classical pathway of bile-acid synthesis, a series of 12 enzymatic reactions that convert cholesterol, which is insoluble, into BAs, which are water soluble. The cholesterol is first converted to 7 alpha-hydroxy cholesterol, followed by the series of enzymatic transformations, eventually producing cholic and chenodeoxycholic acids (not all steps shown). The rate-limiting enzyme in this pathway is cholesterol 7 alpha-hydroxylase (CYP 7A1), which originates from microsomal cytochrome P-450 enzymes, expressed only in the liver hepatocytes. 

Z-17(20)-Dehydrocholesterol (231) has been prepared from pregnenolone by two independent routes. Dehydration of a mixture of 20a- and 20/3-hydroxy-cholesterol gave the required compound (231) along with the previously prepared... 

The initial steps in BA synthesis are characterised by the introduction of a hy-droxylic group in the la position, or in position 27, followed by another in the la position into the cholesterol nucleus. Both synthetic pathways (the neutral and the acidic pathways) possess a distinct microsomal 7-oxysterol hydroxylase, which is regulated by different genes. The most recently described disorder of BA synthesis is cholesterol 7a-hydroxylase deficiency, in which their decreased production through the classical pathway is partially balanced by activation of the alternative pathway. Cholesterol levels increase in the liver, with a consequent low-density lipoprotein hypercholesterolemia, and cholesterol gallstones may result, although there is no liver disease. In contrast, a defect in the conversion of 27-hydroxy-cholesterol to la,27-dihydroxy-cholesterol due to deficiency of the oxysterol 7a-hydroxylase specific for the alternate pathway, causes severe neonatal liver disease [8]. 

Helminthogermacrene, 304 Hirsutene, 153 Hirsutic acid, 117 26-Hydroxy cholesterol, 151... 

Two other new nuclear receptors have been shown to increase epidermal differentiation the LXR and the FXR. Farnesol and juvenile hormone activate the FXR leading to improved epidermal differentiation. Two genes encode for the LXR proteins, LXR alpha and LXR beta, and both are activated by various oxysterols the most potent being 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol, 24(S) 25-epoxycholesterol and 7-hydroxy cholesterol. Cholestenoic acid also acts on this receptor. In vitro these agents also increased epidermal filaggrin levels.129,130... 

Fig. 5. Pattern of product formation during single turnover cycles of anaerobic reduction/oxygenation of the cytochrome P-450,.,. - cholesterol - adrenodoxin complex. The results are expressed as a percentage conversion of the total [14C]cholesterol added to the incubation. Cholesterol, o 22-hydroxy-cholesterol, 20,22-dihydroxycholesterol. a pregnenolone, (from Ref. 38, with permission).

Wieber and co-workers [202] utilized unimolecular decomposition spectra (as MIKE), as well as high-resolution measurements for this type of structural study. This methodology was applied to the following derivatives 5a-6-dihydroergosterol (1), cholest-l-ene-3-one (2), methyl trinor-5-cholestane-3-one-24-oate (3), 22 -hydroxy-cholesterol acetate (4) and 22-ketocholesterol acetate (5) (Fig. 50). 

During bile acid biosynthesis, modifications to the cyclopentanophen-anthrene (steroid) nucleus are thought to precede the oxidation and cleavage of the cholesterol side chain. The first and rate-controlling step in bile acid synthesis is the 7o-hydroxylation of cholesterol (I) to form 7a-hydroxy-choles-terol (II) (Fig. 3). This step is catalyzed by cholesterol 7a-monooxygenase (cholesterol 7a-hydroxylase) (EC 1.14.13.17), a microsomal enzyme (M37). Further metabolism of 7a-hydroxy-cholesterol involves oxidation of the 3p-hydroxyl group and isomerization of the double bond from C-5,6 to C-4,5,... 

S2). More recent studies have shown that patients with cirrhosis are able to efficiently convert 7a-hydroxy-cholesterol into cholic acid (G8, P8), suggesting that 12a-hydroxylase activity is near normal. Other evidence from in vivo studies in man with labeled preciusors suggests that 12a-hydroxylase activity is not important in the regulation of the ratio between cholic acid and chenodeoxycholic add in human bile (B21). The possibility that different pools of cholesterol are utilized for the biosynthesis of cholic acid and chenodeoxycholic acid is now being investigated. 

During the 1960 s, the above sequence of reactions was confirmed by different in vitro studies. Mendelsohn and Staple showed that labelled cholesterol could be converted into 5j8-cholestane-3a,7a,12 -triol by 20000 X g supernatant fluid of rat liver homogenates [23]. The enzymatic conversion of cholesterol into 7a-hydroxy-cholesterol was first shown by Danielsson and Einarsson using the microsomal fraction fortified with NADPH [24]. The conversion of 7 -hydroxycholesterol into 7 -hydroxy-4-cholesten-3-one was found to be catalysed by the microsomal fraction fortified with NAD [25]. The latter steroid was converted into 7a,12a-dihydroxy-4-cholesten-3-one by the microsomal fraction and NADPH [26]. The conversion of 7 -hydroxy-4-cholesten-3-one and 7a,12a-dihydroxy-4-cholesten-3-one into the corresponding 3a-hydroxy-5/8-saturated steroids was catalysed by soluble NADPH-de-pendent enzymes [25,27,28]. Since Hutton and Boyd found that 4-cholestene-3 ,7 -diol was a product of 7a-hydroxy-4-cholesten-3-one in vitro [25], it was first... 

There was a significant isotope effect in the conversion of [3a- H]7a-hydroxy-cholesterol into 7 -hydroxy-4-cholesten-3-one, indicating that abstraction of a hydride ion from the 3a position may be rate limiting in the reaction. 

The idea that only one enzyme is involved in the conversion of 7a-hydroxy-cholesterol into 7a-hydroxy-4-cholesten-3-one was recently supported by an investigation by Wikvall [97]. He solubilized the enzyme activity from rabbit liver micro-somes by treatment with a mixture of sodium cholate and the non-ionic detergent Renex 690. The enzyme was purified about 200-fold by different chromatographic steps. The purified enzyme showed only one protein band with an apparent molecular weight of 46 000. Whereas the microsomal fraction had a broad substrate... 

J23-22-Acetoxycholcstcrol derivatives can be converted stereoselectively with bis(aceto-nitrile)dichloropalladium(II) as a catalyst to 24-hydroxycholesterol derivatives435. The (225,23 )- and (225,23Z)-diastereomers react more rapidly than the (22i ,23 >isomer, and the (22i ,23Z)-compound does not rearrange under the reaction conditions. The difference in the reactivity is attributed to the different conformations of the side chains of the 22-hydroxy-cholesterol derivatives, as the coordination of the palladium complex with the 7t-electrons of the double bond from the side opposite the acetoxy group is sterically hindered in the less reactive derivative. 

However, other experimental observations do not quite fit this model that centers on the physical interaction of sphingomyelin with cholesterol and current data suggest at least one other level of regulation. For example, 25-hydroxy-cholesterol and ceramide both increase sphingomyelin mass but decreases SREBP and SRE-mediated gene transcription (Ridgway, 1995 Sakai et al., 1996).Ceramide dose-dependently decreases SREBP and HMG-CoA synthase activity by a mechanism independent of a cholesterol-mediated decrease of SREBP (Ridgway and Merriam, 1995). 

By the time comprehensive reviews by Wilds (29) in 1944 and Djerassi (30) in 1953 became available, MPV reactions were a standard reductive technique in the organic chemistry community. For example, a 1945 patent (20) describes the utility of using aluminum alkoxides in the presence of an organic nitrogen as a weak base for the reduction of carbonyl groups on oxo compounds such as 7-hydroxy-cholesterol acetate, benzaldehyde, cinnamic aldehyde, and citronellal. 

Synthesis of sterol side chain. Polish chemists have prepared 25-hydroxy-cholesterol (6) from zl -3jS-acetoxyandrostene-17-one (2) in 42% overall yield as formulated. The key step involves alkylation of the anion of the ester (3)... 

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... 

A third approach [scheme (16)] involves deconjugation of a 1,4-dien-3-one to the 1,5-diene followed by selective hydroboration of the 1,2-olefin. This scheme gives both the la-hydroxy and 2a-hydroxy cholesterol derivatives. Application of this method to l,4,6-cholestatrien-3-one leads directly to the la-hydroxy provitamin D skeleton ... 

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