Steroid 22-hydroxycholesterol

The preparation of estranes via 19-hydroxy steroids has also been accompHshed microbially. 19-Hydroxycholesterol, prepared chemically (39), is incubated with JS icardia restrictus to provide estrone (20) direcdy (44). Similarly, 19-hydroxyandrost-4-en-3,17-dione is converted to estrone (20) (45). 

In work published in 1989, Hylemon et al. used cholesterol oxidase to convert 7a-hydroxycholesterol to 7a-hydroxy-4-cholesten-3-one. Cholesterol that remained was converted to 4-cholesten-3-one. 7/3-Cholesterol, which was added as an internal steroid recovery standard, was oxidized to 7/3-hydroxy-4-cholesten-3-one. These steroid products were analyzed by Qg reversed-phase chromatography on an Altex Ultrasil-ODS column (4.6 mm x 25 cm) using 70 30 (v/v) mixture of acetonitrile and methanol (Fig. 9.83). The eluate was monitored at 240 nm, and the amount of product determined from a calibration curve. 

The second step in the synthesis of bile acids, according to Hylemon et al. (1991), is the conversion of 7a-hydroxycholesterol to 7a-hydroxy-4-cholesten-3-one by NAD+-dependent 3/3-hydroxy-A5-C27-steroid oxidoreductase. This enzyme is located in the endoplasmic reticulum of liver, and its catalysis of the 3/3-hydroxy group also results in isomerization of the double bond from A5 to A4. 

Addition of alkenylzirconium (51) to steroid-derived allylpalladium (52), followed by hydrogenation and hydrolysis, gives 25-hydroxycholesterol in 66% overall yield from (52 Scheme 15). Finally,... 

B. Both pregnenolone and progesterone are intermediates in the synthesis of steroid hormones from cholesterol. 7-Hydroxycholesterol is an intermediate in bile salt synthesis, and aldosterone is a mineralocorticoid produced well beyond the branch point for the synthesis of the adrenal and gonadal steroids. Retinoic acid is derived from vitamin A. 

Cholesterol and many of its biosynthetic precursors are highly insoluble in aqueous media. Yet, cholesterol biosynthesis, utilization and intracellular transfers occur in environments which involve both aqueous and nonaqueous components. For example, the enzymes involved in the conversion of squalene to cholesterol, the conversion of cholesterol to cholesterol esters, and the conversion of cholesterol to 7a-hydroxycholesterol are associated with the endoplasmic reticulum (microsomes). The conversion of cholesterol to pregnenolone, an essential first step in steroid hormone biosynthesis, occurs in mitochondria. In addition, transfers of cholesterol from cytoplasmic lipid inclusion droplets through the cytosol to the mitochondria are essential for steroid hormone production. 

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... 

Thus, product 4 may have a dual origin, arising from autoxidation as well as from en2ymatic monooxygenation [16] However, autoxidation always yields a mixture of 3 and 4 [16]. Also 20-hydroxycholesterol can be formed both by autoxidation and by enzymatic means. 20(S)- and 22(R)-hydroxycholesterol are intermediates in the synthesis of steroid hormones [16]. Cholesterol 5a,6a-epoxide 6 is mostly associated with autoxidation, but has also been shown to be enzymatically formed in different systems. It is also formed by the action of agents such as hydroxyl radical or hydrogen peroxide on cholesterol [9]. 

A number of oxysterols, which suppress HMGR when added to cell cultures, are natural precursors of cholesterol or products of its metabolism to bile acids or steroid hormones [18]. These sterols include the bile acid intermediates 7a- and 26-hydroxycholesterol, the steroid hormone precursors 20a- and 22R-hydroxycholesterol. 24(s)-hydroxycholesterol (cerebrosterol) was foimd by Kandutch et al [18] that other oxysterol metabolites capable of repressing the reductase may also be produced. Kandutch et al [18] foimd several unidentified oxysterols capable of suppressing activity in extracts of cultured cells. Inhibitory oxysterols are, therefore, produced in all cells that synthesize cholesterol as an end product, as well as in cells that synthesize cholesterol and metabolize it to other steroid products. 

Stereoselective Wittig reaction. The reagent has been used in a novel synthesis of 25-hydroxycholesterol (5). Thus the /-steroid aldehyde 2 reacts with 1 to form 3. Two features of the reaction are noteworthy. The 20-methyl group does not epimerize and the A -double bond of the major product has the E-con-... 

Steroid standards were purchased from the following sources Cholestane - 3/9,5a, 6/9-triol and 7a-hydroxycholesterol from Research Plus Laboratories, Inc. (Denville, NJ) Cholesterol, 25-hydroxycholesterol, 7-... 

Sevanian et al. (1994) applied GLC and LC/TS/MS for the analysis of plasma cholesterol-7-hydroperoxides and 7-ketocholesterol. Analysis of human and rabbit plasma identified the commonly occurring oxidation products, yet dramatic increases in 7-ketocholesterol and cholesterol-5p, 6P-epoxide were observed. The study failed to reveal the presence of choles-terol-7-hydroperoxides, which were either too unstable for isolation, metabolized or further decomposed. The principal ions of cholesterol oxides monitored by LC/TS/MS were m/z 438 (cholestane triol) m/z 401 (cholesterol-7-hydroperoxide) m/z 401 (7-ketocholesterol) m/z 367 (7a-hydroxycholesterol) m/z 399 (cholesta-3,5-dien-7-one) and m/z 385 (choles-terol-5a,6a-epoxide). The major ions were supported by minor ions consistent with the steroid structure. Kamido et al. (1992a, b) synthesized the cholesteryl 5-oxovaleroyl and 9-oxononanoyl esters as stable secondary oxidation products of cholesteryl arachidonate and linoleate, respectively. These compounds were identified as the 3,5-dinitrophenylhydrazone (DNPH) derivatives by reversed-phase LC/NICI/MS. These standards were used to identify cholesteryl and 7-ketocholesteryl 5-oxovaleroyl and 9-oxononanoyl esters as major components of the cholesteryl ester core aldehydes generated by copper-catalysed peroxidation of low-density lipoprotein (LDL). In addition to 9-oxoalkanoate (major product), minor amounts of the 8, 9, 10, 11 and 12 oxo-alkanoates were also identified among the peroxidation products of cholesteryl linoleate. Peroxidation of cholesteryl arachidonate yielded the 4, 6, 7, 8, 9 and 10 oxo-alkanoates of cholesterol as minor products. The oxysterols resulting from the peroxidation of the steroid ring were mainly 7-keto, 7a-hydroxy and 7P-... 

A new era in the regirlation of P450 7A1 began with reports of the involvement of some of the orphan steroid receptors. The proximal promoter region interacts with LXRa. The oxy-sterols 24(5)-hydroxycholesterol and 24(5)-epoxycholesterol activate LXRa (and LXRj ... 

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