Microsomes hydroxysteroid dehydrogenase

This interconversion is catalaysed by 17)3-hydroxysteroid dehydrogenase (17/3-HSD), an enzyme generally found in the ER of numerous tissues such as adrenal, liver, testis, ovary and kidney. Like many of the enzymes described above, there appear to be different forms [52,87]. For example, rat adrenal cytosol and ER contain separate 17/3-HSDs, with NADH as the preferred cofactor. The rat testicular enzyme, however, prefers NADPH. Guinea-pig liver also contains two 17j3-HSDs, one solubilized from cytosol, the other associated with the ER [88], These enzymes exhibit different activities towards C19 steroids, the cytosolic one preferring 5/3-reduced 17-oxosteroids and the microsomal counterpart being involved with 5a-reduced steroids, such as 5a-DHT. In this case, the product of the reaction would be 5a-an-drostane-3,17-dione. 

Fig. 2. Pathway of biosynthesis of the glucocorticoid, cortisol, in the adrenal cortex. Cholesterol, from stores in cholesteryl esters or from other sources (see text) is converted via mitochondrial cytochrome P-450SCC (cholesterol side-chain cleavage enzyme) to pregnenolone, which then is successively converted by the microsomal enzymes cytochrome P-450,7 (17a-hydroxylase), 3 j8-hydroxysteroid dehydrogenase/ isomerase and cytochrome P-450c2, (21-hydroxylase) to 11-deoxycortisol, followed by conversion by the mitochondrial cytochrome P-450ll(3 (11/3-hydroxylase) to cortisol. The short-term action of ACTH in stimulation of steroidogenesis is to increase the availability of cholesterol for conversion by cytochrome P-450scc. From Ref. 9.

A microsomal retinol dehydrogenase catalyzes the oxidation of CRBP-hound all-trans-retinol to retinaldehyde it also acts as a 3a-hydroxysteroid dehydrogenase. A similar enzyme catalyzes the oxidation of 9-cis- and 11-ds-retinol, but not all-trans-retinol again, it has 3a-hydroxysteroid dehydrogenase activity. In the eye, the major product of this enzyme is 11-cts-retinaldehyde, whereas in other tissues it is 9-cts-retinaldehyde, which is then oxidized to 9-cis-retinoic acid (Section 2.3.2.1 Chen et al., 2000 Duester, 2000, 2001 Gamble et al., 2000 NapoU, 2001). Although there is known to be an isomerase in the eye for the formation of 11-cts-retinaldehyde as a... 

Nordlie RC, Jorgenson RA. Latency and inhibitability by metabolites of glucose-6-phosphatase of permeable hepatocytes from fasted and fed rats. J. Biol. Chem. 1981 256 4768-4771. McCornuck KL, Wang X, Mick GJ. Evidence that the 11 P-hydroxysteroid dehydrogenase is regulated by pentose pathway flux. Studies in rat adipocytes and microsomes. J. Biol. Chem. 2006 281 341-347. 

Microsomal fractions contain 3a- and 3 -hydroxysteroid dehydrogenases which do not appear to be derived from cytosol [171]. Thus, microsomal 3a-hydroxysteroid dehydrogenase preferentially utilized [4B- H]NADH instead of [4A- H]NADPH preferred by the cytosolic enzyme. Microsomal 3)3-hydroxysteroid dehydrogenase was inactive toward ethanol, and thus is not an active component of alcohol dehydrogenase [172]. These dehydrogenases were active with ,9, C21, C24 and C27... 

Although cholesterol is the major source of 5)9-bile acids, an unsaturated acid, 3)8-hydroxy-5-cholenic acid [174] has been found in meconium, mainly as the sulfate [175], in bile of a boy with a deficiency of 3)8-hydroxysteroid dehydrogenase [176], and in urine of healthy persons and individuals with liver disease [164]. The details of metabolism of 3)8-hydroxy-5-cholenic acid to lithocholate have not been entirely elucidated, but the mechanism for conversion of the 3/8-hydroxy-A to the 3-oxo-A derivative has been formulated in the C27 series (cf. Chapter 9). Briefly, the 3)8-ol is dehydrogenated by a microsomal enzyme fortified with NAD to provide the 3-oxo-A system [177,178]. Whether a A - A" isomerase is essential is not known, since there is no direct evidence for the formation of the intermediary 3-oxo-A system the rate-limiting step is the dehydrogenation of the 3)8-ol which may prevent accumulation of the 3-oxo-A system [177]. The reduction of the double bond at 4-5 to the 5)8- or 5a-bile acid is catalyzed by the respective A -3-oxosteroid 5)8- or 5 -reductase obtained from liver cytosol [170], and has been purified about 10-fold [178]. The formation of the 3-oxo-5/9 derivative requires the enzyme and NADPH the proton from the A side (4A-NADPH) appeared in the product as the 5)8-H, whereas the proton at C-4 is derived from the aqueous medium. Formation of the 5a derivative requires (4B-NADPH) in a similar mechanism (Fig. 4) [179], Reduction of the 3-0X0 product is then catalyzed by 3a-hydroxysteroid dehydrogenase as discussed above. 

The next major step in the transformation of cholesterol to cholic acid is the conversion of the 5-choles-tene-3j8, 7a-diol into 7a-hydroxy-4-cholestene-3-one. Such conversion may occur through at least two different pathways. In the first, the substrate (5-choles-tene-3j8, 7a-diol) is sequentially attacked by a microsomal 3j8-hydroxysteroid dehydrogenase and then... 

In the next step, the conversion of 7a-hydroxy-4-cholestene-3-one into 5j8-cholestene-3a, 7a, 12a-triol involves the 12 hydroxylation of the substrate by a NADPH microsomal enzyme. The product of the reaction, the 7a, 12a-dihydroxy-4-cholestene-3-one, is converted into the 7a, 12a-dihydroxy-5j8-cholestene-3-one by a soluble A, 3-oxysteroid-5j8-reductase, and 3a-hydroxysteroid dehydrogenase yields the final intermediate 5j -cholestene-3a, 7a, 12a-triol. 

Fig. 25 Mouse liver microsome (MLM) half-life data for piperidine-based 11 P-hydroxysteroid dehydrogenase type I (lip-HSDl) inhibitors...

Gough WH, VanOoteghem S, Sint T, Kedishvili N Y (1998) cDNA cloning and characterization of a new human microsomal NAD -dependent dehydrogenase that oxidizes all-rra 5-retinol and 3a-hydroxysteroids. JBiol Chem 273 19 778-19 785... 

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