7a-Hydroxysteroid dehydrogenase

Assays have also made use of 7a-hydroxysteroid dehydrogenase that can measure the primary bile acids, or for more specialised purposes such as differentiating between pathways of bile-acid synthesis to determine the proportion derived from the acid pathway. 

We thank Drs. Tanaka, Nonaka, Nakanishi, Deyashiki, , and Mitsui for providing us with the x-ray crystallographic coordinates of carbonyl reductase and 7a-hydroxysteroid dehydrogenase. The support of the Supercomputer Center of the University of California, San Diego is gratefully acknowledged. 

As well as the enzyme 3a-hydroxysteroid dehydrogenase, a 7a-enzyme has been isolated from E. coli and a 12a enzyme from a strain of Clostridium (Ml). These group-specific enzymes can be used to measure the amounts and ratios of the three main bile acids (cholic, chenodeoxycholic, and deoxy-cholic acids) in bile specimens. Bioluminescent assays suitable for serum bile acid analysis have been described using 7a-hydroxysteroid dehydrogenase (R6) and 12a-hydroxysteroid dehydn enase (S12), as well as for the 3a enzyme (S13, S44). 

The degradation of cholesterol leads to the production of bile acids which are structurally closely related to various steroid hormones. (3-Hydroxysteroid dehydrogenase (EC 1.1.1.51) catalyzes the NAD+-de-pendent oxidation of 3(3-, 17(3- and some l6(3-hydroxysteroids to the respective ketosteroids. The enzyme has been adsorbed on a carbon electrode modified by NMP+TCNQ and the NADH liberated in the reaction oxidized anodically (Albery et al., 1987a). Campanella et al. (1984) employed an enzyme sequence electrode composed of NAD+-de-pendent steroid dehydrogenase and horseradish peroxidase for assay of 7a-hydroxysteroids. 

A 3a-hydroxysteroid dehydrogenase active on 7a-hydroxy-5 -cholestan-3-one and 7a,12a-dihydroxy-5 -cholestan-3-one (cf. Fig. 3), was partially purified (about 300-fold) from rat Uver cytosol by Berseus [112,113]. NADPH was required as cofactor and hardly any activity was observed with NADH. The preparation was also active towards 3-oxo steroids of the Cjg, C21 and C24 series, and in these cases appreciable activity was obtained also with NADH. The mechanism of reduction involves a stereospecific transfer of a hydride ion from the 4A position of NADPH to the 3)S position of the steroid [114],... 

The 3a-hydroxysteroid dehydrogenase preparation described by Berseus (42) has been found to catalyze the oxidation of 4-cholestene-3a,7a-diol into 7a-hydroxy-4-cholesten-3-one (52). It is interesting that the reverse reaction proceeds at a rate which is at least ten times slower. Similarly, the rate of oxidation of 5j -cholestane-3a,7a,12a-triol into 7a,12a-dihydroxy-5j -choles-tan-3-one, catalyzed by 3a-hydroxysteroid dehydrogenase, is about ten times slower than the rate of reduction (42). 

The reduction of a 3-oxosteroid, catalyzed by 3a-hydroxysteroid dehydrogenase from rat liver, has been shown to involve transfer of a hydride ion from the A-side of NADPH to the 3j -position of the steroid (44,52). The oxidation of 4-cholestene-3a,7a-diol into 7a-hydroxy-4-cholesten-3-one, catalyzed by the same enzyme preparation, has also been shown to involve the A-side of NADPH (52). It remains to be established whether or not Zl -3a-hydroxysteroid dehydrogenase activity is due to enzyme(s) different from 3a-hydroxysteroid dehydrogenase(s). 

CH25H5 followed by oxysterol 7a-hydroxylase (CYP7Bl)/and hydroxysteroid dehydrogenase 3B7 (HSD3B7)... 

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. 

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