The sulfate specific hydroxylase system in liver microsomes from rats has been investigated with respect to its substrate specificity. 18 different C18, C19, C21, and C27 steroid sulfates and the corresponding free steroids have been incubated with microsomal preparations from male and female rats. The sulfate specific enzyme system was only present in preparations from female rats and primarily catalyzed hydroxylation in position 15β but also in position 7β. In contrast to this, male liver microsomes were more efficient than female liver microsomes in hydroxylating free steroids; these were hydroxylated in positions 2α,2β,6α,6β,7α,7β,16α, and 18. The sulfate specific hydroxylase system in female liver microsomes was found to have rigid requirements concerning the structure of ring D in the substrate molecule; only 17β sulfates (C18 and C19 steroids) and 21 sulfates (C21 steroids) were hydroxylated. Less rigid criteria, however, exist concerning the structure of ring A. The following K(m) values were determined for microsomal 15β hydroxylation: 5α androstane 3α, 17β diol disulfate, 17.2 μM; 5β androstane 3α, 17β diol disulfate, 16 μM; 5α androstane 3α, 17β diol 17 sulfate, 26 μM; and estradiol 17 sulfate, 181 μM. Some of the regulatory mechanisms controlling the activity of the sex specific 15β hydroxylase system also have been studied and compared to the mechanisms controlling the activities of the less specific 2α, 7α, and 18 hydroxylase systems active on 5α [4 14C]androstane 3α, 17β diol. Biliary drainage did not affect the 15β hydroxylase activity, whereas the 2α and 7α hydroxylase activities decreased. Ligation of the common bile duct led to a depression of the 2α, 7α, and 18 hydroxylase activities but to an increase in the 15β hydroxylase activity. Administration of 5α androstane 3α, 17β diol 3,17 disulfate stimulated the 15β hydroxylase activity, whereas no significant changes were seen in the activities of the 2α, 7α, and 18 hydroxylase systems. Hypophysectomy totally abolished the 15β hydroxylase activity in female rats. Furthermore, the induction of the 15β hydroxylase system seen in castrated male rats after estradiol benzoate treatment was not seen in hypophysectomized male rats. The results indicate the existence in the female rat liver of a hydrophilic species of cytochrome P 450, specifically catalyzing the hydroxylation of steroid sulfates and with quite different properties from the ''bulk'' of cytochrome P 450 participating in hydroxylations of hydrophobic substrates. Furthermore, the results indicate that the 15β hydroxylase system active on 5α androstane 3α, 17β diol 3,17 disulfate is regulated by other control mechanisms than the hydroxylase systems active on free 5α androstane 3α, 17β diol. Of special interest are the results regarding the stimulatory effect of sulfate administration on the 15β hydroxylase activity, possibly implying a physiological substrate regulating control mechanism for hepatic steroid sulfate metabolism, and regarding the obligatory role of the hypophysis in maintaining measurable levels of the 15β hydroxylase activity indicating that the liver could serve as a target organ for a hypophyseal hormonal factor regulating hydroxylation of steroid sulfates.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Biological Chemistry|
|State||Published - 1975|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology