Regulation and properties of a sex specific hydroxylase system in female rat liver microsomes active on steroid sulfates. I. General characteristics

The general characteristics and gonadal hormonal regulation of a new, sex specific hydroxylase system present in female rat liver microsomes and active on steroid sulfates but not on free steroids is described. The substrate studied was 5α [4-¹²C]androstane 3α,17β diol 3,17 disulfate which was converted to 5α [4-¹²C]androstane 3α,15β,17β triol 3,17 disulfate. The 15β hydroxylase system could not be detected in liver microsomes active in female rats. The spectral dissociation constant (K(s)) of 5α androstane 3α,17β diol 3,17 disulfate was 9.52 μM for male liver microsomes and 23.8 μM for female liver microsomes, whereas the apparent K(m) value calculated for female liver microsomes was 25.8 uM. Incubations carried out in the presence of carbon monoxide led to more than 98% inhibition of the 15β hydroxylase system, whereas presence of SKF 525 A in a concentration of 10^-3M inhibited the 15β hydroxylase activity by 60%. Treatment of female rats with phenobarbital and 3 methylcholanthrene led to a significant decrease in the activity of the 15β hydroxylase system, whereas treatment with 16α cyanopregnenolone did not significantly affect the enzyme activity. Postpubertal gonadectomy did not significantly affect the 15β hydroxylase activity in female rats and did not lead to the appearance of detectable enzyme levels in liver microsomes from male rats. On the other hand, neonatal castration of male rats resulted in completely feminized levels of 15β hydroxylase in the adult animals and this feminization was totally inhibited by one single dose of 1.45 μmoles of testosterone propionate administered on the day after neonatal testectomy. Treatment of postpubertally castrated male rats with estradiol benzoate led to an at least transient, partial feminization of the liver with a 15β hydroxylase activity of about 30% of that present in normal female rats. When postpubertally castrated female rats were treated with testosterone propionate, the activity of the 15β hydroxylase system was suppressed. The 15β hydroxylase activity was also studied in male and female rats 0, 10, 20, 30, 35, 40, 45 and 55 days of age and in rat fetuses. The 15β hydroxylase activity was not measurable until at 20 days of age and at this time no sexual difference in enzyme activity was apparent. Already at 30 days of age, however, female rats tended to hydroxylate more effectively than male rats and after this time the 15β hydroxylase activity increased rapidly in female rats so that it had almost reached adult levels at 40 days of age. During the same period the hydroxylase activity decreased successively in male rats and from 45 days and onward it was no longer detectable. It is speculated that the physiological role of the highly sex specific 15β hydroxylase system is to ascertain efficient hepatic deactivation of potential androgenic compounds in female rats.