Structure-activity relationships in the in vitro modulation of rat hepatic microsomal androst-4-ene-3,17-dione hydroxylase activities by derivatives of 5α- and 5β-androstane

Abstract

The relationships between structure and inhibitory potency toward microsomal cytochrome P-450 ( P-450)-mediated androst-4-ene-3,17-dione hydroxylase activities were investigated in rat liver with a series of 5α- and 5β-androstane derivatives. 5β -Reduced steroids (containing a cis-A/B ring junction) were more potent inhibitors than the 5α-reduced epimers (containing a trans-A/B ring junction) except in the case of the 17β-hydroxy-substituted derivatives. The most effective inhibitor was 5β-androstane-3β-ol which exhibited I 50 values of 7 and 27 μM against androstenedione 16α- and 6β -hydroxylase activities, which are catalysed by P-450 IIC11 and IIIA2, respectively. In general, these two pathways of steroid hydroxylation were more susceptible to inhibition than the 7α- and 16β -hydroxylase pathways. The 7α-hydroxylase enzyme ( P-450 IIA1) was only inhibited by 5β -reduced steroids that contained an oxygenated function at C17. All of the test compounds elicited type I spectral binding interactions with P-450 in oxidised microsomes. The most effective steroid inhibitors generally exhibited the greatest capacity to interact with P-450. Additional studies with one of the more potent compounds, 5β-androstane-3β-ol-17-one, revealed that the inhibition kinetics were competitive and that preincubation of the inhibitor with NADPH-supplemented microsomes prior to substrate (androstenedione) addition decreased the extent of inhibition observed. These findings are consistent with the assertion that the inhibition of hepatic steroid hydroxylases by 5β-androstanes involves an effective competitive interaction with the steroid substrate at the P-450 active site. Since the relative overproduction of 5β-reduced metabolites of certain androgens has been reported in clinical conditions, such as androgen insensitivity, it now appears important to investigate the hepatic drug oxidation capacity of patients with hormonal abnormalities.