Effects of drugs and sterols on cholesterol 7 α-hydroxylase activity in rat liver microsomes

This study examined the effects of various drugs and sterols on the rate 7 α-hydroxycholesterol synthesis in isolated rat liver microsomes. Cholesterol 7 α-hydroxylase activity was significantly inhibited by proadifen (98%), metyrapone (67%), and aminoglutethimide (45%) at concentrations of 1 mM and by ascorbic acid (40%) at a concentration of 10 mM. Cimetidine had no significant effect. The activity of cholesterol 7 α-hydroxylase was also significantly inhibited by 7 β-hydroxycholesterol (38%) and 7-ketocholesterol (35%) at concentrations of 1 μM, and by 10 μM 7α-hydroxycholesterol (35%), 25 α-hydroxycholesterol (32%), and 5-cholenic acid-3β-ol (27%). Two bile acids, cholate and lithocholate, as well as a geometric isomer of cholesterol, coprostanol, had little influence on 7 α-hydroxylase activity at concentrations of 10 μM. The inhibitory effect of metyrapone was additive with that of either 7 β-hydroxycholesterol or proadifen; the effects of 7 β-hydroxycholesterol and proadifen were not additive. These results suggest that proadifen and 7β-hydroxycholesterol interact with the same enzyme site while metyrapone binds at a different location. Proadifen inhbited 7 α-hydroxylase irreversibly, while kinetic studies demonstrated noncompetitive inhibition by metyrapone (K(I) = 0.55 mM) and competitive inhibition by 7 β-hydroxycholesterol (K(I) = 2.4 μM). The inhibition of 7 α-hydroxylase activity by metyrapone and aminoglutethimide, drugs used to manage patients with excessive cortisol production, suggest that such treatment may also alter bile acid synthesis.