Effects of drugs and sterols on cholesterol 7 alpha-hydroxylase activity in rat liver microsomes.
This study examined the effects of various drugs and sterols on the rate of 7alpha-hydroxycholesterol synthesis in isolated rat liver microsomes. Cholesterol 7alpha-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 7alpha-hydroxylase was also significantly inhibited by 7beta-hydroxycholesterol (38%) and 7-ketocholesterol (35%) at concentrations of 1 micro M, and by 10 micro M 7alpha-hydroxycholesterol (35%), 25alpha-hydroxycholesterol (32%), and 5-cholenic acid-3beta-ol (27%). Two bile acids, cholate and lithocholate, as well as a geometric isomer of cholesterol, coprostanol, had little influence on 7alpha-hydroxylase activity at concentrations of 10 micro M. The inhibitory effect of metyrapone was additive with that of either 7beta-hydroxycholesterol or proadifen; the effects of 7beta-hydroxycholesterol and proadifen were not additive. These results suggest that proadifen and 7beta-hydroxycholesterol interact with the same enzyme site while metyrapone binds at a different location. Proadifen inhibited 7alpha-hydroxylase irreversibly, while kinetic studies demonstrated noncompetitive inhibition by metyrapone (K(I) = 0.55 mM) and competitive inhibition by 7beta-hydroxycholesterol (K(I) = 2.4 micro M). The inhibition of 7alpha-hydroxylase activity by metyrapone and aminoglutethimide, drugs used to manage patients with excessive cortisol production, suggests that such treatment may also alter bile acid synthesis.-Schwartz, M. A., and S. Margolis. Effects of drugs and sterols on cholesterol 7alpha-hydroxylase activity in rat liver microsomes.
D000616:Aminoglutethimide / Q000494:pharmacology; D000818:Animals; D001205:Ascorbic Acid / Q000494:pharmacology; D001647:Bile Acids and Salts / Q000494:pharmacology; D002790:Cholesterol 7-alpha-Hydroxylase / Q000378:metabolism*; D002927:Cimetidine / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D008797:Metyrapone / Q000494:pharmacology; D008862:Microsomes, Liver / Q000201:enzymology*; D011335:Proadifen / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013250:Steroid Hydroxylases / Q000378:metabolism*; D013261:Sterols / Q000494:pharmacology*
D001647:Bile Acids and Salts; D013261:Sterols; D000616:Aminoglutethimide; D002927:Cimetidine; D011335:Proadifen; D013250:Steroid Hydroxylases; D002790:Cholesterol 7-alpha-Hydroxylase; D001205:Ascorbic Acid; D008797:Metyrapone