Bile acids produce a generalized reduction of the catalytic activity of cytochromes P450 and other hepatic microsomal enzymes in vitro: Relevance to drug metabolism in experimental cholestasis

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Abstract
In bile duct-ligated male rats, there is a reduction of total hepatic microsomal cytochrome P450 (P450) levels and of NADPH-cytochrome P450 reductase (P450-reductase) activity, but the changes in activity of individual microsomal enzymes are nonuniform. We have proposed that the initial effect of cholestasis on microsomal proteins is a non-specific reduction caused by bile acid-mediated destruction, whereas the disproportionate lowering of male-specific P450 enzymes results from secondary down-regulation of some cytochrome P450 (CYP) genes. We report herein the results of experiments to test the first part of this hypothesis, at least as indicated by enzyme inhibition. Hepatic microsomal fractions from normal male rats were incubated at 37 degrees C with increasing concentrations of a range of bile acids selected for their varying physicochemical properties. The endpoints were catalytic activity of three individual CYP proteins, CYP 2A1 (measured as testosterone 7 alpha-hydroxylase activity), 2C11 (testosterone 2 alpha-hydroxylase and 16 alpha-hydroxylase) and 3A2 (testosterone 6 beta-hydroxylase), and the non-CYP enzymes, steroid 17 beta-dehydrogenase and P450-reductase. With 0.25 mmol/L cholic acid, a concentration exceeded in serum following bile duct ligation, there was a significant reduction in the activity of all enzymes at 4 h. Cholic acid-mediated inhibition was dose-dependent and there was no difference in inhibitory activity towards the male sex-dependent CYP 2C11 and 3A2 and the non-sex-dependent CYP 2A1 and other microsomal enzymes. Taurocholic acid was twice as potent an inhibitor as unconjugated cholic acid, the respective apparent I50 values being approximately 0.6 mmol/L compared with approximately 1.2 mmol/L. The dihydroxy bile acids, chenodeoxycholic acid and deoxycholic acid, were also more potent inhibitors than cholic acid, exhibiting I50 values in the range of 0.3-0.5 mmol/L, but the monohydroxy bile acid, lithocholic acid, was the most potent inhibitor (I50 approximately 0.2 mmol/L). Thus, the inhibitory potential of bile acids towards microsomal enzymes was inversely related to their extent of hydroxylation, while taurine conjugation enhanced the inhibitory potential of cholic acid. These data confirm the potential of bile acids to inhibit the activity of microsomal enzymes in livers of bile ductligated rats and indicate that such changes can occur with concentrations of bile acids that are physiologically relevant. Further, the results are consistent with the proposal that the disproportionately greater reduction of the male sex-dependent CYP, 2C11 and 3A2, is not explained by a destructive mechanism.