Studies on the pregnenolone-16 alpha-carbonitrile-inducible form of rat liver microsomal cytochrome P-450 and UDP-glucuronosyltransferase.

Treatment of rats with pregnenolone-16 alpha-carbonitrile (PCN) markedly induces rat liver microsomal cytochrome P-450p and UDP-GT-dt1, a glucuronosyltransferase active towards the digitoxin metabolite, digitoxigenin monodigitoxoside. The present study characterizes the regulation of these two enzymes in rats treated with different xenobiotics. Like PCN, treatment of rats with dexamethasone, spironolactone, troleandomycin or erythromycin estolate markedly induced both UDP-GT-dt1 and cytochrome P-450p (measured as erythromycin demethylase and testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activities). However, compared to PCN and dexamethasone, both troleandomycin and erythromycin estolate preferentially induced cytochrome P-450p, whereas spironolactone preferentially induced UDP-GT-dt1. Treatment of rats with the polychlorinated biphenyl mixture, Aroclor 1254, increased both cytochrome P-450p and UDP-GT-dt1 activity to about 40% of that in liver microsomes from rats induced with PCN or dexamethasone. Treatment of rats with phenobarbital or chlordane caused a relatively small increase in cytochrome P-450p and UDP-GT-dt1 activity. Neither enzyme was induced by treatment of rats with 3-methylcholanthrene, rifampin or digitoxin. The induction of cytochrome P-450p and UDP-GT-dt1 by PCN followed similar dose-response curves. Although cytochrome P-450p and UDP-GT-dt1 are differentially affected by the age and the sex of rats, the enzymes responded similarly, but not identically, to xenobiotic treatment. This suggests that cytochrome P-450p and UDP-GT-dt1 are co-inducible but not coordinately regulated.