REGIOSELECTIVE O-DEMETHYLATION OF SCOPARONE (6,7-DIMETHOXYCOUMARIN) TO ASSESS CYTOCHROME-P450 ACTIVITIES INVITRO IN RAT - EFFECTS OF GONADAL-STEROIDS AND THE INVOLVEMENT OF CONSTITUTIVE P450 ENZYMES

1. In the rat, scoparone (6,7-dimethoxycoumarin) is regioselectively O -demethylated in vitro by the hepatic cytochrome P450 (P450) system, y ielding isoscopoletin and scopoletin. 2. Scoparone has been proposed a s an indicator substrate for the assessment of P450 differentiation in vitro in rat. It has been suggested that isoscopoletin formation main ly reflects activity of enzymes of the P4501A subfamily, whereas scopo letin formation has been associated with P4502B activity. 3. In the pr esent study, rate of formation of scopoletin and isoscopoletin were me asured in hepatic microsomes from male, female and castrated male rats , in castrates treated with testosterone, in males treated with oestra diol, and in females treated with testosterone. Furthermore, effects o f induction by phenobarbital (PB), beta-naphthoflavone (BNF), isoniazi d, triacetyloleandomycin, and dexamethasone were studied in both sexes . 4. Scoparone metabolism was partly sex- and steroid-dependent. Varia tion of isoscopoletin formation with sex or hormonal status correlated well with ethylmorphine demethylation. 5. Scoparone-0-demethylation w as regioselectively induced by PB and BNF. Induction effects were not very large and showed no sex differences. 6. Microsomal metabolism of scoparone was partly inhibited by a monoclonal antibody against P4502C 11. Scopoletin and isoscopoletin formation were inhibited when ethylmo rphine was added to the incubation mixture. Scoparone competitively in hibited testosterone 6beta-hydroxylation. 7. It is concluded that a nu mber of P450 enzymes, including those which are constitutively express ed, contribute to the biotransformation of scoparone. This lack of sel ectivity limits the usefulness of scoparone as a general indicator sub strate for P450 differentiation in rats.