A series of in vitro studies was performed, in rat liver microsomes, in which metabolite intermediate (MI) complexation of cytochrome P-450 (P-450) by the methylenedioxyphenyl compound isosafrole was related to P-450 isozyme-specific inhibition of drug oxidation. The C19-steroid androst-4-ene-3,17-dione was selected for initial study because the stereoselective hydroxylation of this substrate is specific for certain P-450s. In control microsomes only the 6 beta- and 16 beta-hydroxylations of the steroid (catalyzed, respectively, by the P-450s PCN-E and PB-B) were inhibited by isosafrole (I50 = 100 and 110 microM). In contrast, the 7 alpha- and 16 alpha-hydroxylases (P-450 UT-F- and UT-A-mediated, respectively) were refractory to inhibition. After phenobarbital (PB) induction, steroid 6 beta- and 16 beta-hydroxylase activities were again inhibited (I50 = 170 and 190 microM) but, in addition, the 16 alpha-hydroxylase pathway was also inhibited (I50 = 200 microM). Spectral studies revealed that MI complexation of P-450 in untreated microsomes was minimal but was enhanced markedly after PB induction (up to 50% of the total P-450 content complexed). Thus, it is apparent that a PB-inducible P-450 is involved in MI complex formation under these conditions. Indeed the I50 of isosafrole toward steroid 16 beta-hydroxylase activity was decreased if the inhibitor was preincubated with NADPH-fortified PB-induced microsomes prior to substrate addition; the preincubation step did not enhance the inhibition of any other steroid hydroxylase pathway by isosafrole.(ABSTRACT TRUNCATED AT 250 WORDS)