N-Monoalkyl substituted tricyclic antidepressants like desipramine (DES) undergo cytochrome P450 (P450)-mediated biotransformation in liver to produce inhibitory metabolite-intermediate (MI) complexes with the enzyme. However, additional oxidation pathways that generate isolable metabolites have also been identified, so that the relationship between MI complexation and total oxidative metabolism is unclear. The present study investigated the capacity of DES and three putative metabolites (2-hydroxy- and 10-hydroxy-DES and N,N-didesmethylimipramine; DIDES) to elicit MI complexation and inhibit P450-dependent activities in rat liver. MI complexation of P450 was produced by DES, but not with the three metabolites, in NADPH-supplemented microsomes. Consistent with this finding, inhibition of testosterone hydroxylation pathways was enhanced markedly by prior incubation of DES with NADPH and microsomes. Direct addition of DIDES to incubations resulted in significant inhibition of P450 activities (IC50s of 35 and 29 microM against estradiol 6 beta- and 16 alpha-hydroxylation mediated by P450s 3A2 and 2C11, respectively). Neither 2-hydroxy- nor 10-hydroxy-DES directly inhibited testosterone hydroxylation (IC50s > 100 microM). However, after a preincubation step between these metabolites and NADPH-fortified microsomes, enhanced inhibition of reactions mediated by P450 3A2 and P450 2C11/2A1 was produced by 2-hydroxy-DES and 10-hydroxy-DES, respectively. Metabolism of DES to DIDES and 2-hydroxy-DES was estimated as 7.77 +/- 0.48 nmol/mg protein/hr (10-hydroxy-DES was not detected). It is likely that secondary oxidized metabolites derived from 2-hydroxy-DES, as well as the primary metabolite DIDES, may contribute to the inhibition of P450 activity during DES biotransformation. These results indicate that the 2-hydroxy-, 10-hydroxy-, and N-desmethyl-metabolites of DES are not involved in MI complexation, but complexation is not the sole mechanism by which DES inhibits microsomal drug oxidation that may lead to pharmacokinetic drug interactions.