Currently, there are no selective, well characterized inhibitors for CYP2A6. Therefore, the effects of trans-(+/-)-2-phenylcyclopropylamine (tranylcypromine), a potent CYP2A6 inhibitor, on human liver microsomal cytochromes P450 (CYP) were studied to elucidate its selectivity. The IC50 value of tranylcypromine in coumarin 7-hydroxylation (CYP2A6 model activity) was 0.42 +/- 0.07 microM and in chlorzoxazone 6-hydroxylation (CYP2E1 model activity) 3.0 +/- 1.1 microM. The IC50 values for CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 activities were >10 microM. Potency and selectivity of tranylcypromine were strongly dependent on the amine group, because its nonamine analog cyclopropylbenzene was much less potent inhibitor of CYP1A, CYP2A6, CYP2C19, and CYP2E1 activities and did not inhibit at all CYP2C9, CYP2D6, or CYP3A4 activities. In human liver microsomes tranylcypromine induced type II and cyclopropylbenzene type I difference spectrum. According to the double reciprocal analysis of these spectral responses both tranylcypromine and cyclopropylbenzene may have at least two P450-related binding sites in liver microsomes. The K(a) values of tranylcypromine varied from 4.5 to 15.1 microM and -34.3 to 167 microM in microsomes derived from three different livers and of cyclopropylbenzene from -1.6 to 10.1 microM and -34.6 and 75.2 microM in the same liver microsomes. Based on these results, tranylcypromine seems an adequately selective CYP2A6 inhibitor for in vitro use.