Aristolochic acid (AA), a naturally occurring nephrotoxin and carcinogen, has been associated with the development of urothelial cancer in humans. Understanding which human enzymes are involved in AA activation and/or detoxication is important in the assessment of an individual's susceptibility to this plant carcinogen. Using the (32)P postlabeling assay, we examined the ability of microsomal samples from 8 human livers and from 1 human kidney to activate AAI, the major component of the plant extract AA, to metabolites forming adducts in DNA. Microsomes of both organs generated DNA adduct patterns reproducing those found in renal tissues from humans exposed to AA. 7-(deoxyadenosin-N(6)-yl)aristolactam I, 7-(deoxyguanosin-N(2)-yl)aristolactam I and 7-(deoxyadenosin-N(6)-yl)aristolactam II were identified as AA-DNA adducts formed from AAI by all human hepatic and renal microsomes. To define the role of human microsomal enzymes in the activation of AAI, we investigated the modulation of AAI-DNA adduct formation by cofactors and selective inhibitors of microsomal reductases, cytochrome P450 (CYP) enzymes, NADPH:CYP reductase and NADH:cytochrome b(5) reductase. We also determined whether the activities of CYP and NADPH:CYP reductase in different human hepatic microsomal samples correlated with the levels of AAI-DNA adducts formed by the same microsomal samples. On the basis of these studies, we attribute most of the activation of AAI in human hepatic microsomes to CYP1A2. In contrast to human hepatic microsomes, in human renal microsomes NADPH:CYP reductase is more effective in AAI activation. In addition, prostaglandin H synthase is another enzyme activating AAI in renal microsomes. The results demonstrate for the first time the potential of microsomal enzymes in human liver and kidney to activate AAI by nitroreduction.