Human cytosolic enzymes involved in the metabolic activation of carcinogenic aristolochic acid: evidence for reductive activation by human NAD(P)H:quinone oxidoreductase

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, indicating a possible demethoxylation reaction of AAI, were identified as AA‐ DNA adducts formed from AAI by all human hepatic and renal cytosols. To define the role of human cytosolic reductases in the activation of AAI, we investigated the modulation of AAI‐DNA adduct formation by cofactors or selective inhibitors of the NAD(P)H:quinone oxidoreductase (NQO1), xanthine oxidase (XO) and aldehyde oxidase. We also determined whether the activities of NQO1 and XO in different human hepatic cytosolic samples correlated with the levels of AAI‐DNA adducts formed by the same cytosolic samples. Based on these studies, we attribute most of the activation of AA in human cytosols to NQO1, although a role of cytosolic XO cannot be ruled out. With purified NQO1 from rat liver and kidney and XO from buttermilk, the major role of NQO1 in the formation of AAI‐DNA adducts was confirmed. The orientation of AAI in the active site of human NQO1 was predicted from molecular modeling based on published X-ray structures. The results demonstrate for the first time the potential of human NQO1 to activate AAI by nitroreduction.