The ubiquitous aldehyde reductase (AKR1A1) oxidizes proximate carcinogen trans-dihydrodiols to o-quinones: Potential role in polycyclic aromatic hydrocarbon activation

Polycyclic aromatic hydrocarbons (PAHs) are metabolized to trans-dihydrodio l proximate carcinogens by human epoxide hydrolase (EH) and CYP1A1. Human d ihydrodiol dehydrogenase isoforms (AKR1C1-AKR1C4), members of the aldo-keto reductase (AKR) superfamily, activate trans-dihydrodiols by converting the m to reactive and redox-active o-quinones. We now show that the constitutiv ely and widely expressed human AKR, aldehyde reductase (AKR1A1), will oxidi ze potent proximate carcinogen trans-dihydrodiols to their corresponding o- quinones. cDNA encoding AKR1A1 was isolated from HepG2 cells, overexpressed in Escherichia coli, purified to homogeneity, and characterized. AKR1A1 ox idized the potent proximate carcinogen (+/-)-trans-7,8-dihydroxy-7,8-dihydr obenzo[a]pyrene with a higher utilization ratio (V-max/K-m) than any other human AKR. AKR1A1 also displayed a high V-max/K-m for the oxidation of 5-me thylchrysene-7,8-diol, benz[a]anthracene-3,4-diol, 7-methylbenz[a]anthracen e-3,4-diol, and 7,12-dimethylbenz[a]anthracene-3,4-diol. AKR1A1 displayed r igid regioselectivity by preferentially oxidizing non-K-region trans-dihydr odiols. The enzyme was stereoselective and oxidized 50% of each racemic PAH trans-dihydrodiol tested. The absolute stereochemistries of the reactions were assigned by circular dichroism spectrometry. AKR1A1 preferentially oxi dized the metabolically relevant (-)-benzo[a]pyrene-7(R),8(R)-dihydrodiol. AKR1A1 also preferred (-)-benz[a]anthracene-3(R),4(R)-dihydrodiol, (+)-7-me thylbenz[a]anthracene-3(S),4(S)-dihydrodiol, and (-)-7,12-dimethylbenz[a]an thracene-3(R),4(R)-dihydrodiol. The product of the AKR1A1-catalyzed oxidati on of (+)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene was trapped with 2- mercaptoethanol and characterized as a thioether conjugate of benzo[a]pyren e-7,8-dione by LC/MS. Multiple human tissue expression array analysis showe d coexpression of AKR1A1, CYP1A1, and EH, indicating that trans-dihydrodiol substrates are formed in the same tissues in which AKR1A1 is expressed. Th e ability of this general metabolic enzyme to divert trans-dihydrodiols to o-quinones suggests that this pathway of PAH activation may be widespread i n human tissues.