| Abstract
| - Polycyclic aromatic hydrocarbons (PAHs) are metabolized to trans-dihydrodiol proximatecarcinogens by human epoxide hydrolase (EH) and CYP1A1. Human dihydrodiol dehydrogenase isoforms(AKR1C1−AKR1C4), members of the aldo−keto reductase (AKR) superfamily, activate trans-dihydrodiolsby converting them to reactive and redox-active o-quinones. We now show that the constitutively andwidely expressed human AKR, aldehyde reductase (AKR1A1), will oxidize potent proximate carcinogentrans-dihydrodiols to their corresponding o-quinones. cDNA encoding AKR1A1 was isolated from HepG2cells, overexpressed in Escherichia coli, purified to homogeneity, and characterized. AKR1A1 oxidizedthe potent proximate carcinogen (±)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene with a higherutilization ratio (Vmax/Km) than any other human AKR. AKR1A1 also displayed a high Vmax/Km for theoxidation of 5-methylchrysene-7,8-diol, benz[a]anthracene-3,4-diol, 7-methylbenz[a]anthracene-3,4-diol,and 7,12-dimethylbenz[a]anthracene-3,4-diol. AKR1A1 displayed rigid regioselectivity by preferentiallyoxidizing non-K-region trans-dihydrodiols. The enzyme was stereoselective and oxidized 50% of eachracemic PAH trans-dihydrodiol tested. The absolute stereochemistries of the reactions were assigned bycircular dichroism spectrometry. AKR1A1 preferentially oxidized the metabolically relevant (−)-benzo[a]pyrene-7(R),8(R)-dihydrodiol. AKR1A1 also preferred (−)-benz[a]anthracene-3(R),4(R)-dihydrodiol,(+)-7-methylbenz[a]anthracene-3(S),4(S)-dihydrodiol, and (−)-7,12-dimethylbenz[a]anthracene-3(R),4(R)-dihydrodiol. The product of the AKR1A1-catalyzed oxidation of (±)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene was trapped with 2-mercaptoethanol and characterized as a thioether conjugate ofbenzo[a]pyrene-7,8-dione by LC/MS. Multiple human tissue expression array analysis showed coexpressionof AKR1A1, CYP1A1, and EH, indicating that trans-dihydrodiol substrates are formed in the same tissuesin which AKR1A1 is expressed. The ability of this general metabolic enzyme to divert trans-dihydrodiolsto o-quinones suggests that this pathway of PAH activation may be widespread in human tissues.
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