The Ubiquitous Aldehyde Reductase (AKR1A1) Oxidizes Proximate Carcinogen trans-Dihydrodiols to o-Quinones:  Potential Role in Polycyclic Aromatic Hydrocarbon Activation

• Published in: Biochemistry (Easton) 2001-09, Vol.40 (36), p.10901-10910
• Main Authors: Palackal, Nisha T, Burczynski, Michael E, Harvey, Ronald G, Penning, Trevor M
• Format: Article
• Language: English
• Subjects: Aldehyde Reductase - metabolism; Biotransformation; Carcinogens - pharmacokinetics; Cloning, Molecular; Cytochrome P-450 CYP1A1 - metabolism; Epoxide Hydrolases - metabolism; Escherichia coli; Humans; Isoenzymes - metabolism; Models, Chemical; Oxidation-Reduction; Polycyclic Aromatic Hydrocarbons - pharmacokinetics; Quinones - pharmacokinetics; Recombinant Proteins - metabolism; Substrate Specificity
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi010872t

Polycyclic aromatic hydrocarbons (PAHs) are metabolized to trans-dihydrodiol proximate carcinogens by human epoxide hydrolase (EH) and CYP1A1. Human dihydrodiol dehydrogenase isoforms (AKR1C1−AKR1C4), members of the aldo−keto reductase (AKR) superfamily, activate trans-dihydrodiols by converting them to reactive and redox-active o-quinones. We now show that the constitutively and widely expressed human AKR, aldehyde reductase (AKR1A1), will oxidize 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 oxidized the potent proximate carcinogen (±)-trans-7,8-dihydroxy-7,8-dihydrobenzo[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-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 preferentially oxidizing non-K-region trans-dihydrodiols. 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 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 of benzo[a]pyrene-7,8-dione by LC/MS. Multiple human tissue expression array analysis showed coexpression of AKR1A1, CYP1A1, and EH, indicating that trans-dihydrodiol substrates are formed in the same tissues in which AKR1A1 is expressed. The ability of this general metabolic enzyme to divert trans-dihydrodiols to o-quinones suggests that this pathway of PAH activation may be widespread in human tissues.