title: The ubiquitous aldehyde reductase (AKR1A1) oxidizes proximate carcinogen trans-dihydrodiols to o-quinones: potential role in polycyclic aromatic hydrocarbon activation.
abstract: 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.
authors.affiliation: Department of Biochemistry, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA.
authors.forename: N T
authors.initials: NT
authors.lastname: Palackal
authors.forename: M E
authors.initials: ME
authors.lastname: Burczynski
authors.forename: R G
authors.initials: RG
authors.lastname: Harvey
authors.forename: T M
authors.initials: TM
authors.lastname: Penning
mesh_terms: D000449:Aldehyde Reductase / Q000378:metabolism*; D001711:Biotransformation; D002273:Carcinogens / Q000493:pharmacokinetics*; D003001:Cloning, Molecular; D019363:Cytochrome P-450 CYP1A1 / Q000378:metabolism; D004851:Epoxide Hydrolases / Q000378:metabolism; D004926:Escherichia coli; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D008956:Models, Chemical; D010084:Oxidation-Reduction; D011084:Polycyclic Aromatic Hydrocarbons / Q000493:pharmacokinetics; D011809:Quinones / Q000493:pharmacokinetics*; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D002273:Carcinogens; D007527:Isoenzymes; D011084:Polycyclic Aromatic Hydrocarbons; D011809:Quinones; D011994:Recombinant Proteins; D000449:Aldehyde Reductase; D019363:Cytochrome P-450 CYP1A1; D004851:Epoxide Hydrolases