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Polycyclic aromatic hydrocarbons (PAH) are environmental pollutants and suspected human lung carcinogens. In patients with non-small cell lung carcinoma, differential display shows that aldo-keto reductase (AKR1C) transcripts are dramatically overexpressed. However, whether AKR1C isoforms contribute to the carcinogenic process and oxidize potent PAH trans-dihydrodiols (proximate carcinogens) to reactive and redox active o-quinones is unknown; nor is it known whether these reactions occur in human lungs. We now show that four homogeneous human recombinant aldo-keto reductases (AKR1C1-AKR1C4) are regioselective and oxidize only the relevant non-K region trans-dihydrodiols. However, these enzymes are not stereo-selective, since they oxidized 100% of these racemic substrates. The highest utilization ratios (V(max)/K(m)) were observed for some of the most potent proximate carcinogens known (e.g. 7,12-dimethylbenz[a]anthracene-3,4-diol (DMBA-3,4-diol) and benzo[g]chrysene-11,12-diol). In vitro, DMBA-3,4-diol was oxidized by AKR1C4 to the highly reactive 7,12-dimethylbenz[a]anthracene-3,4-dione (DMBA-3,4-dione), which was trapped in situ as its mono- and bis-thioether conjugates, which arise from the sequential 1,6- and 1,4-Michael addition of thiol nucleophiles. Human multiple tissue expression array analysis showed that AKR1C isoform transcripts were highly expressed in the human lung carcinoma cell line A549. Isoform-specific reverse transcriptase-PCR showed that AKR1C1, AKR1C2, and AKR1C3 transcripts were all expressed. Western blot analysis and functional assays confirmed high expression of AKR1C protein and enzyme activity in these lung cells. A549 cell lysates were found to convert DMBA-3,4-diol to the corresponding o-quinone. In trapping experiments, LC/MS analysis identified peaks in the cell lysates that corresponded to the synthetically prepared mono- and bis-thioether conjugates of DMBA-3,4-dione. This quinone is one of the most electrophilic and redox-active o-quinones produced by AKRs. Its unique ability to form bis-thioether conjugates parallels the formation of bis- and tris-glutathionyl conjugates of hydroquinone, which display end organ toxicity. The ability to measure DMBA-3,4-dione formation in A549 cells implicates the AKR pathway in the metabolic activation of PAH in human lung.