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  • Title: Metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in human kidney epithelial cells transfected with rat CYP2B1 cDNA.
    Author: Lacroix D, Desrochers M, Castonguay A, Anderson A.
    Journal: Carcinogenesis; 1993 Aug; 14(8):1639-42. PubMed ID: 8353848.
    Abstract:
    In all species where it has been tested, the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) has been shown to be a potent carcinogen, and NNK and other nitrosamines may play a role in human tobacco-related carcinogenesis. Purified rat CYP2B1 has been shown to metabolize NNK, and the CYP2B1 gene is expressed constitutively in rat lung. The objectives of this study were to test the capacity of CYP2B1, synthesized from a rat hepatic cDNA in Ad293 cells, to metabolize NNK, and to define the type and the proportions of the final metabolites produced. Ad293 cells were transfected with a CYP2B1 expression vector (pMT2-2B1), or with a control vector and incubated in culture medium containing [3H]NNK, after which alpha-carbon hydroxylation and pyridine N-oxidation metabolites were identified by HPLC analysis and quantitated by scintillation counting. pMT2-2B1-transfected cells were capable of catalyzing alpha-carbon hydroxylation and pyridine N-oxidation of NNK, although the reduction product 4-(methylnitrosamino)-1-(3-pyridyl)-1-butan-1-ol(NNAL) was the major metabolite formed in cells regardless of transfection treatment. The total amount of alpha-carbon hydroxylation metabolites produced by pMT2-2B1-transfected cells was greater than that of pyridine N-oxidation metabolites. However, pMT2-2B1 transfected cells produced approximately ten-fold more pyridine N-oxidation metabolites and only two-fold more alpha-carbon hydroxylation metabolites than control cells. Furthermore, the amount of NNAL-N-oxide was much lower than that of NNK-N-oxide in the medium of pMT2-2B1-transfected cells, even though the amount of available NNAL, resulting from carbonyl reduction of NNK, was very high; this suggests that NNAL is poorly N-oxidized by CYP2B1 compared to NNK. These results show that within living cells NNK was metabolized by CYP2B1 via both the pyridine N-oxidation and alpha-carbon hydroxylation pathways. However, CYP2B1 preferentially catalyzed pyridine N-oxidation, which is considered to be a deactivation reaction.
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