| Abstract
| - The tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), is apotent pulmonary carcinogen. This unsymmetric nitrosamine can be metabolically activated to lungDNA methylating and pyridyloxobutylating intermediates. The methyl DNA adducts are wellcharacterized. The pyridyloxobutyl adducts are unstable under DNA hydrolysis conditions anddecompose to release 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB). One of the HPB-releasing adducts,O6-[4-oxo-4-(3-pyridyl)butyl]guanine (O6-pobG), has been detected in DNA reacted in vitrowith the model pyridyloxobutylating agent, 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone(NNKOAc). To determine whether this adduct was formed in vivo, A/J mice were treated with 10μmol of [5-3H]NNK and sacrificed 24 h postinjection. The mutagenic O6-pobG was detected in liverbut not lung DNA from these animals. Since 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL)is a major metabolite of NNK, it is also possible that these animals are activating NNAL to apyridylhydroxybutylating agent. Therefore, we also measured the levels of O6-[4-hydroxy-4-(3-pyridyl)butyl]guanine (O6-phbG) in these DNA samples. While radioactivity did coelute withsynthetic standard for this potential NNAL adduct in one lung DNA sample, significant levels ofO6-phbG were not detected in any other lung or liver DNA samples. The pyridyloxobutyl adduct,O6-pobG, was also observed in lung and liver DNA from mice treated with 4.2 μmol of [5-3H]NNKOAcin the presence but not absence of 2.5 μmol of O6-benzylguanine, a known depletor of the repairprotein O6-alkylguanine-DNA alkyltransferase (AGT). These data indicate that this adduct is formedin vivo but is repaired in part by AGT. Cell-free extracts from A/J mouse lung and liver were usedto determine the relative rate of O6-alkylguanine repair. O6-mG and O6-pobG were removed fromDNA to the same extent in a competitive assay, suggesting that low levels of O6-pobG in lungs ofNNK-treated mice did not result from preferential repair of O6-pobG by AGT. It is more likely thatinitial levels of O6-pobG are much lower than initial levels of O6-mG in lung DNA from NNK-treated A/J mice. These data are consistent with previous studies, which indicate that DNAmethylation is the critical pathway for NNK-induced lung carcinogenesis in A/J mice.
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