Pyridyloxobutyl adduct O⁶-[4-oxo-4-(3-pyridyl)butyl]guanine is present in 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone-treated DNA and is a substrate for O⁶-alkylguanine-DNA alkyltransferase

The lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is activated to reactive metabolites that methylate or pyridyloxobutylate DNA. Previous studies demonstrated that pyridyloxobutylated DNA interferes with the repair of O⁶-methylguanine (O⁶-mG) by O⁶-alkylguanine-DNA alkyltransferase (AGT). The AGT reactivity of pyridyloxobutylated DNA was attributed to (pyridyloxobutyl)guanine adducts. One potential AGT substrate adduct, 2'-deoxy-O⁶-[4-oxo-4-(3-pyridyl)butyl]guanosine (O⁶-pobdG), was prepared. This adduct was stable at pH 7.0 for greater than 13 days and to neutral thermal hydrolysis conditions (pH 7.0, 100 °C, 30 min). Under mild acid hydrolysis conditions (0.1 N HCl 80 °C), O⁶-pobdG was depurinated to yield O⁶-[4-oxo-4-(3-pyridyl)butyl]guanine (O⁶-pobG). O⁶-pobdG was hydrolyzed to 4-hydroxy-1-(3-pyridyl)-1-butanone and guanine under strong acid hydrolysis conditions (0.8 N HCl, 80 °C). O⁶-pobG was detected in 0.1 N HCl hydrolysates of DNA alkylated with the model pyridyloxobutylating agent 4-(acetoxymethylnitrosamino)-1-(3-[5-³H]pyridyl)-1-butanone ([5-³H]NNKOAc). When [5-³H]NNKOAc-treated DNA was incubated with either rat liver or recombinant human AGT, O⁶-pobG was removed, presumably a result of transfer of the pyridyloxobutyl group from the O⁶-position of guanine to AGT's active site.