Analysis for N²-(Pyridyloxobutyl)deoxyguanosine Adducts in DNA of Tissues Exposed to Tritium-Labeled 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone and N'-Nitrosonornicotine

The tobacco-specific carcinogens 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) are metabolically activated to DNA binding intermediates, partially via 4-(3-pyridyl)-4-oxobutanediazohydroxide (7) or related carbonium ions. Previous studies have shown that generation of 7 from 4-(carbethoxynitrosamino)-1-(3-pyridyl)-1-butanone (11) in the presence of deoxyguanosine yields a major adduct identified as 2'-deoxy-N-[1-methyl-3-oxo-3-(3-pyridyl)propyl]guanosine (adduct 1). These results suggested that adduct 1 should be present in DNA of tissues that can metabolically activate NNK and NNN. In the present study, we evaluate the formation of adduct 1 and its structurally related straight-chain analogue 2'-deoxy-N-[4-oxo-4-(3-pyridyl)butyl]guanosine (adduct 2) in DNA of tissues of rats treated with [5-³H]NNK or [5-³H]NNN, and in DNA of nasal mucosa that had been cultured in medium containing [5-³H]NNK or [5-³H]NNN. Hepatic DNA from rats treated with [5-³H]NNK was enzymatically hydrolyzed to deoxyribonucleosides and analyzed by HPLC. One of the radioactive peaks, peak E, coeluted with adduct 1. However, treatment of peak E with NaBH₄ resulted in the formation of products different from those produced by NaBH₄ treatment of adduct 1, demonstrating that adduct 1 could not be detected under these conditions. Hydrolysis of peak E with acid produced 4-hydroxy-1-(3-pyridyl)-1-butanone (9), suggesting that peak E might be adduct 2. Therefore, adduct 2 was synthesized by reaction of deoxyguanosine with 1-(3-pyridyl)butane-1,4-dione (5) in the presence of NaCNBH₃. Its HPLC retention time, however, was different from that of peak E. Peak E was also detected in DNA of nasal mucosa incubated with [5-³H]NNK or [5-³H]NNN. The HPLC chromatograms obtained upon enzymatic hydrolysis of DNA exposed to [5-³H]NNK or [5-³H]NNN were generally similar, indicating the operation of a common alkylation pathway via diazohydroxide 7 or a related carbonium ion. This pathway does not however lead to detectable levels of adducts 1 or 2 in DNA (detection limit using [5-³H]NNK or [5-³H]NNN, approximately 0.05 pmol/mg of DNA).