Comparative Metabolism of N'-Nitrosonornicotine and 4-(Methylnitrosamino)-l-(3-pyridyl)-l-butanone by Cultured F344 Rat Oral Tissue and Esophagus

The metabolism and DNA binding of N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino-1-(3-pyridyl)-l-butanone (NNK) by cultured F344 rat oral tissue and esophagus were investigated over a range of concentrations. The metabolites present in the culture media were separated by high performance liquid chromatography and were identified by comparison to standards. α-Hydroxylation of NNN, an esophageal carcinogen, was the major pathway for metabolism of this nitrosamine in both tissues. The metabolites formed from 2'-hydroxylation were between 3.0 and 3.9 times those formed from 5'-hydroxylation. 2'-Hydroxylation results in a pyridyloxobutylating species. DNA from esophagus cultured with 5-3H NNN contained a pyridyloxobutylated adduct which upon acid hydrolysis released 3.8 pmol 5-3H-4-hydroxy-l-(3-pyridyl)-l-butanone/ µmol guanine. DNA from oral tissue cultured under the same conditions, where the extent of metabolism was the same, contained no measurable [5-3H]NNN DNA adduct. This suggests that factors, as yet unknown, cause the DNA of oral cavity tissue to be protected from pyridyloxobu-tylation by NNN. The metabolism of NNK by a-hydroxylation was as much as 10-fold less than the metabolism of NNN by this pathway in both tissues. a-Hydroxylation of NNK results in either a methylating species or a pyridyloxobutylating species. DNA from oral tissue cultured with C3H3 NNK contained between 1.7 and 4.3 pmol 7-methylguanine/ εmol guanine, respectively. No pyridyloxobutylated DNA (<0.2 pmol/ Mmol guanine) was detected in oral tissue incubated with 5-3H)NNK. The DNA from esophagi incubated with [C3H3JNNK contained no 7-methylguanine (<0.4 pmol/Mmol guanine). The level of pyridyloxobuty-lation of DNA from esophagi incubated with [5-3H]NNK was 0.17 pmol/ µmol guanine. The ability of the esophagus to metabolize NNN to a greater extent than NNK to a reactive species which pyridyloxobutylates DNA may be important in determining the carcinogenicity of NNN in the esophagus. In contrast, the metabolism of NNK to a methylating species by oral cavity tissue suggests that this tobacco-specific nitrosamine is important in tobacco-related oral cavity carcinogenesis.