Methylation of DNA in rat liver and intestine by dimethylnitrosamine and N-methylnitrosourea

A chromatographic procedure for improved separation of deoxyribonucleosides and methylated deoxyribonucleosides is described. DNA was isolated from liver and small intestine of rats treated with [ 14C]dimethylnitrosamine ([ 14C]DMN) or N-[ 3 H] methyl-N- nitrosourea ([ 3H]MNU), and the purified DNA was hydrolyzed enzymatically. The deoxyribonucleosides were chromatographed on an Aminex A-6 cation exchange column at 37°C with 0.4 M ammonium formate, pH 4.5, as eluant. In addition to showing the presence of the expected alkylated products, N 7-methyldeoxyguanosine (determined as N 7-methylguanine) and O 6-methyldeoxyguanosine, several other minor methylated products were found in liver and intestinal DNA of rats treated with DMN or MNU. Two of these products are believed to be N 3-methylthymidine and O 4-methylthymidine.