Formation and persistence of N7-methylguanine DNA adducts in the target pyloric tissue following chronic exposure to N-methyl-N′-nitro-N-nitrosoguanidine

Outbred 7-week old male Wistar rats were exposed for 21 days to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) via the drinking water and N7-methyl deoxyguanosine 3'-monophosphate (N7-MedGp) levels in DNA from the pyloric mucosa (target tissue) and white blood cells (wbc: non-target tissue) were determined by ³²P-postlabelling. Exposure to MNNG resulted in the non-linear, dose-related formation of N7-medGp in both tissues. Adduct levels in the pyloric mucosa were determined to be 1058, 5.4 and 1.1 μmole N7-medGp mole^-1 deoxyguanosine 3'-monophosphate (dGp) after exposure to 4.1, 0.62 and 0.006 mg MNNG kg^-1 day^-1 respectively whereas adduct levels in the wbc DNA were lower at 5.2, 0.52 and 0.68 μmoles N7-medGp mole^-1 dGp after exposure to 4.1, 0.62 and 0.062 mg MNNG kg^-1 day^-1 respectively. In addition, the persistence of N7-medGp was investigated. Loss of adduct occurred rapidly, with a decrease of 87 and 97% respectively in target tissue and wbc DNA by 48 h after cessation of 4.1 mg MNNG kg^-1 day^-1 exposure; 14 days post-MNNG treatment, however, N7-medGp was still detectable (0.46 μmole N7-medGp mole^-1 dGp) in pyloric mucosal DNA. The quantitation of N7-medGp after exposure to low doses of carcinogen, i.e. 0.006 mg MNNG kg^-1 day^-1, approaching environmentally relevant levels has not been previously reported, and indicates that the ³²P-postlabelling assay developed here possesses sufficient sensitivity to quantitate N7- medGp in human DNA arising from environmental exposure to methylating agents.