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 deoxy guanosine 3'-monophosphate (N7-MedGp)levels in DNA from the pyloric mucosa (target tissue) and white blood cells (wbc: non-target tissue) were determi ned by P-32-postlabelling. Exposure to MNNG resulted in the non-linear, dos e-related formation of N7-medGp in both tissues. Adduct levels in the pylor ic mucosa were determined to be 1058, 5.4 and 1.1 mu mole N7-medGp mole(-1) deoxyguanosine 3'-monophosphate (dGp) after exposure to 4.1, 0.62 and 0.00 6 mg MNNG kg(-1) day(-1) respectively whereas adduct levels in the wbc DNA were lower at 5.2, 0.52 and 0.68 mu moles N7-med Gp mole(-1) dcp after expo sure to 4.1, 0.62 and 0.062 mg MNNG kg(-1) day(-1) respectively. In additio n, the persistence of N7-medGp was investigated. Loss of adduct occurred ra pidly, 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 mu mole 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 rep orted, and indicates that the 32P-postlabelling assay developed here posses ses sufficient sensitivity to quantitate N7-medGp in human DNA arising from environmental exposure to methylating agents.