Formation of O6 -mythylguanine in rat liver DNA by nitrosamines does not predict initiation of preneoplastic foci

Nitrosodimethylamine (NDMA) at 26 μmol/kg, and nitro somethylbenzylamine (NMBzA) at 33.5 μmol/kg were equally potent in producing 7-methylguanine and O6 -methylguanine in rat liver DNA. These doses were used to compare the abilities of the two nitrosamines to initiate production of putative preneoplastic foci in rat liver. Whereas NMBzA resulted in no increase above the background level of foci (0.9 ± 0.1 foci/cm2), NDMA produced 7.5 ± 0.6 foci/cm2. Co-administration of NDMA and NMBzA produced no more foci than NDMA alone, even though the combined effect of the two nitrosamines on DNA methylation was additive. The results suggest that methylation of hepatic DNA by nitrosamines does not predict initiation of preneo plastic foci.