Formation of 06-Methylguanine by Alkylation of Rat Liver, Colon, and Kidney DNA following Administration of

cinogen (3, 31). Tumors were also produced in the duo denum, small intestine, liver, and kidney by 1,2-dimethyl hydrazine (3, 10, 13, 31). The distribution of tumors de pended on the dosage and method of application, but even a single injection resulted in the formation of some tumors in the colon and kidney (3, 10). The mechanism by which 1,2-dimethylhydrazine exerts its carcinogenic action has been postulated to be similar to that of cycasin and simple aliphatic nitroso compounds in that metabolism of the carcinogen leads to the formation of an active alkylating agent (3, 10, 31, 35, 36). In confirma tion of this hypothesis, it has been shown that, like dimeth ylnitrosamine and N-methyl-N-nitrosourea, 1,2-dimethylhy drazine leads to the methylation of nucleic acids in tissues in which tumors are produced (11, 12). In these experi ments, it was found that 7-methylguanine was formed in the DNA of liver, colon, and kidney of rats and mice given 1,2-dimethylhydrazine by s.c. injection. For some time al kylation of DNA has been considered the likely means by which cancer is induced by cycasin and the nitroso corn pounds (16—i 8, 22, 23, 29). However, more recent studies have suggested that the 06-position of guanine rather than the 7-position may be the critical site for the alkylation (17— 20, 29). The presence of 06-methylguanine, but not 7methylguanine, causes mutation in phage (21) and miscod ing by nucleic acid polymerases (8). In studies with simple aliphatic nitrosamines and nitrosamides, the formation and persistence of 06-alkylguanine in various tissues including brain, kidney, and liver correlated well with tumorigenesis (9, 24, 27, 30). Conversely, the alkylating agents methyl methanesulfonate and ethyl methanesulfonate, which are only weakly carcinogenic, resulted in much less 0-alkyl guanine formation (18, 30, 33, 34). These experiments support the hypothesis that the formation and persistence of 06-alkylguanine until cell replication leads to the herita ble, usually irreversible, change to neoplastic growth. There is evidence that this abnormal purine can be enzymatically removed in some organs, and it appears that persistence results from either a lack of this enzyme System or its inhibition rsaturation (25,28,29). Since it was not known to what extent 1,2-dimethylhydra zine alkylated DNA at the 0-position of guanine in target cells or whether cells in the colon were able to carry out the removal of 06-methylguanine seen in the liver (9, 24, 28, 29), the production and persistence of this methylated purine was measured in rat colon, kidney, and liver after