The methylation of hamster liver DNA was studied as a function of dose of dimethylnitrosamine. 7-Methylguanine levels were proportional to a dose over the range of 10 μg/kg to 25 mg/kg when measured 5 to 24 hr after treatment. This product was lost from the DNA at a rate greater than expected from spontaneous depurination at neutral pH, suggesting that enzyme-catalyzed excision takes place. O6-Methylguanine levels were not proportional to doses over this range but were much lower than expected (based on 7-methylguanine levels) when measured 5 to 24 hr after doses of dimethylnitrosamine below 0.5 mg/kg. It is suggested that this result may be due to the presence of an enzyme capable of removing O6-methylguanine from DNA efficiently, provided the level of methylation was low. The presence of such an enzyme in hamster liver extracts was demonstrated by incubation with methylated DNA. The extracts brought about a significant decrease in the content of O6-methylguanine present in acid-precipitable DNA. However, when doses of dimethylnitrosamine above 0.5 mg/kg were used, removal of O6-methylguanine occurred much more slowly, and the capacity of hamster liver to carry out removal of O6-methylguanine from DNA in vivo was considerably lower than that of rat liver. The possible relevance of these findings to the relative susceptibility of these species to liver cancer induction by single doses of dimethylnitrosamine is discussed.


This research was supported in part by Grants CA18137 and 1P30 CA18450 and by Contracts CP55630 and CP6-1063 from the National Cancer Institute, Department of Health, Education, and Welfare.

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