Effect of Chronic Treatment of Rats with Dimethylnitrosamine on the Removal of O⁶-Methylguanine from DNA

Rats were treated chronically for up to 6 weeks with unlabeled dimethylnitrosamine at daily doses of 0.2 to 2 mg/kg/day. The effect of this pretreatment on the persistence of O⁶-methylguanine, 7-methylguanine, and 3-methyladenine in DNA from liver, lung, and kidney was determined after administration of a single 2-mg/kg dose of [¹⁴C]dimethylnitrosamine. Pretreatment did not affect the formation or the rate of loss of 3-methyladenine or 7-methyguanine in any organ but resulted in a lower initial amount of O⁶-methylguanine in hepatic DNA as measured at 1 or 2 hr. The decrease in the initial amount of O⁶-methylguanine was dependent on the dose of dimethylnitrosamine used for pretreatment, with 2 mg/kg/day giving a maximal effect, although a significant decrease could still be observed when 0.2 mg/kg/day was used. It was also related to the time of pretreatment, requiring at least 2 weeks of exposure to 2 mg/kg/day for a maximal response. There was no effect of O⁶-methylguanine removal from DNA of kidney or lung at any dose used. Pretreatment with dimethylnitrosamine led to an increased activity of an enzyme removing O^e-methyl-guanine from methylated DNA in vitro which could be detected,. in liver extracts but had no effect on a similar activity in kidney extracts. These results suggest that the lower amount of O⁶-methylguanine found in hepatic DNA of rats pretreated with dimethylnitrosamine was due to an increased activity of the enzyme system responsible for this reaction. This increase could be the result of an induction or activation of the enzyme in response to the chronic administration of the carcinogen. The results are discussed in relation to the carcinogenicity of dimethylnitrosamine for rat liver and to the observation of an inducible enzyme system in Escherichia coli, which carries out an analogous reaction and protects against mutagenesis by alkylating agents.