Removal of methylated purines from rat liver DNA after administration of dimethylnitrosamine.

Dimethylnitrosamine administration led to the produc tion of abnormal methylated purines in rat liver DNA. All of the methylated purines formed were lost from DNA to some extent over a 24-hr period although the rate of removal varied with the purine considered. The rates of loss of 1-methyladenine, 3-methyladenine, 7-methyladenine, 3-methylguanine, and 7-methylguanine were not changed significantly when the extent of methylation was increased more than 20-fold by increasing the dose of dimethylnitrosamine from 0.75 to 20 mg/kg. In contrast, O' -methylguamne was lost rapidly from the DNA after the lower dose of dimethylnitrosamine but at a much slower rate after the higher dose. When chromatin was prepared from livers of rats treated with labeled dimethylnitrosa mine and partially digested with DNase I or micrococcal nuclease, no difference in the content of 7-methyl guanine or O'-methylguanine between the nucleaseresistant and nuclease-sensitive portions of the chro matin DNA was seen. Changing the dose of dimethyl nitrosamine from 25 Mg/kg to 20 mg/kg did not alter the relative distribution of these methylated guanines. Al though at the lower doses tested a substantial amount of the O6-methylguanine was lost from the chromatin DNA within 24 hr, there was no indication that this purine was lost more readily from either fraction of the chromatin DNA. When labeled O' -methylguanine that had been pro duced in hepatic DNA by administration of a small dose (0.75 mg/kg) of [MC]dimethylnitrosamine was diluted by unlabeled O6-methylguanine formed by a (20-mg/kg) dose of unlabeled dimethylnitrosamine given 1 hr later, the rate of loss of labeled O' -methylguanine was greatly reduced. It is concluded that O' -methylguanine is removed from hepatic DNA via a specific mechanism that is more effi cient after lower doses of dimethylnitrosamine either because the removal system becomes saturated at higher levels of methylation of the DNA or because it is inhibited by the larger doses of the carcinogen.