Removal of Methylated Purines from Rat Liver DNA after Administration of Dimethylnitrosamine

Dimethylnitrosamine administration led to the production 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, O⁶-methylguanine 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 dimethylnitrosamine and partially digested with DNase I or micrococcal nuclease, no difference in the content of 7-methylguanine or 0⁶-methylguanine between the nuclease-resistant and nuclease-sensitive portions of the chromatin DNA was seen. Changing the dose of dimethylnitrosamine from 25/ug/kg to 20 mg/kg did not alter the relative distribution of these methylated guanines. Although 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 O⁶-methylguanine that had been produced in hepatic DNA by administration of a small dose (0.75 mg/kg) of [¹⁴C]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 0⁶-methylguanine was greatly reduced. It is concluded that O⁶-methylguanine is removed from hepatic DNA via a specific mechanism that is more efficient 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.