Repair of 06-Methylguanine and 04-Methylthymidine in F344 Rat Liver following Treatment with 1,2-Dimethylhydrazine and 06-Benzylguanine

Concentrations of 06-methylguanine, 04-methyl thymidine, and N-7-methylguanine were measured in the livers of Fischer 344 rats following treatment with 1,2-dimethylhydrazine (20 mg/kg, s.c.) alone or in combination with the 06-alkylguanine transferase inhibitor 06-benzylguanine (100 mg/kg, i.p., daily). Animals were sacrificed at 12, 24, 36, or 48 h following 1,2-dimethylhydrazine exposure. Direct measurement of alkyl-transferase demonstrated that daily treatment with 06-benzylguanine completely eliminated detectable alkyltransferase activity in the livers of treated rats. Adducts in liver DNA were quantitated by high performance liquid chromatography separation followed by fluorescence detection, UV absorbance, and/or specific radioimmunological assays. In animals exposed to 1,2-dimethylhydrazine alone 06-methylguanine concentrations declined rapidly, whereas animals exposed to both 06-benzy(guanine and 1,2-dimethylhydrazine showed less removal of 06-methylguanine, with significant differences between the two populations appearing at 36 and 48 h. 04-Methylthymidine removal also differed significantly between the two groups, with 06-benzylgua nine-treated animals exhibiting higher concentrations of adducts at 36 and 48 h. 06-Benzylguanine treatment had no effect on the removal of JV-7-methylguanine. These results show that the rate of disappearance of both 06-methylguanine and 04-methyl thymidine is slower following alkyltransferase depletion, suggesting that mammalian alkyltransferase is involved in the removal of O4-methyl thymidine lesions as well as 06-methylguanine lesions.