Repair of O6-methylguanine and O4-methylthymidine in F344 rat liver following treatment with 1,2-dimethylhydrazine and O6-benzylguanine.

Concentrations of O6-methylguanine, O4-methylthymidine, 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 O6-alkylguanine transferase inhibitor O6-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 alkyltransferase demonstrated that daily treatment with O6-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 O6-methylguanine concentrations declined rapidly, whereas animals exposed to both O6-benzylguanine and 1,2-dimethylhydrazine showed less removal of O6-methylguanine, with significant differences between the two populations appearing at 36 and 48 h. O4-Methylthymidine removal also differed significantly between the two groups, with O6-benzylguanine-treated animals exhibiting higher concentrations of adducts at 36 and 48 h. O6-Benzylguanine treatment had no effect on the removal of N-7-methylguanine. These results show that the rate of disappearance of both O6-methylguanine and O4-methylthymidine is slower following alkyltransferase depletion, suggesting that mammalian alkyltransferase is involved in the removal of O4-methylthymidine lesions as well as O6-methylguanine lesions.