REPAIR OF O(6)-METHYLGUANINE AND O(4)-METHYLTHYMIDINE IN F344 RAT-LIVER FOLLOWING TREATMENT WITH 1,2-DIMETHYLHYDRAZINE AND O(6)-BENZYLGUANINE

Concentrations of O6-methylguanine, O4-methylthymidine, and N-7-methyl guanine were measured in the livers of Fischer 344 rats following trea tment with 1,2-dimethylhydrazine (20 mg/kg, s.c.) alone or in combinat ion with the 06-alkylguanine transferase inhibitor O6-benzylguanine (1 00 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 O6-benzylguanine co mpletely eliminated detectable alkyltransferase activity in the livers of treated rats. Adducts in liver DNA were quantitated by high perfor mance liquid chromatography separation followed by fluorescence detect ion, UV absorbance, and/or specific radioimmunological assays. In anim als exposed to 1,2-dimethylhydrazine alone O6-methylguanine concentrat ions declined rapidly, whereas animals exposed to both O6-benzylguanin e and 1,2-dimethylhydrazine showed less removal of O6-methylguanine, w ith significant differences between the two populations appearing at 3 6 and 48 h. O4-Methylthymidine removal also differed significantly bet ween the two groups, with O6-benzylguanine-treated animals exhibiting higher concentrations of adducts at 36 and 48 h. O6-Benzylguanine trea tment had no effect on the removal of N-7-methylguanine. These results show that the rate of disappearance of both O6-methylguanine and O4-m ethylthymidine is slower following alkyltransferase depletion, suggest ing that mammalian alkyltransferase is involved in the removal of O4-m ethulthymidine lesions as well as O6-methylguanine lesions.