Determination of N7- and O6-methylguanine in Rat Liver DNA after Oral Exposure to Hydrazine by Use of Immunochemical and Electrochemical Detection Methods

Hydrazine belongs to a group of compounds for which there is evidence that the in vivo genotoxic effects become manifest only upon exposure to toxic dose levels. The present study was performed to investigate whether this phenomenon is also reflected in the pattern of DNA methylation. The induction of N7- and O6-methylguanine (MeGua) was studied in liver DNA of rats, 16 hr after treatment with various doses of hydrazine. After DNA isolation, the presence of N7-MeGua in DNA was assessed with an immunochemical method and with a physicochemical technique (HPLC with electrochemical detection). Application of these two methods resulted in almost identical patterns of dose-dependent induction of guanine N7-methylation in rats dosed orally with 0.1 to 10 mg hydrazine per kilogram of body weight, increasing from 1.1-1.3 to 39-45 N7-MeGua per 10(6) nucleotides. At lower dosages a constant adduct level was observed, equivalent to that in untreated rats (background level). The O6-MeGua level was analyzed by a combination of HPLC separation and competitive radioimmunoassay. A background level was observed for untreated rats and no increase was visible up to the 0.2 mg/kg dose group. After hydrazine doses from 0.2 to 10 mg/kg, O6-MeGua increased from 0.29 to 134 per 10(9) nucleotides. These data show that even at dosages below the maximum tolerated dose (0.6 mg/kg/day), for which carcinogenic effects have not been described, DNA adducts are formed. A comparison is made of the data obtained in this study with models that describe the mechanism of hydrazine-induced DNA methylation.