S-Adenosylmethionine metabolism and DNA methylation in hydrazine-treated rats

The treatment of rats with hepatotoxic doses of hydrazine (NH₂-NH₂)induces the rapid formation of 7-methylguanine and O⁶-methylguaninein liver DNA. The methyl moiety in these reactions might bederived from the cellular S-adenosyl-methionine pool becauseradioactivity administered to these rats as methionine rapidlyappears in the DNA as methylated guanine. An increased incorporationof radioactivity into 5-methylcytosine was previously reportedfollowed by subsequent suppression. This increased radiolabelingof 5-methylcytosine coincided with time of maximal DNA guaninemethylation. To determine the nature of S-adenosyl-methioninemetabolism during the period of DNA methylation induced by hydrazinetreatment, and to determine if the increased radiolabeling of5-methylcytosine at this time reflected an actual increase in5-methylcytosine synthesis, liver DNA synthesis and S-adenosylmethioninelevels and turnover were assayed. Liver S-adenosylmethionineconcentrations varied slightly between control rats and hydrazine-treatedrats during the first five hours after hydrazine ad ministration,and no difference was detectable in the incorporation of administered[³H]methionine into S-adenosylmethionine. Because S-adenosylmethioninespecific radio activity in hydrazine-treated rats was not differentfrom control rats, the previously observed increased radiolabelingof 5-methylcytosine appeared to represent an actual increasein synthesis. This conclusion was supported by finding thatin corporation of radioactive thymidine into DNA was also acceleratedimmediately following hydrazine administration, again followedby a decrease. 5-Methylcytosine synthesis, therefore, appearsto follow DNA synthesis during hydrazine toxicity, and formationof 7-methylguanine and O⁶-methylguanine in liver DNA of hydrazine-treatedrats occurs during a short period of increased DNA synthesisand 5-methylcytosine formation very early in hydrazine toxicity.