Methylation of RNA purine bases by methyl radicals.

During the metabolism of 1,2-dimethylhydrazine (a colon carcinogen) in rats, methyl radicals (.CH3) are produced. To gain an insight into the nature of .CH3 interactions with biomolecules in vivo, we conducted in vitro studies using RNA as a model compound. RNA was incubated in a system where .CH3 was generated from the oxidation of dimethyl sulfoxide by hydroxyl radicals (.OH); .OH was produced from a Fenton-type reaction between Fe(2+)-EDTA and H2O2. Four new products were detected from acid hydrolysates of .CH3-treated RNA: 8-methylguanine, 2-methyladenine, 8-methyladenine, and a highly unstable compound of unknown structure. The production was significantly affected by pH. The concentrations of Fe2+ and H2O2 influenced the production of methylated purine residues in a dose-dependent manner. Catalase, ethanol, alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone, and O2 inhibited the production of methylated nucleobases.