Reaction of DNA with the carcinogens N-methyl-N-nitrosourea and N- nitroso-N,N-dimethylamine produces several methylated species including the premutagenic O6-methylguanine. The mechanism of methylation is believed to be through a methanediazonium ion. We have studied the mechanism of methylation of DNA by these carcinogens by analyzing the stereochemistry of the methyl transfer. DNA was methylated in vitro by (R)- and (S)-N- [2H1,3H]methyl-N-nitrosourea and in vivo by (R)- and (S)-N- [2H1,3H]methyl-N-methyl-N-nitrosamine and (R)-and (S)-N- [2H1,3H]methyl-N-nitrosourea. 7-Methylguanine, 3-methyladenine, O6- methylguanine, and the methylated phosphate backbone were isolated. The methyl groups were converted into acetic acid, and the stereochemistry was analyzed. The identity of the nucleophile did not influence the stereochemistry of the methylation reaction. It was found that the methyl group was transferred with an average of 73% inversion and 27% retention of configuration. The most likely mechanism for the retention of configuration is through multiple methylation events in which nucleophiles which initially react with the methanediazonium ion react as electrophiles with DNA.
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