STEREOCHEMISTRY OF THE IN-VITRO AND IN-VIVO METHYLATION OF DNA BY (R)-N-[H-2(1),H-3]METHYL-N-NITROSOUREA AND (S)-N-[H-2(1),H-3]METHYL-N-NITROSOUREA AND (R)-N-NITROSO-N-[H-2(1),H-3]METHYL-N-METHYLAMINE AND (S)-N-NITROSO-N-[H-2(1),H-3]METHYL-N-METHYLAMINE

Reaction of DNA with the carcinogens N-methyl-N-nitrosourea and N-nitr oso-N,N-dimethylamine produces several methylated species including th e premutagenic O-6-methylguanine. The mechanism of methylation is beli eved to be through a methanediazonium ion. We have studied the mechani sm of methylation of DNA by these carcinogens by analyzing the stereoc hemistry of the methyl transfer. DNA was methylated in vitro by (R)- a nd (S)-N-[H-2(1),H-3]methyl-N-nitrosourea and in vivo by (R)- and (S)- N-[H-2(1),H-3]methyl-N-methyl-N-nitrosamine and (R)-and (S)-N-[H-2(1), H-3]methyl-N-nitrosourea. 7-Methylguanine, 3-methyladenine, O-6-methyl guanine, and the methylated phosphate backbone were isolated. The meth yl groups were converted into acetic acid, and the stereochemistry was analyzed. The identity of the nucleophile did not influence the stere ochemistry of the methylation reaction. It was found that the methyl g roup was transferred with an average of 73% inversion and 27% retentio n of configuration. The most likely mechanism for the retention of con figuration is through multiple methylation events in which nucleophile s which initially react with the methanediazonium ion react as electro philes with DNA.