Spratt T E, Zydowsky T M, Floss H G
Department of Chemistry, Ohio State University, Columbus 43210, USA.
Chem Res Toxicol. 1997 Dec;10(12):1412-9. doi: 10.1021/tx970097b.
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.
DNA与致癌物N-甲基-N-亚硝基脲和N-亚硝基-N,N-二甲基胺反应会产生多种甲基化产物，包括致突变前体O6-甲基鸟嘌呤。甲基化机制被认为是通过甲二铵离子进行的。我们通过分析甲基转移的立体化学研究了这些致癌物对DNA的甲基化机制。DNA在体外被（R）-和（S）-N-[2H1,3H]甲基-N-亚硝基脲甲基化，在体内被（R）-和（S）-N-[2H1,3H]甲基-N-甲基-N-亚硝胺以及（R）-和（S）-N-[2H1,3H]甲基-N-亚硝基脲甲基化。分离出了7-甲基鸟嘌呤、3-甲基腺嘌呤、O6-甲基鸟嘌呤以及甲基化的磷酸骨架。甲基被转化为乙酸，并对立体化学进行了分析。亲核试剂的身份不影响甲基化反应的立体化学。结果发现，甲基转移时平均73%发生构型翻转，27%构型保持。构型保持最可能的机制是通过多次甲基化事件，其中最初与甲二铵离子反应的亲核试剂会作为亲电试剂与DNA反应。
