Postlabeling can be one of the most sensitive methods for the measurement of DNA adducts. However, for the determination of alkylated adducts, essential requirements are standards which must be fully chemically characterized. In order to develop a postlabeling assay for monitoring exposure to genotoxic ethylating agents, the reaction of diethyl sulfate with 2'-deoxynucleoside 3'- and 5'-monophosphates was examined. The adducts generated were fully characterized using HPLC, electrospray tandem mass spectrometry, UV, and postlabeling. The major product was the phosphodiester derived from alkylation of the phosphate, and alkylation of the base occurred to a lesser extent. The phosphodiester standard, 2'-deoxyguanosine 3'-(mono-O-ethyl phosphate) (3'Et-pdG), was used to develop a postlabeling assay for the detection of this adduct in DNA samples. Since alkylated phosphodiesters in DNA are not susceptible to the actions of micrococcal nuclease and calf spleen phosphodiesterase, they can be obtained as alkylated phosphodiester dinucleosides from DNA. Nuclease P1 was used as enhancement step which allowed the separation of these adducted phosphotriesters from the unmodified nucleotides by HPLC. Subsequent hydrolysis of the phosphotriester dinucleosides in alkali yielded phosphodiesters, including 3'Et-pdG, which was efficiently postlabeled. This approach was shown to be capable of detecting this adduct in liver DNA from mice treated intraperitoneally with N-nitrosodiethylamine.
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