Analysis of DNA-phosphate adducts in vitro using miniaturized LC-ESI-MS/MS and column switching: Phosphotriesters and alkyl cobalamins

DNA-phosphate adducts are known to be formed by a variety of alkylating agents. Due to little or no repair of DNA-phosphate adducts, these adducts may offer increased possibilities of both identifying and quantifying DNA adducts. The formation of DNA-phosphate adducts leads to a complete esterification of the phosphate group giving rise to a phosphotriester configuration. This work consists of the characterization of ethyl phosphotriesters (Ethyl PTE) using miniaturized LC-ESI-MS/MS and column switching in enzymatic hydrolysate of DNA treated in vitro with the model compound N-ethyl-N-nitrosourea (ENU). In vitro ENU-treated DNA was enzymatically degraded using nuclease P1, phosphodiesterase, and alkaline phosphatase. The use of column switch allowed for large-volume injections, where unmodified nucleosides were discarded in the loading step. The analytes were forward flushed to the analytical column in the eluting step and separated using a linear gradient. Ten different ethyl PTEs (dGpEtdG, dApEtdA, dCpEtdC, TpEtT, dGpEtdA, dGpEtdC, dGpEtT, dApEtdC, dApEtT, and dCpEtT) were characterized by their masses and CAD product ion spectra. Measurements of accurate masses were carried out yielding experimental masses within 5 ppm of the calculated masses for 9 of the 10 ethyl PTEs. For comparison, the enzymatic hydrolysate of ENU-treated DNA was subjected to transalkylation of the DNA-phosphate adducts by cob(I)alamin. Formed ethyl-cobalamins were analyzed according to earlier developed methods. The limit of detection of an alkyl-cobalamin standard and an alkyl PTE standard was 2 fmol and 5 fmol, respectively.