Radiation-induced formation of purine 5',8-cyclonucleosides in isolated and cellular DNA: high stereospecificity and modulating effect of oxygen

The present work is aimed at gaining conclusive mechanistic insights into the radiation-induced formation of the 5?R and 5?S diastereomers of both adenine and guanine 5?,8-cyclo-2?-deoxyribonucleosides, with emphasis on the delineation of the inhibitory effect of O2 in isolated and cellular DNA. The levels of purine 5?,8-cyclo-2?- deoxyribonucleosides as assessed by HPLC-MS/MS were found to decrease steadily with the increase of O2 concentration, the 5?,8-cyclo-2?- deoxyguanosine being produced more efficiently than the 5?,8-cyclo- 2?-deoxyadenosine for low O2 concentrations. A high stereoselectivity was observed in the intramolecular addition of the C5? radical to the C8 of the purine leading, after the creation of the C5?-C8 bond and a subsequent oxidation step, to the predominant formation of the 5?R diastereomer for both purine 5?,8-cyclonucleosides. The reduced formation yield of the 4 tandem lesions in the presence of O2 explains, at least partly, the low efficiency of radiation-induced yields of the purine 5?,8-cyclo-2?-deoxyribonucleosides in cellular DNA, which are about two orders of magnitude lower than the previously reported data obtained from HPLC-MS analysis.