The effects of secondary structure and O2 on the formation of direct strand breaks upon UV irradiation of 5-bromodeoxyuridine-containing oligonucleotides

Background: 5-Bromodeoxyuridine is a radiosensitizing agent that is currently being evaluated in clinical trials as an adjuvant in the treatment of a variety of cancers. gamma-Radiolysis and UV irradiation of oligonucleotides containing 5-bromodeoxyuridine result in the formation of direct strand breaks at the 5'-adjacent nucleotide by oxidation of the respective deoxyribose, We investigated the effects of DNA secondary structure and O-2 on the induction of direct strand breaks in 5-bromodeoxyuridine-containing oligonucleotides. Results: The efficiency of direct strand break formation in duplex DNA is dependent upon O-2 and results in fragments containing 3'-phosphate and the labile 3'-ketodeoxyadenosine termini, The ratio of the 3'-termini is also dependent upon O-2 and structure. Deuterium product isotope effects and tritium-transfer studies indicate that hydrogen-atom abstraction from the C1'- and C2'-positions occurs in an O-2- and structure-dependent manner. Conclusions: The reaction mechanisms by which DNA containing 5-bromodeoxyuridine is sensitized to damage by UV irradiation are dependent upon whether the substrate is hybridized and upon the presence or absence of O-2. Oxygen reduces the efficiency of direct strand break formation in duplex DNA, but does not affect the overall strand damage. It is proposed that the sigma radical abstracts hydrogen atoms from the C1'- and C2'-positions of the 5'-adjacent deoxyribose moiety, whereas the nucleobase peroxyl radical selectively abstracts the C1'-hydrogen atom from this site. This is the second example of DNA damage amplification by a nucleobase peroxyl radical, and might be indicative of a general reaction pattern for this family of reactive intermediates.