The effects of secondary structure and O-2 on the formation of direct strand breaks upon UV irradiation of 5-bromodeoxyuridine-containing oligonucleotides

Background: 5-Bromodeoxyuridine is a radiosensitizing agent that is current ly 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 br eaks at the 5'-adjacent nucleotide by oxidation of the respective deoxyribo se, We investigated the effects of DNA secondary structure and O-2 on the i nduction of direct strand breaks in 5-bromodeoxyuridine-containing oligonuc leotides. Results: The efficiency of direct strand break formation in duplex DNA is d ependent upon O-2 and results in fragments containing 3'-phosphate and the labile 3'-ketodeoxyadenosine termini, The ratio of the 3'-termini is also d ependent upon O-2 and structure. Deuterium product isotope effects and trit ium-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-bromodeoxyur idine 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 r adical abstracts hydrogen atoms from the C1'- and C2'-positions of the 5'-a djacent deoxyribose moiety, whereas the nucleobase peroxyl radical selectiv ely abstracts the C1'-hydrogen atom from this site. This is the second exam ple of DNA damage amplification by a nucleobase peroxyl radical, and might be indicative of a general reaction pattern for this family of reactive int ermediates.