5 '-nicked apurinic/apyrimidinic sites are resistant to beta-elimination by beta-polymerase and are persistent in human cultured cells after oxidative stress

Genomic DNA is continuously exposed to oxidative stress. Whereas reactive o xygen species (ROS) preferentially react with bases in DNA, free radicals a lso abstract hydrogen atoms from deoxyribose, resulting in the formation of apurinic/apyrimidinic (AP) sites and strand breaks. We recently reported h igh steady-state levels of AP sites in rat tissues and human liver DNA (Nak amura, J., and Swenberg, J. A. (1999) Cancer Res. 59, 2522-2526), These AP sites were predominantly cleaved 5' to the lesion. We hypothesized that the se endogenous AP sites were derived from oxidative stress. In this investig ation, AP sites induced by ROS were quantitated and characterized. A combin ation of H2O2 and FeSO4 induced significant numbers of AP sites in calf thy mus DNA, which were predominantly cleaved 5' to the AP sites (75%) of total aldehydic AP sites). An increase in the number of 5'-AP sites was also det ected in human cultured cells exposed to H2O2, and these 5'-AP sites were p ersistent during the post-exposure period. beta-Elimination by DNA beta-pol ymerase efficiently excised 5'-regular AP sites, but not 5'-AP sites, in DN A from cells exposed to H2O2. These results suggest that 5'-oxidized AP sit es induced by ROS are not efficiently repaired by the mammalian short patch base excision repair pathway.