Two distinct human DNA diesterases that hydrolyze 3′-blocking deoxyribose fragments from oxidized DNA

Mammalian cells were investigated for enzymes that help correct oxidative damages in DNA. We focused on 3'-repair diesterases, which process DNA ends at oxidative strand breaks by removing 3′-blocking fragments of deoxyribose that prevent DNA repair synthesis. Two enzymes were found in a variety of mouse, bovine and human tissues and cultured cells. The two activities were purified to differing degrees from HeLa cells. One enzyme had the properties of the known HeLa AP endonuclease (M_r -38,000, with identical substrate specificity and reaction requirements, and cross-reactivity with anti-HeLa AP endonuclease antiserum) and is presumed identical to that protein. The second activity did not interact with anti-HeLa AP endonuclease antibodies and had relatively less AP endonuclease activity. This second enzyme may have been detected in other studies but never characterized. In addition to the 3′-repair diesterase and AP endonuclease, this partially purified preparation also harbored DNA 3′-phosphatase and 3′-deoxyribose diesterase activities. It is unknown whether all activities detected in the second preparation are due to a single protein, although activity against undamaged DNA was not detected. The in vivo roles of these two widely distributed 3′-repair diesterase/AP endonucleases have not been determined, but with the haracterizations presented here such questions may now be focused.