This disclosure provides methods for catalyzing the release of ADP-ribose from poly(ADP-ribose) or O-acetyl-ADP-ribose. Also provided are methods for modifying DNA repair or chromatin structure by introducing into the cell an agent that modifies the activity of an ARH3 polypeptide, or variant or fragment thereof. Further provided are methods for screening molecules involved in the poly(ADP-ribosyl)ation of proteins or O-acetyl-ADP-ribose content, and method for treating disorders by altering activity of an ARH3 protein.