Fig. 3. Biochemical properties of the N-terminal apoptotic fragment of PARP-1. The catalytic activity of native PARP-1 was determined in the presence of increasing amounts of the 24 kDa apoptotic fragment of PARP-1. The results are expressed as percentage of maximal activity obtained in the absence of apoptotic fragment. Experiments were performed in triplicate at subsaturating concentrations of DNA strand-breaks, as described in Materials and Methods.