DISSECTION OF ADP-RIBOSE POLYMER SYNTHESIS INTO INDIVIDUAL STEPS OF INITIATION, ELONGATION, AND BRANCHING

We have studied the automodification reaction of poly(ADP-ribose)polymerase (PARP) (EC 2.4.2.30). The individual reactions of initiation, elongation, and branching catalyzed by this enzyme have been dissected out by manipulating the concentration of beta NAD, the ADP-ribosylation substrate. While PARP-mono(ADP-ribose) conjugates were the predominant products of automodification at 200 nM NAD (initiation), highly branched and complex polymers were synthesized at 200 mu M NAD (polymerization). Initial rates of automodification increased with second order kinetics as a function of the enzyme concentration at both 200 nM and 200 mu M NAD. These results are consistent with the conclusion that two molecules of PARP are required for ADP-ribose polymer synthesis during enzyme automodification. Thus, the auto-poly(ADP-ribosyl)ation reaction of PARP is intermolecular. In agreement with this notion, we observed that initial rates of the initiation reaction with 3'-deoxyNAD as a substrate also increased with the square of the enzyme concentration. In addition, the auto-poly(ADP-ribosyl)ation reaction of PARP increased with second order kinetics as a function of the NAD concentration at nanomolar levels (0.2-1.6 mu M). Therefore, the dimeric structure of PARP also requires two molecules of bound NAD for efficient ADP-ribose polymerization.