MODE OF ACTION OF POLY(ADP-RIBOSE) GLYCOHYDROLASE

The turnover of the homopolymer of ADP-ribose, which is known to be in volved in many DNA-related functions, is controlled by 2 principal enz ymes. Poly(ADP-ribose) polymerase (EC 2.4.2.30) synthesizes the polyme r from NAD, and poly(ADP-ribose) glycohydrolase (PARG) is the major en zyme responsible for its catabolism (Thomassin et al. (1992) Biochim. Biophys. Acta 1137, 171-181). In vivo, poly(ADP-ribose) polymers const itute a heterogeneous population of branched polymers attaining sizes of 200-400 residues. They are rapidly degraded by PARG, displaying var iable kinetic parameters as a function of polymer size. Several studie s have suggested that PARG acts exoglycosidically on its substrate but others observed that it could act endo/exo-glycosidically. We analyse d the mode of action of PARG under conditions most suitable for expres sion of all the activities of PARG, using HPLC purified long free poly mer and very pure PARG. We conclusively show that on large free polyme rs, PARG exhibits endoglycosidic activity along with exoglycosidic act ivity. This endoglycosidic activity could have a significant role duri ng cellular response to DNA damage.