Distinct roles for recombinant cytosolic 5 '-nucleotidase-I and -II in AMPand IMP catabolism in COS-7 and H9c2 rat myoblast cell lines

Catabolism of AMP during ATP breakdown produces adenosine, which restores e nergy balance. Catabolism of IMP may be a key step regulating purine nucleo tide pools. Two, cloned cytosolic 5'-nucleotidases (cN-I and cN-II) have be en implicated in AMP and IMP breakdown. To evaluate their roles directly, w e expressed recombinant pigeon cN-I or human cN-II at similar activities in COS-7 or H9c2 cells. During rapid (more than 90% in 10 min) or slower (30- 40% in 10 min) ATP catabolism, cN-I-transfected COS-7 and H9c2 cells produc ed significantly more adenosine than cN-II-transfected cells, which were si milar to control-transfected cells, Inosine and hypoxanthine concentrations increased only during slower ATP catabolism. In COS-7 cells, 5'-nucleotida se activity was not rate-limiting for inosine and hypoxanthine production, which was therefore unaffected by cN-II- and actually reduced by cN-I- over expression. In H9c2 cells, in which 5'-nucleotidase activity was rate-limit ing, only cN-II overexpression accelerated inosine and hypoxanthine formati on. Guanosine formation from GMP was also increased by cN-II, Our results i mply distinct roles for cN-I and cN-II, Under the conditions tested in thes e cells, only cN-I plays a significant role in AMP breakdown to adenosine, whereas only cN-II breaks down IMP to inosine and GMP to guanosine.