THE BIFUNCTIONAL CYTOSOLIC 5'-NUCLEOTIDASE - REGULATION OF THE PHOSPHOTRANSFERASE AND NUCLEOTIDASE ACTIVITIES

The cytosolic 5'-nucleotidase specific for IMP, GMP, and their deoxyde rivatives has been purified approximately 1000 times from calf thymus. The enzyme, in the presence of a suitable nucleoside, can act as a ph osphotransferase, catalyzing the transfer of the phosphate moiety from a nucleoside monophosphate donor to a nucleoside acceptor, thus opera ting as an interconverting activity. This phosphorylating activity has drawn the attention of several research groups because the cytosolic 5'-nucleotidase represents the only cellular enzyme able to phosphoryl ate inosine and guanosine analogs, which are not substrates of known c ellular nucleoside kinases. In this paper, we report the kinetic param eters of the bifunctional enzyme and its response to variations in ade nylate energy charge. The results seem to indicate that in the presenc e of physiological concentrations of ATP and phosphate, the enzyme beh aves mainly as a phosphotransferase, its activity being dependent only on the availability of a suitable nucleoside. (C) 1994 Academic Press , Inc.