A novel human phosphotransferase highly specific for adenosine.

A novel nucleoside phosphotransferase, referred to as adenosine phosphotransferase (Ado Ptase), was partially purified 1230-fold from human placenta. This enzyme differed from other known nucleoside phosphotransferases in its substrate specificity. Using AMP as the phosphate donor, it readily phosphorylated Ado. Changes in the sugar moiety were tolerated. dAdo and ddAdo were phosphate acceptors and dAMP was a donor. No other nucleotide or nucleoside common in nature displayed appreciable activity as donor or acceptor substrate, respectively. In the absence of nucleoside, the enzyme catalyzed the hydrolysis of AMP, typical of other nucleoside phosphotransferases. However, in the presence of Ado, little, if any, hydrolysis occurred. Ado Ptase had an absolute requirement for a metal cation, with Mg2+ and, to a lesser extent, Mn2+ fulfilling this requisite. The apparent Km for Ado was 0.2 mM. However, the donor AMP displayed cooperativity in both transfer and hydrolytic reactions. This cooperativity was eliminated by nucleotides, 2,3-diphosphoglycerate, and inorganic phosphate. ADP and 2,3-diphosphoglycerate were especially potent. In the presence of these effectors, the apparent Km for AMP was 3.0 mM in the transfer reaction and 4.0 mM in the hydrolytic reaction. Kinetic data suggest that there are two nucleotide binding sites on Ado Ptase, one for the donor, the other for an effector. AMP appeared to bind to both sites. Although this novel enzyme might play a role in the anabolism of nucleoside analogues, the normal physiological role of this nucleoside phosphotransferase is not understood.