Kinetic studies on phosphorylation of 5-azacytidine with the purified uridine-cytidine kinase from calf thymus.

Summary Uridine-cytidine kinase was purified 330-fold from calf thymus. This enzyme catalyzes the phosphorylation of uridine, cytidine (CR), and the nucleoside analog, 5-azacytidine (5-aza-C) to their respective nucleoside 5′-monophosphates in the presence of Mg ++ and adenosine 5′-triphosphate. The K m values for CR uridine, and 5-aza-C were 40, 50, and 200 µm respectively. Uridine 5′-triphosphate or cytidine 5′-triphosphate inhibited the phosphorylation of these nucleosides; the inhibition was noncompetitive with respect to the nucleoside substrates and competitive with respect to adenosine 5′-triphosphate. CR and uridine were potent competitive inhibitors of 5-aza-C (K i values of 40 and 50 µm, respectively), whereas 5-aza-C was a weak competitive inhibitor of CR (K i value of 200 µm).