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Kinetic Studies on Phosphorylation of 5-Azacytidine with the Purified Uridine-Cytidine Kinase from Calf Thymus¹

Division of Hematology-Oncology, University of Southern California School of Medicine and Childrens Hospital of Los Angeles, Los Angeles, California 90027

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).

¹ This publication was supported in part by Grant CI-85-D from the American Cancer Society and NIH Grants CA 11050 and CA 14089.

² Scholar of Leukemia Society of America.

Received 3/19/74. Accepted 6/ 7/74.

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