Phosphorylation of uridine and cytidine nucleoside analogs by two human uridine-cytidine kinases

Uridine-cytidine kinases (UCK) have important roles for the phosphorylation of nucleoside analogs that are being investigated for possible use in chem otherapy of cancer. We have cloned the cDNA of two human UCKs. The approxim ate to 30- kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides. UCK1 mRNA was detected as two isoforms of approximate to1.8 and approximate to2 .7 kb. The 2.7-kb band was ubiquitously expressed in the investigated tissu es. The band of approximate to1.8 kb was present in skeletal muscle, heart, liver, and kidney. The two isoforms of UCK2 mRNA of 1.2 and 2.0 kb were on ly detected in placenta among the investigated tissues. The genes encoding UCK1 and UCK2 were mapped to chromosome 9q34.2-9q34.3 and 1q22-1q23.2, resp ectively. We tested 28 cytidine and uridine nucleoside analogs as possible substrates of the enzymes. The enzymes phosphorylated several of the analog s, such as 6-azauridine, 5-fluorouridine, 4-thiouridine, 5-bromouridine, N- 4-acetylcytidine, N-4-benzoylcytidine, 5-fluorocytidine, 2-thiocytidine, 5- methylcytidine, and N-4-anisoylcytidine. The cloning and recombinant expres sion of the two human UCKs will be important for development of novel pyrim idine ribonucleoside analogs and the characterization of their pharmacologi cal activation.