Tiazofurin is phosphorylated by three enzymes from Chinese hamster ovary cells

The growth inhibitory activity of tiazofurin toward adenosine kinase deficient Chinese hamster ovary (CHO) cells was partially reversed by the presence of nicotinamide riboside. Similarly, the formation of tiazofurin 5'-monophosphate and the active metabolite, tiazofurin 5'-adenine dinucleotide could be partially inhibited by 100 microM nicotinamide riboside in CHO cells and substantially inhibited (80-90%) in adenosine kinase deficient cells. Tiazofurin phosphorylating activity from CHO cell extracts was resolved into two peaks by DEAE-cellulose chromatography. The first peak of activity was identified as adenosine kinase (ATP:adenosine 5'-phosphotransferase, EC 2.7.1.20). The second peak of activity correlated with a previously described 3-deazaguanosine phosphorylating activity that was identified as a nicotinamide ribonucleoside kinase. Contaminating purine nucleoside phosphorylase was removed by sedimentation through a sucrose density gradient which also resolved the tiazofurin phosphorylating activity into two peaks, one requiring just ATP and the other requiring both ATP and IMP. Of the substrates tested with the lower density peak, nicotinamide riboside was most efficient and was the only natural substance that competed well with tiazofurin for phosphorylation, substantiating its suggested identity as a nicotinamide ribonucleoside kinase. The apparent Km value for nicotinamide riboside (2 microM) was significantly less than that for tiazofurin (13.6 microM). ATP was the best phosphate donor; CTP and UTP were utilized less efficiently and IMP did not support the reaction. The best substrate for the higher density peak of tiazofurin phosphorylation was inosine and both ATP and IMP were required for the reaction, suggesting its identity as a 5'-nucleotidase. In summary, it appears that adenosine kinase, nicotinamide ribonucleoside kinase, and 5'-nucleotidase may all contribute to the phosphorylation of tiazofurin in CHO cells.