Activation of 5’-Deoxy-5-fluorouridine by Thymidine Phosphorylase in Human Tumors

Activities of pyrimidine nucleoside phosphorylases were assayed in extracts of human tumors, normal tissues of the same organs and tumors of mice Sarcoma-180) and guinea pigs Line-10), with thymidine dThd), uridine Urd), and 5’-deoxy-5-fluorouridine 5’-DFUR) as substrates. The nucleoside cleaving activities were higher in extracts of human tumor tissues than in those of normal tissues of the same organs. In human tissues, phosphorolytic activitiy towards dThd was high, while that towards Urd was low. In animal tumors, Urd was the best substrate. 1-2’-Deoxy-β-D-glucopyranosyl)-thymine GPT), a specific inhibitor of uridine phosphorylase, inhibited the phosphorolysis of Urd and 5’-DFUR in extracts of animal tumors, but not that of dThd and 5’-DFUR in extracts of human tumors. A thymidine phosphorylase preparation was partialy purified from human lung cancer. K_mvalues of the preparation were 2.43X10^-4M and 1.69X10^-3M for dThd and 5’-DFUR, respectively. We conclude that in human tumors a thymidine phosphorylase activity converts 5’-DFUR to 5-fluorouracil, an activated form.