Substrate Specificity of a Thymidine Phosphorylase in Human Liver Tumor

A thymidine phosphorylase preparation was partially purified from human liver tumor tissues (poorly differentiated adenocarcinoma). The substrate specificity of the enzyme was investigated with eleven pyrimidine nucleosides. Thymidine and 2`-deoxyuridine were good substrates, while uridine, 3’-deoxyuridine, 5’-deoxyuridine and 2’,3’-dideoxy-3’-hydroxy-methyluridine were not. Uridines substituted at the 5-position by a cyano, bromo, or chloro group were also phosphorolyzed by the enzyme, but the activity for 5-fluorouridine was much lower. 5’-Deoxy-5-fluorouridine was also cleaved. Either a 5-substituent or a 2’-deoxy structure seems to be essential for a good substrate.