I. Thymidine Phosphorylase and Nucleoside Deoxyribosyltransferase in Normal and Malignant Tissues*

Nucleoside : purine (pyrimidine) deoxyribosyltransferase activity (EC 2.4.2.6.) has been demonstrated in bacteria by a number of investigators (l-5). Similar activity in mammalian tissues was reported by deVerdier and Potter (6). With the mammalian enzyme, however, the substrate specificity of the transfer reaction appeared to be restricted to pyrimidine bases and their deoxyribonucleosides. Transferase activity was demonstrated in normal rat liver but not in the Novikoff hepatoma or a number of other rat tissues. Since the Novikoff hepatoma has thymidine phosphorylase activity (EC 2.4.2.4. ) which catalyzes Reaction 1, the absence of Reaction 2, catalyzed by the transferase, was not expected since reversal of phosphorolysis should theoretically result in equilibration between the labeled pyrimidine base and the pyrimidine of the nucleoside.