The activities of enzymes responsible for activating 5-fluorouracil (FUra) to 5-fluorouridine-5′-monophosphate (FUMP) were compared in normal and tumor tissues of rodents to assess the potential capacity of uridine phosphorylase to anabolize FUra to the nucleoside in the presence of ribose-1-phosphate (R-1-P). The activity of the alternative pathway to FUMP with a pyrimidine phosphoribosyltransferase [FUra + 1-pyrophosphoribosyl-5-phosphate (PPRP)] was approximately 15 to 17 nmoles/mg protein/hr in bone marrow from mice and rats and ranged from 28 to 47 nmoles/mg protein/hr tumor tissues. Uridine phosphorylase [measured as the formation of 5-fluorouridine (FUrd) from FUra and R-1-P] was 35-230 nmoles/mg/hr in bone marrow and in two FUra-sensitive solid tumors, colon tumor No. 38 in mice and RPMI colon tumor in rats; the activity of uridine phosphorylase from L5178Y ascites leukemic cells was notably lower, 8 nmoles/mg/hr. Levels of uridine kinase ranged from 55 to 187 nmoles/mg protein/hr. Thus, the activities of the enzymes of the two-step FUra activating pathway were high compared to the PPRP-dependent activity in all tissues except L5178Y; also, the FUra-sensitive tumors yielded extracts with 1.5 to 6.5 times greater enzyme activity than the corresponding activity in bone marrow. Uridine phosphorylase was partially purified from rat liver, RPMI rat tumor and colon tumor No. 38; the apparent Km of FUra averaged 50 μM, almost 9-fold lower than that of uracil, and the apparent Km of R-1-P for condensation with FUra was 33 μM. The tissue concentration of R-1-P was >70 μM in kidney and liver of rodents and somewhat less in spleen. Colon tumor No. 38 and RPMI colon tumor had 12 and 20 μM R-1-P, respectively, but these low values may reflect low tumor viability. The high levels of uridine phosphorylase and uridine kinase activities in normal tissues and even higher levels in tissues from FUra-sensitive tumors, as well as the sufficient concentration of R-1-P relative to its kinetic constant, suggest that FUra metabolism by the two-step pathway to FUMP may be a significant factor in the activity and selectivity of FUra.
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