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 in
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 microM, almost 9-fold lower than that of
uracil, and the apparent Km of R-1-P for condensation with FUra was 33 microM. The tissue concentration of R-1-P was greater than 70 microM in kidney and liver of rodents and somewhat less in spleen. Colon
tumor No. 38 and RPMI colon
tumor had 12 and 20 microM 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.