The metabolism of
5-fluorouracil (5-FU) was studied in biopsy specimens of primary
colorectal cancer and healthy colonic mucosa obtained from previously untreated patients immediately after surgical removal. The conversion of
5-FU to anabolites was measured under saturating substrate (5-FU) and cosubstrate concentrations. For all
enzymes, the activity was about threefold higher in
tumor tissue compared with healthy mucosa of the same patient. The activity of
pyrimidine nucleoside phosphorylase with deoxyribose-1-phosphate (dRib-1-P) was about tenfold higher (about 130 and 1200 nmol/hr/mg
protein in
tumors) than with ribose-1-phosphate (Rib-1-P), both in
tumor and mucosa. Synthesis of the active
nucleotides (5-fluoro-
uridine-5'-monophosphate [FUMP] and
5-fluoro-2'-deoxyuridine-5'-monophosphate [
FdUMP]) was studied by adding physiologic concentrations of
adenosine triphosphate (
ATP) to the reaction mixture; the rate of
FdUMP synthesis was 50% of that of FUMP (about 4 and 7 nmol/hr/mg
protein in
tumors). Direct synthesis of FUMP from
5-FU in the presence of 5-phosphoribosyl-1-pyrophosphate (PRPP) was about 2 nmol/hr/mg
protein. With the natural substrate for this reaction,
orotic acid, the activity was about 14-fold higher. To obtain insight into the recruitment of precursors for these cosubstrates, the authors also tested the
enzyme activity of
pyrimidine nucleoside phosphorylase with
inosine and
ribose-5-phosphate (Rib-5-P, as precursors for Rib-1-P) and
deoxyinosine (as a precursor for dRib-1-P);
enzyme activities were approximately 7%, 7%, and 3%, respectively, of that with the normal substrates, both in
tumors and mucosa. However, when
ATP and Rib-5-P were combined, the synthesis of FUMP was about 70% of that with PRPP, but only in
tumors. In normal tissues no activity was detectable. These data suggest a preference of colon
tumor over colon mucosa for the conversion of
5-FU to active
nucleotides by a direct pathway; a selective antitumor effect of
5-FU may be related to this difference.