Pyrimidine nucleoside phosphorylase activity in tumour and matched normal gastrointestinal mucosa.

5-Fluorouracil (5-FU) requires activation to the metabolites FdUMP and FUTP for its cytotoxic effect. This activation involves the intracellular enzymes uridine and thymidine phosphorylase. We have assayed the levels of these enzymes in colorectal and gastric cancers and have shown that, in the majority of cases, the enzyme levels were higher than in the adjacent normal mucosa. It is suggested that, while the ratio tumour/normal mucosa enzyme activity may give some indication of the relative toxicity of 5-FU in these tissues, the absolute activities of the phosphorylases in tumour tissue could give a better indication of tumour responsiveness.