No abstract

During the course of the study of 5′‐deoxy‐5‐fluorouridine (5′dFUrd) plasma protein binding using 19F NMR spectroscopy, phosphorolytic cleavage of 5′dFUrd into 5‐fluorouracil (5FU) was observed. This transformation was due to the enzymatic content of residual blood cells present in plasma, since the percentage conversion of 5′dFUrd into 5FU was lower as the number of residual blood cells fell, and an ‘acellular’ plasma or a serum effected a negligible phosphorolyis of 5′dFUrd. As platelets were the contaminants of the plasma samples studied, and a concentrate of human platelets demonstrated a high phosphorolytic activity towards 5′dFUrd, it was concluded that these blood cells were responsible for the 5′dFUrd cleavage. Thymidine phosphorylase, being the only pyrimidine nucleoside phosphorylase in human platelets, is suggested by the present results to catalyse 5′dFUrd phosphorolysis and is therefore not as specific for 2′‐deoxyribonucleosides as has been reported.