INHIBITION OF MAMMALIAN THYMIDYLATE SYNTHASE BY 10-FORMYLTETRAHYDROPTEROYLPOLYGLUTAMATE

Reduced derivatives of 10-formylfolate have been evaluated as inhibitors of mammalian thymidylate synthase (EC 2.1.1.45) from H35 hepatoma cells. With 5,10-methylenetetrahydrofolylheptaglutamate as the substrate, 10-formyltetrahydrofolylmonoglutamate is a competitive inhibitor with a ki of 2.4 μm, which is reduced to 0.1 μm for the heptaglutamate derivative. 10-Formyldihydrofolylmono- and -heptaglutamate are approximately threefold less inhibitory than the tetrahydro analog. The concentrations of 10-formyltetrahydrofolate and 10-formyldihydrofolate were measured in dividing hepatoma cells by a novel enzymatic assay and were found to be 5 μm and undetectable, respectively. These results suggest that the concentration of 10-formyltetrahydrofolate within the dividing cells has the potential to severely inhibit or modulate thymidylate biosynthesis. © 1991.