Abstracts Biological Sciences

Methotrexate inhibition of one carbon fragment metabolism in ascites tumor cells.

by John R. Perrone

One carbon transfer from N5,l0-methylenetetrahydrofolate to deoxyuridylate to form thymidylate leads to dihydrofolate formation, and depends upon dihydrofolate reductase to regenerate the active coenzyme. Methotrexate inhibits dihydrofolate reductase and consequently blocks one carbon fragment metabolism. In Ehrlich ascites cells methotrexate depresses the rate of formate-14C labelling of purine nucleotides, RNA and serine. Deoxyuridine and deoxycytidine, both of which can give rise to thymidylate and therefore lead to dihydrofolate formation, cause an increased inhibition of formate-14C metabolism by methotrexate. Addition of 5-fluorodeoxyuridine, which is phosphorylated to 5-fluorodeoxyuridine monophosphate an inhibitor of thymidylate synthetase, or addition of thymidine, reverse the inhibitory action of deoxyuridine plus methotrexate on one carbon fragment metabolism. Additional evidence suggests that the thymidylate synthetase reaction can regulate the intracellular level of dihydrofolate and hence regulate the effects of methotrexate on the one carbon fragment metabolism of ascites tumor cells in vitro.