Interaction of thymidylate synthetase and dihydrofolate reductase enzymes in vitro and in vivo.

Summary Thymidylate synthetase and dihydrofolate reductase modify each other9s activities in vitro . Folate, dihydrofolate, and methotrexate inhibit thymidylate synthetase in the absence of dihydrofolate reductase, but dihydrofolate and methotrexate appear to prevent the interaction between the two enzymes, thus increasing the activity of thymidylate synthetase. Dihydrofolate reductase, inhibited by thymidylate synthetase, is also reactivated by deoxyuridine 5′-monophosphate, a substrate of thymidylate synthetase. The former behavior seems significant in vivo , but not the latter. Crude extracts of methotrexate-resistant L1210 cells, which have high dihydrofolate reductase activity and low thymidylate synthetase activity, show increased activity of thymidylate synthetase in the presence of methotrexate. With intact cells, the rate of incorporation of deoxyuridine into DNA is enhanced in the presence of low concentrations of methotrexate. Wild-type L1210 cells show neither effect. Methotrexate may thus increase thymidylate synthetase activity in cells possessing relatively large amounts of dihydrofolate reductase and small amounts of thymidylate synthetase.