Direct experimental evidence for competitive inhibition of dihydrofolate reductase by methotrexate.

Steady-state enzyme kinetic techniques at very low enzyme concentration (0.4 nM) were used successfully to measure the inhibition constant (53 pM) for the dissociation of methotrexate from the ternary complex of methotrexate, NADPH and dihydrofolate reductase from Lactobacillus casei; and to demonstrate unequivocally that the inhibition was, indeed, competitive with respect to dihydrofolate.