Which experimental result would best demonstrate that trimethoprim is a competitive inhibitor of dihydrofolate reductase (enzyme B)?

• A. Increasing dihydrofolic acid concentration in enzyme kinetics assays reduces the degree of inhibition by trimethoprim and increases apparent Km without changing Vmax.
• B. Trimethoprim increases the transcription of dihydrofolate reductase, and simultaneous addition of large substrate amounts enhances inhibition, demonstrating competitive binding.
• C. Trimethoprim permanently denatures dihydrofolate reductase, which can be reversed by dialysis, proving competitive reversible binding.
• D. Trimethoprim reduces Vmax but does not change Km in enzyme assays, which is the hallmark of competitive inhibition.

Answer: (A)