Reduced Nicotinamide Adenine Dinucleotide Phosphate-Dependent Binding of Competitive Inhibitors to Dihydrofolate Reductase

The enzyme dihydrofolate reductase (EC 1.5.1.3), which catalyzes the reduced pyridine nucleotide-dependent reduction of dihydrofolate to tetrahydrofolate, is inhibited by 6- N -ω-( N -ethyl- N -2-chloroethyl)propyl-2,4,6-triamino-5-(dichlorophenylazo)pyrimidines acting as active site-directed irreversible inhibitors. A requirement for the binding of these inhibitors to the enzyme prior to alkylation is the presence of NADPH. Apparently the binding of NADPH to the enzyme causes a conformational change in the protein that makes the substrate-binding site accessible to either the substrate or a competitive inhibitor.