Dihydropteridine reductase from Escherichia coli exhibits dihydrofolate reductase activity.

E. coli Dihydropteridine reductase, known to have a pterin-independent oxidoreductase activity with potassium ferricyanide as electron donor, has now been shown to possess also dihydrofolate reductase activity. The kinetic parameters for dihydrofolate reductase activity have been determined. The ratio of the three activities, dihydropteridine reductase, dihydrofolate reductase and pterin-independent oxidoreductase activity is 1.0, 0.05 and 4.3, respectively. The enzyme, a flavoprotein which is unstable in the presence of dithiothreitol, was shown to be a monomer with a molecular mass of 25.7 kDa. The apparent lack of discrimination between hydride transfer from the pyridine nucleotide to N5 of the pterin in the dihydropteridine reductase reaction and C6 of folate in the dihydrofolate reaction suggested that the FAD prosthetic group may be involved in the hydride transfers. The flavoprotein inhibitor N,N- dimethylpropargylamine inhibited the dihydropteridine reductase and oxidoreductase reactions differently and did not affect the dihydrofolate reductase activity however.