Dihydrofolic Acid Reductase1

Abstract

THE enzymatic reduction of dihydrofolic acid (FH2) to tetrahydrofolic acid (FH4) has been reported by several investigators1–3. Futterman1 noted that dihydrofolic acid was reduced by chicken liver extracts when either reduced di- or tri-phospho-pyridine nucleotide served as the co-factor. We have found that the rate of reduction of dihydrofolic acid by reduced diphosphopyridine nucleotide in the presence of a partially purified sheep liver preparation proceeds most favourably at pH values less than 5.5, whereas reduction by reduced triphosphopyridine nucleotide is favoured at pH values greater than 6 (Fig. 1). The possibility that two enzymes are involved in the reduction of dihydrofolic acid was tested by noting the effect of reduced diphosphopyridine nucleotide on dihydrofolic acid-reductase which had been saturated with reduced triphosphopyridine nucleotide at pH 5.7, a pH at which both nucleotides were found to be equally effective electron donors. No increase in activity was observed, suggesting that only one enzyme is involved in the reduction. Reduced diphosphopyridine nucleotide also had no effect upon dihydrofolic acid-reductase which had been saturated with reduced triphosphopyridine nucleotide.