Evidence for a single enzyme reducing folate and dihydrofolate.

each step being catalyzed by a separate enzyme, El (folic acid reductase) and Et (dihydrofolic acid reductase) (2). The purification of dihydrofolic acid reductase from chicken liver (3) and from sheep liver (4) has been reported. In each case the final preparation reduced only dihydrofolate; no mention was made of the distribution in different fractions of folic acid reductase which was presumably present in the starting material. In this laboratory all attempts to resolve the system reducing folate to folate-H4 into its components, E1 and Ez, were unsuccessful. Moreover, evidence has accumulated that both reactions (reduction of folate and folate-Hz) are carried out by the same enzyme (5). The basis for differences between these results and those of earlier investigations (2-4) is discussed.