On the mechanism of chemical and enzymatic reduction of folate and dihydrofolate.

The incorporation of tritium from tritiated water, which occurs during the chemical reduction of folate to dihydrofolate, folate to tetrahydrofolate, and dihydrofolate to tetrahydrofolate, has been determined. Also, the transfer of tritium from enzymatically reduced triphosphopyridine nucleotide during the enzymatic reduction of dihydrofolate to tetrahydrofolate has been investigated. By degradation of the reaction products it was established that in all of these reactions tritium was most probably incorporated only into position 7 of the pteridine ring, suggesting that the reduction of dihydrofolate to tetrahydrofolate is preceded by intramolecular rearrangement. This was confirmed by experiments in which dihydrofolate labeled with deuterium in position 7 was reduced enzymatically or chemically to tetrahydrofolate.