Large-scale and small-scale methods for the preparation of 5,10-methylenetetrahydrofolate

Two procedures have been developed for the synthesis and isolation of 5,10-methylenetetrahydrofolate, the cofactor for the reaction catalyzed by thymidylate synthetase, one of which can be used for large-scale preparations of the cofactor and the other for small-scale syntheses especially suitable for obtaining the radiolabeled cofactor. The large-scale procedure involves treatment of folic acid with dithionite to give dihydrofolate, which is then converted to tetrahydrofolate by dihydrofolate reductase (L. casei). The small-scale method involves a direct enzymatic reduction of folic acid to tetrahydrofolate by dihydrofolate reductase, and has been used to prepare the double-labeled 5,10-[14C]methylene[3',5',7,9-3H]tetrahydrofolate. In both procedures, after the reduction steps have been performed, the tetrahydrofolate is treated in situ with formaldehyde prior to purification by DEAE-cellulose chromatography, thus allowing the isolation of 5,10-methylenetetrahydrofolate as a dry powder after lyophilization. This product is active in the enzyme reaction without the further addition of excess formaldehyde as in previous procedures. The cofactor prepared in this manner has much improved stability toward oxidation compared to free tetrahydrofolate.