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. Author manuscript; available in PMC: 2013 Sep 19.
Published in final edited form as: Hematol Oncol Clin North Am. 2012 Jun;26(3):629–ix. doi: 10.1016/j.hoc.2012.02.002

Figure 1.

Figure 1

The structures of folic acid, dihydrofolate, tetrahydrofolate, and 5-methyltetrahydrofolate. Folic acid is not a physiological folate but is an important source of folate in foods and vitamins. Within cells folic acid is reduced to dihydrofolate by dihydrofolate reductase (DHFR-see Figure 2), albeit at a very slow rate since it is a very poor substrate for this enzyme. Dihydrofolate is the major oxidized form of folates within cells, and the preferred substrate for DHFR, mediating the formation of tetrahydrofolate that goes on to form a variety of tetrahydrofolate cofactors. Seen here is the major dietary folate, and the major folate in the blood, 5-methyltetrahydrofolate.