5,10-Methylenetetrahydrofolate (N5,N10-Methylenetetrahydrofolate; 5,10-CH2-THF) is the substrate used by the enzyme methylenetetrahydrofolate reductase (MTHFR) to generate 5-methyltetrahydrofolate (5-MTHF, or levomefolic acid).
The diastereoisomer [6R]-5,10-methylene-THF is the key endogenous one-carbon donor and a co-substrate of the TS (thymidylate synthase) enzyme for methylation of 2-deoxy-uridine-5-monophosphate (dUMP) to 2-deoxy-thymidine-5-monophosphate (dTMP). The coenzyme is necessary for the biosynthesis of thymidine and is the C1-donor in the reactions catalyzed by TS and thymidylate synthase (FAD).
[6R]-5,10-methylene-THF is a biomodulator that has proven to enhance the desired cytotoxic antitumor effect of Fluorouracil (5-FU) and can bypass the metabolic pathway required by other folates (such as leucovorin) to achieve necessary activation. The active metabolite is being evaluated in clinical trials for patients with colorectal cancer in combination with 5-FU.
It also acts as a cofactor in the synthesis of serine from glycine via the enzyme serine hydroxymethyltransferase.