methyltetrahydrofolate

**Formation:** Dietary folate (polyglutamate) is deconjugated in the jejunum, absorbed, and reduced to dihydrofolate (DHF) and then tetrahydrofolate (THF) by dihydrofolate reductase (DHFR). THF is converted to 5,10-methylene-THF by serine hydroxymethyltransferase (B6-dependent). Methylenetetrahydrofolate reductase (MTHFR; requires FAD/riboflavin B2) irreversibly reduces 5,10-methylene-THF to 5-methyl-THF (5-MTHF), the main form in plasma.

**Key reaction:** Homocysteine + 5-MTHF → methionine + THF, catalyzed by methionine synthase (homocysteine methyltransferase), which requires methylcobalamin (vitamin B12) as a cofactor. Methionine is then converted to SAM (S-adenosylmethionine), the universal methyl donor.

**Methyl folate trap:** Because the MTHFR reaction is irreversible, 5-MTHF can only be used by the B12-dependent methionine synthase reaction. In B12 deficiency, 5-MTHF accumulates and cannot regenerate THF, so folate is "trapped." The result is functional folate deficiency: reduced THF for purine and thymidylate (dTMP) synthesis, impaired DNA synthesis, and megaloblastic anemia with hypersegmented neutrophils.

**Lab pattern:** - Folate deficiency: ↑ homocysteine, normal methylmalonic acid (MMA), low folate. - B12 deficiency: ↑ homocysteine and ↑ MMA, low or normal serum folate (due to trap). The MMA rise reflects impaired methylmalonyl-CoA mutase, which also needs B12 (adenosylcobalamin). That impairment causes the neurologic findings (subacute combined degeneration), which are not seen in folate deficiency.

**Clinical associations:** - Folic acid supplementation can correct the megaloblastic anemia of B12 deficiency but leaves the neurologic damage to progress. Always check B12 before treating with folate. - MTHFR polymorphisms (C677T, A1298C) reduce enzyme activity, which can mildly raise homocysteine. The clinical significance is limited, and routine testing is not recommended. Homocysteine elevation is associated with thrombosis and atherosclerosis. Homocystinuria (cystathionine beta-synthase deficiency) is a separate, more severe cause. - Folate supplementation before conception and in early pregnancy (400 µg/day, 4 mg for high risk) prevents neural tube defects. - Drugs affecting folate: methotrexate (DHFR inhibitor), trimethoprim (bacterial DHFR), pyrimethamine, phenytoin, and sulfonamides (inhibit bacterial folate synthesis via PABA). - Leucovorin (folinic acid, 5-formyl-THF) bypasses DHFR and rescues cells from methotrexate toxicity. L-methylfolate (5-MTHF) is available as a supplement that bypasses MTHFR. - 5-FU inhibits thymidylate synthase, which uses 5,10-methylene-THF as the methyl donor (the folate is oxidized to DHF in the process). Leucovorin potentiates 5-FU by stabilizing the enzyme-FdUMP complex.

**Mnemonic:** "Methyl trap" = B12 deficiency traps folate as 5-MTHF, so the cell is folate-starved despite normal total folate.