Translations:Folate/39/en

Vitamin B₁₂ activation

Simplified schematic diagram of the folate methionine cycle

Methyl-THF converts vitamin B₁₂ to methyl-B₁₂ (methylcobalamin). Methyl-B₁₂ converts homocysteine, in a reaction catalyzed by homocysteine methyltransferase, to methionine. A defect in homocysteine methyltransferase or a deficiency of B₁₂ may lead to a so-called "methyl-trap" of THF, in which THF converts to methyl-THF, causing a deficiency in folate. Thus, a deficiency in B₁₂ can cause accumulation of methyl-THF, mimicking folate deficiency.