Editing Folate (section)

=== Vitamin B<sub>12</sub> activation ===
[[File:Folate methionine cycle.svg|thumb|class=skin-invert-image|Simplified schematic diagram of the folate methionine cycle<ref name="Froese_2019">{{cite journal | vauthors = Froese DS, Fowler B, Baumgartner MR | title = Vitamin B12, folate, and the methionine remethylation cycle-biochemistry, pathways, and regulation | journal = Journal of Inherited Metabolic Disease | volume = 42 | issue = 4 | pages = 673–685 | date = July 2019 | pmid = 30693532 | doi = 10.1002/jimd.12009 | doi-access = free }}</ref>|360px]]
Methyl-THF converts vitamin B<sub>12</sub> to methyl-B<sub>12</sub> ([[methylcobalamin]]). Methyl-B<sub>12</sub> converts homocysteine, in a reaction catalyzed by [[homocysteine methyltransferase]], to [[methionine]]. A defect in homocysteine methyltransferase or a deficiency of B<sub>12</sub> may lead to a so-called "methyl-trap" of THF, in which THF converts to methyl-THF, causing a deficiency in folate.<ref name = Hoffbrand2001/> Thus, a deficiency in B<sub>12</sub> can cause accumulation of methyl-THF, mimicking folate deficiency.