Fig. 1 - Site of folate trap

Several forms of folate are shown here, all of which must pass thru tetrahydrofolate (THF), the central intermediate. The form trapped by a B₁₂ deficiency is N⁵-methyl THF, for the following two reasons: i) without B₁₂, N⁵-methyl THF cannot donate its methyl group to make methyl-B₁₂; ii) this is the only way N⁵-methyl THF can be converted back to THF for recycling. So, sooner or later, a significant portion of the body's folate winds up in this dead end and becomes useless. The next question explores the N⁵-methyl THF-requiring reaction a bit further...