Methylcobalamin (mecobalamin, MeCbl, or MeB12) is a cobalamin, a form of vitamin B12. It differs from cyanocobalamin in that the cyano group at the cobalt is replaced with a methyl group.[1] Methylcobalamin features an octahedral cobalt(III) centre and can be obtained as bright red crystals.[2] From the perspective of coordination chemistry, methylcobalamin is notable as a rare example of a compound that contains metal–alkyl bonds. Nickel–methyl intermediates have been proposed for the final step of methanogenesis.
Methylcobalamin that is ingested is not used directly as a cofactor, but is first converted by MMACHC into cob(II)alamin. Cob(II)alamin is then later converted into the other two forms, adenosylcobalamin and methylcobalamin for use as cofactors. That is, methylcobalamin is first dealkylated and then regenerated.[5][6][7]
The efficacy of methylcobalamin administration in treating vitamin B12 deficiency remains uncertain. While directly providing active cobalamin forms to deficient patients is an attractive approach promoted by the manufacturers of methylcobalamin products, it is not known whether methylcobalamin can reach its intracellular targets in its original, unmodified form to function effectively as ready coenzyme. It is also not known whether current level of evidence sufficient to recommend this relatively expensive strategy as an alternative to cyanocobalamin or hydroxocobalamin. There is currently insufficient evidence on comparative effectiveness and safety of various B12 vitamers (methylcobalamin, cyancobalamin, hydroxocobalamin, adenosylcobalamin).[8][9][10]
Production
Methylcobalamin physically resembles the other forms of vitamin B12, occurring as dark red crystals that freely form cherry-colored transparent solutions in water.
Methylcobalamin participates in the Wood-Ljungdahl pathway, which is a pathway by which some organisms utilize carbon dioxide as their source of organic compounds. In this pathway, methylcobalamin provides the methyl group that couples to carbon monoxide (derived from CO2) to afford acetyl-CoA. Acetyl-CoA is a derivative of acetic acid that is converted to more complex molecules as required by the organism.[11]
Methylcobalamin is produced by some bacteria.[citation needed] It plays an important role in the environment, where it is responsible for the biomethylation of certain heavy metals. For example, the highly toxic methylmercury is produced by the action of methylcobalamin.[12] In this role, methylcobalamin serves as a source of "CH3+".
A lack of cobalamin can lead to megaloblastic anemia and subacute combined degeneration of the spinal cord.[13]
^ abDavid D (January 1971). "Preparation of the Reduced Forms of Vitamin B12 and of Some Analogs of the Vitamin B12 Coenzyme Containing a Cobalt-Carbon Bond". In McCormick DB, Wright LD (eds.). Vitamins and Coenzymes. Methods in Enzymology. Vol. 18. Academic Press. pp. 34–54. doi:10.1016/S0076-6879(71)18006-8. ISBN9780121818821.
