Form of vitamin B12
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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. Methylcobalamin features an octahedral cobalt(III) centre and can be obtained as bright red crystals. 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 is equivalent physiologically to vitamin B12, and can be used to prevent or treat pathology arising from a lack of vitamin B12 intake (vitamin B12 deficiency).
Methylcobalamin is also used in the treatment of peripheral neuropathy, diabetic neuropathy, and as a preliminary treatment for amyotrophic lateral sclerosis.
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.
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).
Production.
Methylcobalamin can be produced in the laboratory by reducing cyanocobalamin with sodium borohydride in alkaline solution, followed by the addition of methyl iodide.
Functions.
This vitamer, along with adenosylcobalamin, is one of two active coenzymes used by vitamin B12-dependent enzymes and is the specific vitamin B12 form used by 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), also known as methionine synthase.
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.
Methylcobalamin is produced by some bacteria. 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. 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.
References.
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