Methylcobalamin

Methylcobalamin is a cobalamin (MeCbl or MeB) used in the treatment of peripheral neuropathy, diabetic neuropathy, and as a preliminary treatment for amyotrophic lateral sclerosis. It is a form of vitamin B and differs from cyanocobalamin in that the cyanide is replaced by a methyl group. Methylcobalamin features an octahedral cobalt(III) centre.

This vitamer is one of two active coenzymes used by vitamin B-dependent enzymes and is the specific vitamin B form used by 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), also known as methionine synthase. Methylcobalamin is notable for being one of the few examples in nature of a bona fide organometallic bond, although Ni-CH3 intermediates are proposed for the final step of methanogenesis. In physiological terms, it is equivalent to vitamin B, e.g. for addressing pathologies arising from a lack of vitamin vitamin B, such as pernicious anemia.

Role in biomethylation
Methylcobalamin plays an important role in the environment. It is produced preferentially by some bacteria. In the environment, it is responsible for the biomethylation of certain heavy metals. For example, the highly toxic methylmercury is produced in this way. In this role, methylcobalamin serves as a source "CH3+". In the Wood-Ljungdahl pathway, methylcobalamin provides the methyl group that couples to carbon monoxide to afford acetyl-CoA.

Small clinical studies
Methylcobalamin has been studied in conjunction with sleep-wake rhythm disorders, where it appears to yield benefits, but at a low or inconsistent level.

Supplementation with megadoses of methylcobalamin has been advocated to protect the cognitive function of patients suffering chronic fatigue syndrome, stroke, depression, Alzheimers disease, and other neurological diseases. The rationale is that methylcobalamin may help to remove brain-damaging levels of the neurotransmitter glutamate. Methylcobalamin has been reported to have been used to reduce neurotoxicity and lower excess glutamate levels, resulting in the reduction of fatigue, stabilisation of mood, improvement of memory, and executive function. However, clinical trials using methylcobalamin have been small and, for the most part, have not been replicated or confirmed.