Methylcobalamin/ja: Difference between revisions

'''メチルコバラミン'''（'''Methylcobalamin''', メコバラミン、MeCbl、MeB{{sub|12}}）は、[[Vitamin B12|ビタミンB{{sub|12}}]の一種である[[cobalamin/ja|コバラミン]]である。］ コバルトの[[cyanide/ja|シアノ基]]が[[methyl group/ja|メチル基]]に置換されている点で[[cyanocobalamin/ja|シアノコバラミン]]とは異なる。メチルコバラミンは八面体のコバルト(III)中心を持ち、明るい赤色の結晶として得られる。 [[coordination chemistry/ja|配位化学]]の観点からは、メチルコバラミンが金属-アルキル結合を含む化合物の珍しい例として注目される。[[Nickel/ja|ニッケル]]-メチル中間体は、[[methanogenesis/ja|メタン生成]]の最終段階として提案されている。

Methylcobalamin is equivalent physiologically to vitamin B{{sub|12}}, and can be used to prevent or treat pathology arising from a lack of vitamin B{{sub|12}} intake ([[vitamin B12 deficiency|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.

According to one author, it is important to treat [[vitamin B12|vitamin B{{sub|12}}]] deficiency with [[hydroxocobalamin]] or [[cyanocobalamin]] or a combination of [[adenosylcobalamin]] and methylcobalamin, and not methylcobalamin alone.

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==Production==

[[File:B12 methylcobalamin.jpg|thumb|left|300px|Methylcobalamin physically resembles the other forms of vitamin B₁₂, occurring as dark red crystals that freely form cherry-colored transparent solutions in water.]]

Methylcobalamin can be produced in the laboratory by reducing [[cyanocobalamin]] with [[sodium borohydride]] in alkaline solution, followed by the addition of [[methyl iodide]].

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==Functions==

This [[vitamer]], along with [[adenosylcobalamin]], is one of two active coenzymes used by vitamin B{{sub|12}}-dependent enzymes and is the specific vitamin B{{sub|12}} 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 metal (chemistry)|heavy metal]]s.  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.

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== See also ==

*[[Cobamamide]]

*[[Cyanocobalamin]]

*[[Hydroxocobalamin]]

*[[Vitamin B12]]

*[[Cobalamin biosynthesis]]