Methylcobalamin is a form of vitamin B12, and is also written as MeCBl or MeB12. It is one of four cobalamin compounds that the human body is able to metabolize. A cobalamin compound contains a central cobalt ion, with one of four particular groups attached to its upper ligand. These can be cyanide, hydroxide, adenosine, or a methyl group.
All vitamin B12 compounds are soluble in water, and naturally-occurring forms of B12 — such as methylcobalamin — are produced by bacteria. These bacteria are present in the human liver, but the cobalamin compounds produced there are not usable by the body, and are flushed out with feces. In order to obtain the required amount of vitamin B12 from dietary sources, humans must consume animal products. Plants produce a compound similar in structure to B12, but that actually inhibits B12 activity in humans.
In addition to natural forms of vitamin B12, a synthetic version, cyanocobalamin, can also be used by humans. This form of the vitamin occurs in nature also, but very rarely. It is far cheaper to synthetically produce this version of the vitamin for supplements than to isolate any of the natural forms. Once it is in the body, cyanocobalamin is converted into methylcobalamin.
Vitamin B12 is essential in humans to ensure proper nervous system function. It also plays a part in blood formation. In addition, this vitamin aids in the production of fatty acids, and has an active role in the metabolism of all cells in the body. It is especially important for DNA synthesis. Methylcobalamin has been used medically to treat sleep rhythm disorders, but with limited success.
A lack of vitamin B12 in the body leads to pernicious anemia, a condition that inhibits DNA replication. It also causes low blood pressure, minor cognitive impairment, and jaundice. Pernicious anemia is also known as Addison–Biermer anemia.
There is also a rare genetic disorder known as Arakawa's syndrome II, which causes a deficiency in the enzyme necessary to metabolize methylcobalamin. This disorder leads to the same symptoms as pernicious anemia. Arakawa’s syndrome II is a dominant autosomal disorder, which means that only copy of the defective gene in a body is required for the methylcobalamin metabolizing enzyme to cease functioning.
Humans require between one and two micrograms (mcg) of vitamin B12 per day. A typical North American diet will easily provide this amount. The liver is able to stockpile this vitamin, and because of this it can take up to two years for a deficiency to become apparent.