Vitamin B₁₂-Cyanocobalamin by definition, but can include any substituted cobalamin derivative with similar biological activity. Vitamin B₁₂ is a water-soluble haematopoietic (blood forming) vitamin. Cobalamin contains a corrin ring system with cobalt at the center. Commercial preparations of the vitamin are slightly modified and are known as cyanocobalamin (the cyano=cyanide). The coenzyme forms of the vitamin are 1) methylcobalamin, whereby the cyanide group is replaced by a methyl group and 2) 5^'-deoxyadenosylcobalamin, whereby the cyanide group is replaced with 5^'-deoxyadenosine.

The functions of cobalamin are 1) as coenzymes in the synthesis of methionine from homocysteine: this reaction requires methylcobalamin and 2) carbon atom rearrangement in methylmalonyl CoA to produce succinyl CoA: this reaction requires deoxyadenosylcobalamin.

To be absorbed by the intestine, vitamin B₁₂ must combine with intrinsic factor, which is produced in the stomach by gastric parietal cells. Vitamin B₁₂ is required for the growth and replication of all cells, including blood cells and is required for the proper functioning of the nervous system. Vitamin B₁₂ is also required for purine and pyrimidine (components of DNA) synthesis and is thus needed for DNA production.

Vitamin B₁₂ metabolism is connected with that of folic acid. In vitamin B₁₂ deficiency, reactions requiring this vitamin decrease, resulting in a subsequent decrease in reactions that convert folic acid to other derivatives. This essentially traps a portion of the folic acid pool to an unusable form. Therefore, sufficient amounts of vitamin B₁₂ must be present in order for folic acid to be utilised.

Sources of vitamin B₁₂

Good sources of vitamin B₁₂ are animal flesh, since this vitamin is not found in plant sources. Animals obtain their vitamin B₁₂ from their bacterial gut flora. The bacterial gut flora synthesize vitamin B₁₂ and small amounts of other vitamins. Therefore, we can obtain some of our vitamin B₁₂ from our own gut flora, and the rest from animal products, such as liver, whole milk, eggs, seafoods (oysters, shrimp), pork and chicken.

RDA of vitamin B₁₂

The RDA of cobalamin is 3 micrograms/day. The requirement doubles during pregnancy and breast feeding.

Deficiency of vitamin B₁₂

During the introduction to vitamins we discussed water-soluble and fat-soluble vitamins. Fat-soluble vitamins are stored to a large extent in the body in comparison to water-soluble vitamins, which are excreted more rapidly. Vitamin B₁₂ is a water-soluble vitamin. However, it is the exception to the rule regarding excretion from the body. Vitamin B₁₂ is stored in significant amounts (5 mg) in the body and depletion can take several years, depending on the individual and factors such as weight, activity, physiological state (eg., pregnancy, lactation).

Deficiencies are not normally from the diet, although vitamin B₁₂ can be rapidly depleted during starvation. Most deficiency states of vitamin B₁₂ are from intestinal complications, such as surgery. Gastrectomy, or removal of a portion of the stomach results in a lack of intrinsic factor which is required for vitamin B₁₂ to be absorbed by the ileum of the small intestine.

If the parietal cells fail for any other reason, the signs and symptoms of vitamin B₁₂ deficiency will develop once the stored supply of the vitamin is depleted. Any insult that damages the ileum as well, can cause a vitamin B₁₂ deficiency. Crohn's disease, and other inflammatory conditions that damage the ileum portion of the small intestine can cause vitamin B₁₂ deficiencies. Intestinal infections can also damage the intestinal wall, with the same outcome.

Cells with a high turnover rate such as the intestinal epithelium and haemopoietic cells of the bone marrow are the first to manifest the results of a vitamin B₁₂ deficiency. Since this vitamin is needed for proper purine and pyrimidine synthesis (for DNA), incorrect production occurs and is not packed correctly in cells, resulting in a larger than normal cell. This larger than normal cell is called a megaloblast, and is part of the condition known as megaloblastic anaemia, suffered by the individual lacking vitamin B₁₂.

Toxicity of vitamin B₁₂

Itching, gastrointestinal upset, sensitivity reactions, and thrombosis can be observed in individuals taking chronic excessive doses of vitamin B₁₂.