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Vitamin B12, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins. It is normally involved in the metabolism of every cell of the human body, especially affecting DNA synthesis and regulation, but also fatty acid metabolism and amino acid metabolism. Neither fungi, plants, nor animals (including humans) are capable of producing vitamin B12. Only bacteria and archaea have the enzymes required for its synthesis, although many foods are a natural source of B12 because of bacterial symbiosis. The vitamin is the largest and most structurally complicated vitamin and can be produced industrially only through bacterial fermentation-synthesis. Vitamin B12 consists of a class of chemically related compounds (vitamers), all of which have vitamin activity. It contains the biochemically rare element cobalt sitting in the center of a planar tetra-pyrrole ring called a corrin ring. Biosynthesis of the basic structure of the vitamin is accomplished only by bacteria and archaea (which usually produce hydroxocobalamin), but conversion between different forms of the vitamin can be accomplished in the human body. A common semi-synthetic form of the vitamin is cyanocobalamin, which does not occur in nature but is produced from bacterial hydroxocobalamin. Because of its stability and lower production cost, this form is then used in many pharmaceuticals and supplements as well as a food additive. In the body it is converted to the human physiological forms methylcobalamin and 5'-deoxyadenosylcobalamin, leaving behind the cyanide ion, albeit in a minimal concentration. More recently, hydroxocobalamin, methylcobalamin, and adenosylcobalamin can be found in more expensive pharmacological products and food supplements. Their extra utility is currently debated. Vitamin B12 was discovered from its relationship to disease pernicious anemia, which is an autoimmune disease in which parietal cells of the stomach responsible for secreting intrinsic factor are destroyed (these cells are also responsible for secreting acid in the stomach). Because intrinsic factor is crucial for the normal absorption of B12, its lack in pernicious anemia causes a vitamin B12 deficiency. Many other subtler kinds of vitamin B12 deficiency and their biochemical effects have since been elucidated.〔(【引用サイトリンク】title=Dietary Supplement Fact Sheet: Vitamin B12 )〕 ==Terminology== The names vitamin B, vitamin B12, or vitamin B-12, and the alternative name cobalamin, generally refer to all forms of the vitamin. Some medical practitioners have suggested that its use be split into two categories. *In a broad sense, B12 refers to a group of cobalt-containing vitamer compounds known as cobalamins: these include cyanocobalamin (an artifact formed from using activated charcoal, which always contains trace cyanide, to purify hydroxycobalamin), hydroxocobalamin (another medicinal form, produced by bacteria), and finally, the two naturally occurring cofactor forms of B12 in the human body: 5'-deoxyadenosylcobalamin (adenosylcobalamin—AdoB12), the cofactor of Methylmalonyl Coenzyme A mutase (MUT), and methylcobalamin (MeB12), the cofactor of enzyme Methionine synthase, which is responsible for conversion of homocysteine to methionine and of 5-methyltetrahydrofolate to tetrahydrofolate. *The term B12 may be properly used to refer to cyanocobalamin, the principal B12 form used for foods and in nutritional supplements. This ordinarily creates no problem, except perhaps in rare cases of eye nerve damage, where the body is only marginally able to use this form due to high cyanide levels in the blood due to cigarette smoking; it thus requires cessation of smoking or B12 given in another form, for the optic symptoms to abate. However, tobacco amblyopia is a rare condition, and it is yet unclear whether it represents a peculiar B12 deficiency that is resistant to treatment with cyanocobalamin. Finally, so-called pseudovitamin-B12 refers to B12-like analogues that are biologically inactive in humans and yet found to be present alongside B12 in humans, many food sources (including animals), and possibly supplements and fortified foods. In most cyanobacteria, including ''Spirulina'', and some algae, such as dried Asakusa-nori (''Porphyra tenera''), pseudovitamin-B12 is found to predominate. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vitamin B12」の詳細全文を読む スポンサード リンク
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