16/01/2025
| Vitamin B12
Vitamin B12 is the largest and most complex of all the vitamins. The name vitamin B12 is generic for a specific group of cobalt-containing corrinoids with biological activity in humans. Interestingly it is the only known metabolite to contain cobalt, which gives this water-soluble vitamin its red colour. This group of corrinoids is also known as cobalamins. The main cobalamins in humans and animals are hydroxocobalamin, adenosylcobalamin and methylcobalamin, the last two being the active coenzyme forms. Cyanocobalamin is a form of vitamin B12 that is widely used clinically due to its availability and stability. It is transformed into active factors in the body.
In 1934, three researchers won the Nobel prize in medicine for discovering the lifesaving properties of vitamin B12. They found that eating large amounts of raw liver, which contains high amounts of vitamin B12, could save the life of previously incurable patients with pernicious anaemia. This finding saves 10,000 lives a year in the US alone. Vitamin B12 was isolated from liver extract in 1948 and its structure was elucidated 7 years later.
Functions
Vitamin B12 is necessary for the formation of blood cells, nerve sheaths and various proteins. It is therefore, essential for the prevention of certain forms of anaemia and neurological disturbances. It is also involved in fat and carbohydrate metabolism and is essential for growth. In humans, vitamin B12 functions primarily as a coenzyme in intermediary metabolism. Two metabolic reactions are dependent on vitamin B12:
The methionine synthase reaction with methylcobalamin
The methylmalonyl CoA mutase reaction with adenosylcobalamin
In its methylcobalamin form vitamin B12 is the direct cofactor for methionine synthase, the enzyme that recycles homocysteine back to methionine. There is evidence that vitamin B12 is required in the synthesis of folate polyglutamates (active coenzymes required in the formation of nerve tissue) and in the regeneration of folic acid during red blood cell formation.
Methylmalonyl CoA mutase converts 1-methylmalonyl CoA to succinyl CoA (an important reaction in lipid and carbohydrate metabolism). Adenosylcobalamin is also the coenzyme in ribonucleotide reduction (which provides building blocks for DNA synthesis).
Main functions in a nutshell:
Essential growth factor
Formation of blood cells and nerve sheaths
Regeneration of folic acid
Coenzyme-function in the intermediary metabolism, especially in cells of the nervous tissue, bone marrow and gastrointestinal tract
Dietary sources
Vitamin B12 is produced exclusively by microbial synthesis in the digestive tract of animals. Therefore, animal protein products are the source of vitamin B12 in the human diet, in particular organ meats (liver, kidney). Other good sources are fish, eggs and dairy products. In foods, hydroxo-, methyl- and 5'-deoxyadenosyl-cobalamins are the main cobalamins present. Foods of plant origin contain no vitamin B12 beyond that derived from microbial contamination. Bacteria in the intestine synthesise vitamin B12, but under normal circumstances not in areas where absorption occurs.
Stability
Vitamin B12 is stable to heat, but slowly loses its activity when exposed to light, oxygen and acid or alkali-containing environments. Loss of activity during cooking is due to the water solubility of vitamin B12 (loss through meat juices or leaching into water) rather than to its destruction.
Safety
Large intakes of vitamin B12 from food or supplements have caused no toxicity in healthy people. No adverse effects have been reported from single oral doses as high as 100 mg and chronic administration of 1 mg (500 times the RDA) weekly for up to 5 years. Moreover, there have been no reports of carcinogenic or mutagenic properties, and studies to date indicate no teratogenic potential. The main food safety authorities have not set a tolerable upper intake level (UL) for vitamin B12 because of its low toxicity.
Industrial production
Vitamin B12 is produced commercially from bacterial fermentation, usually as cyanocobalamin.
