Vitamin B12

Vitamin B12
Hbf878 · Public domain · source
Name	Vitamin B12
Caption	Structure of cyanocobalamin, a common synthetic form.
Pronounce	/ˌvaɪtəmɪn ˌbiː ˈtwɛlv/
Synonyms	Cobalamin
Class	Water-soluble vitamin

Vitamin B12. It is a crucial water-soluble vitamin that plays a fundamental role in DNA synthesis, red blood cell formation, and the proper functioning of the nervous system. This essential nutrient is unique among vitamins because it is synthesized exclusively by certain microorganisms and is naturally found in significant amounts only in animal-derived foods. Deficiency can lead to serious hematological and neurological disorders, making its adequate intake a key concern in human nutrition.

Chemical structure and forms

The core structure of this vitamin is a complex corrin ring, which coordinates a central cobalt ion, classifying the compounds collectively as cobalamins. The most common natural forms include methylcobalamin and adenosylcobalamin, which are the active coenzymes in human metabolism. Synthetic forms used in supplements and fortified foods are primarily cyanocobalamin and hydroxocobalamin; the latter is also used in clinical settings for injection. The stability and biological activity of these compounds are influenced by the ligand attached to the cobalt ion, with the cyanide group in cyanocobalamin providing exceptional shelf life.

Sources and bioavailability

Natural dietary sources are almost exclusively of animal origin, including meat (especially liver), poultry, fish (like salmon and tuna), shellfish, eggs, and dairy products such as milk and cheese. Some edible algae and fermented foods may contain analogues with limited activity. Bioavailability is highest from animal sources, as the vitamin is bound to proteins and released by the action of stomach acid and pepsin. Synthetic cyanocobalamin, used in fortified cereals and nutritional yeast, is generally well-absorbed in individuals with normal gastrointestinal function, though absorption efficiency decreases with higher single doses.

Biological role and function

Its primary biological functions are as a cofactor for two essential enzymes. Methionine synthase, which requires methylcobalamin, catalyzes the conversion of homocysteine to methionine, a reaction critical for DNA methylation and nucleotide synthesis. The other, methylmalonyl-CoA mutase, utilizes adenosylcobalamin to convert methylmalonyl-CoA to succinyl-CoA, a key step in the metabolism of certain fatty acids and amino acids. These roles are indispensable for the maturation of erythrocytes in the bone marrow and for maintaining the integrity of the myelin sheath surrounding neurons in the central nervous system.

Deficiency and symptoms

Deficiency arises from inadequate intake (as in strict vegan diets), malabsorption syndromes (like pernicious anemia, Crohn's disease, or after gastrectomy), or increased requirements. Hematological manifestations include megaloblastic anemia, characterized by large, immature red blood cells, leading to fatigue and pallor. Neurological symptoms can range from paresthesia and ataxia to more severe damage, such as optic neuropathy and cognitive disturbances. Long-term deficiency can cause irreversible damage to the spinal cord, a condition historically described as subacute combined degeneration.

Diagnosis and treatment

Diagnosis typically involves measuring serum levels, though elevated methylmalonic acid and homocysteine concentrations are more specific functional markers. The Schilling test, now largely historical, was used to differentiate causes of malabsorption. First-line treatment for deficiency often involves intramuscular injections of hydroxocobalamin or cyanocobalamin to bypass absorption issues, as practiced in managing pernicious anemia. For dietary insufficiency, high-dose oral supplements or intranasal gels are effective, and long-term management includes dietary modification or regular supplementation, often overseen by specialists in hematology or gastroenterology.

History and synthesis

The history of this vitamin is intertwined with the study of pernicious anemia, a fatal disease described by Thomas Addison and later studied by William Bosworth Castle. The crucial anti-anemia factor in liver was isolated in 1948 by teams led by Karl August Folkers at Merck & Co. and Alexander R. Todd in the United Kingdom. Its complex chemical structure was elucidated by Dorothy Hodgkin using X-ray crystallography, for which she was awarded the Nobel Prize in Chemistry in 1964. Total laboratory synthesis was achieved in 1972 by Robert Burns Woodward and Albert Eschenmoser, a landmark feat in organic chemistry, though commercial production relies on bacterial fermentation using species like Propionibacterium freudenreichii. Category:Vitamins Category:Cobalt compounds