- Vitamin B12, also known as cobalamin, is a complex, water-soluble vitamin essential for human health and vital biological functions. It belongs to the B-complex group of vitamins and plays a crucial role in red blood cell formation, DNA synthesis, neurological function, and energy metabolism.
- Unique among vitamins, B12 contains a corrin ring with a central cobalt ion, giving the molecule its name. The most biologically active forms in the human body are methylcobalamin and adenosylcobalamin, while cyanocobalamin and hydroxocobalamin are synthetic or storage forms commonly used in supplements and therapeutics.
- Cobalamin is involved in two major enzymatic reactions in humans: the conversion of homocysteine to methionine, catalyzed by methionine synthase (requiring methylcobalamin), and the conversion of methylmalonyl-CoA to succinyl-CoA, catalyzed by methylmalonyl-CoA mutase (requiring adenosylcobalamin). These reactions are critical for maintaining neurological health, myelin sheath integrity, and amino acid metabolism. A deficiency in B12 impairs these reactions, leading to megaloblastic anemia, peripheral neuropathy, memory loss, and, in severe cases, irreversible nervous system damage.
- Vitamin B12 is synthesized exclusively by certain bacteria and archaea, not by plants or animals. As such, natural dietary sources are limited to animal-derived foods, including meat, liver, fish, eggs, and dairy products. For individuals on vegan or vegetarian diets, or those with malabsorption issues such as pernicious anemia, gastric bypass surgery, or Crohn’s disease, supplementation with oral or injectable B12 is often necessary to prevent deficiency.
- From a biochemical standpoint, the structure of vitamin B12 is remarkably intricate, featuring a corrin ring similar to the porphyrin ring in heme, but with a reduced ring structure and a cobalt ion at the center. This cobalt ion can coordinate to various ligands such as a methyl group, adenosyl group, hydroxyl group, or cyanide ion, depending on the specific form. The complexity and size of the B12 molecule make it the largest and most structurally complicated vitamin known. It is also one of the few vitamins that the human body can store in significant quantities, primarily in the liver, which helps buffer against short-term dietary deficiencies.
- Clinically, vitamin B12 is used to treat and prevent deficiency syndromes, particularly in the elderly, individuals with gastrointestinal disorders, or those taking medications like proton pump inhibitors or metformin, which interfere with B12 absorption. Hydroxocobalamin is often the preferred injectable form in clinical settings due to its longer half-life in the bloodstream. Additionally, B12 is used as part of treatment regimens for cyanide poisoning, as it can bind cyanide ions to form cyanocobalamin, a nontoxic compound excreted in urine.
