The discovery of vitamin B12 (cobalamin) followed intensive studies on pernicious anemia (PA), a previously fatal disease. In the early 1920s Minot and Murphy demonstrated that they were able to cure PA by whole liver extract. Later, it was shown that liver is an important source of B12. Animals can obtain B12 by consuming foods contaminated with synthesizing bacteria and then incorporate the vitamin into their body organs. Foods of animal source are the only natural source of B12 in the human diet. Vitamin B12 is synthesized exclusively in microorganisms. In humans it is an essential cofactor for methyl group transfer and cell division.
Vitamin B12 belongs to a group of compounds of similar chemical structure but completely different biological functions. The vitamin is composed of a corrin ring surrounding a central cobalt atom that is bound to a lower and an upper ligand. The lower ligand consists of a benzimidazole group that through a ribose-phosphate group is attached to the corrin ring. The upper ligand is unique for each form of the vitamin. In the co-factor forms, the upper ligand is either a methyl or a 5’-deoxyadenosyl group. The vitamin is crucially involved in the proliferation, maturation, and regeneration of cells. Vitamin B12 maintains low homocysteine (Hcy) levels by transferring a methyl group from 5-methyltetrahydrofolate (5-methylTHF) to Hcy converting it into methionine.