A vitamin synthesized by microorganisms and conserved in animals in the liver. Deficiency or collective uptake of vitamin B-12 leads to pernicious anemia. It consists of cobalamin, a substituted corrin-Co(III) complex in which the cobalt atom is bound to the four nitrogen atoms of the corrin ring, an axial group A and 5,6-dimethylbenzimidazole. Various forms of the vitamin are known with different A groups such as A = CN, cyanocobalamin; R = OH, hydroxocobalamin; R = CH₃, methylcobalamin; R = adenosyl, coenzyme B-12.