The cobalamins are a group of closely related and interconvertible compounds with a complex structure that are collectively known by the common name of vitamin Bi2. Recommended biochemical nomenclature restricts the term 'vitamin B12' for the particular form of cobalamin known as cyanocobalamin. All cobalamins belong to the broader family of corri-noids, which share the characteristic of consisting of a planar four-member pyrrole ring (corrin ring) containing a central cobalt atom. Cobalamins are distinguished from other corrinoids by possessing both alpha (lower) and beta (upper) axial ligands that are attached to the central cobalt atom (Figure 1). The lower ligand consists of a base (5,6-dimethylben-zimidazole) attached to a sugar (ribose), which in turn is attached to a phosphate and an amino-propyl group that ultimately is tethered back to the corrin ring. In the naturally occurring cobalamins the upper ligand is variably a cyano-, hydroxo-, aquo-, methyl-, or adenosyl- group, giving rise to the correspondingly named chemical forms of the vitamin. Of these, methylcobalamin and deoxyadenosylcobalamin are the forms that function as coenzymes for metabolic reactions. These are sensitive to destruction by light. Cyanocobalamin is a stable form and is therefore used in therapeutic preparations. Hydroxo- or

Corrin ring system

H2C^ vxn oh

NH2 c=o

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