Cobalamin (Cbl, vitamin B12) transport and metabolism

Cbl derivatives function as cofactors in two reactions, AdoCbl in the conversion of homocysteine to methionine and MetCbl in the conversion of L-methylmalonyl CoA to succinyl CoA. Both reactions are essential for normal human function, however, and defects in the steps by which Cbl or CNCbl is taken up from the diet, transported to metabolically active cells, and transformed to AdoCbl and MeCbl are associated with severe defects in blood formation and neural function.

The overall process of Cbl utilization is presented here in three parts: its uptake from the diet into gut enterocytes, its release into the blood, circulation within the body (including renal re-uptake), and delivery to the cells where it is used, and its metabolism in those cells to generate AdoCbl and MeCbl.