Vitamin B12 transport from food to the body's cells-a sophisticated, multistep pathway

Vitamin B-12 (B-12; also known as cobalamin) is a cofactor in many metabolic processes; deficiency of this vitamin is associated with megaloblastic anaemia and various neurological disorders. In contrast to many prokaryotes, humans and other mammals are unable to synthesize B-12. Instead, a sophisticated pathway for specific uptake and transport of this molecule has evolved. Failure in the gastrointestinal part of this pathway is the most common cause of nondietary-induced B-12 deficiency disease. However, although less frequent, defects in cellular processing and further downstream steps in the transport pathway are also known culprits of functional B-12 deficiency. Biochemical and genetic approaches have identified novel proteins in the B-12 transport pathway-now known to involve more than 15 gene products-delineating a coherent pathway for B-12 trafficking from food to the body's cells. Some of these gene products are specifically dedicated to B-12 transport, whereas others embrace additional roles, which explains the heterogeneity in the clinical picture of the many genetic disorders causing B-12 deficiency. This Review describes basic and clinical features of this multistep pathway with emphasis on gastrointestinal transport of B-12 and its importance in clinical medicine.