Absorption and transport of cobalamin (vitamin B12).

This review apprises the reader of recent advances in cobalamin absorption and transport since 1970. This field has been of interest to gastroenterologists, hematologists, and biochemists as well as nutritionists. Clinical conditions leading to cobalamin deficiency present especially to the first two of these specialties. Many recent advances involve biochemical definition of the specific transport proteins themselves. Full understanding of the multiple clinical conditions associated with cobalamin deficiency requires some knowledge of these proteins. We do not discuss every paper in the field, especially the early literature (see 47, 61, 150). Despite rapid strides in understanding the mechanism of absorption and transport of cobalamin both in prokaryotic and eukaryotic cells, the complete sequence of events during cellular absorption at the plasma membrane level and events in the cells that mediate intracellular movement of cobalamin have not been elucidated completely. Here we emphasize the current state of knowledge about cobalamin absorption and transport in man and other mammalian systems, particularly the physiological role and biochemical nature of proteins involved (intrinsic factor, intrinsic factor-cobalamin receptor, transcobalamins).