Digestion and Absorption of Vitamin Bi2

Both gastric acid and pepsin are important in vitamin B12 nutrition, serving to release the vitamin from protein binding and to make it available. Between 10% to 15% of people aged more than 60 years show some degree of vitamin B12 deficiency as a result of impaired absorption due to atrophic gastritis. In the early stages, there is failure of acid secretion, resulting in failure to release the vitamin from dietary proteins. The absorption of free crystalline vitamin B12, as opposed to vitamin that is bound to dietary proteins, is normal. As the condition progresses, there is also failure of the secretion of intrinsic factor.

In the stomach, vitamin B12 binds to cobalophilin, a binding protein secreted in the saliva. The cobalophilins are a group of antigenically related, relatively unspecific, corrinoid binding proteins, formerly known as R-proteins because of their rapid mobility, compared with other cobalamin binding proteins, on electrophoresis.

In the duodenum, cobalophilin is hydrolyzed, releasing vitamin B12 for binding to intrinsic factor. Pancreatic insufficiency can therefore be a factor in the development of vitamin B12 deficiency, because failure to hydrolyze cobalophilin will result in the vitamin remaining bound to cobalophilin and being excreted, rather than being transferred to intrinsic factor (Gueant et al., 1990).

Intrinsic factor binds the various vitamin B12 vitamers with equal affinity, but not other corrinoids. There is one vitamin B12 binding site per mole of intrinsic factor. On binding the vitamin, the protein undergoes a conforma-tional change, resulting in dimerization and greatly enhanced resistance to proteolysis.

Vitamin B12 is absorbed from the distal third of the ileum by receptor-mediated endocytosis. There are intrinsic factor-vitamin B12 binding sites on the brush border of the mucosal cells in this region. Free intrinsic factor does not interact with the receptors (Seetharam, 1999).

The absorption of vitamin B12 is limited by the number of intrinsic factor-vitamin B12 binding sites in the ileal mucosa, so that not more than about 0.7 to 1.1 nmol (1 to 1.5 ^g) of a single oral dose of the vitamin can be absorbed. The absorption is also slow; peak blood concentrations of the vitamin are not achieved for 6 to 8 hours after an oral dose.

Within the ileal mucosal cell, the vitamin is released by lysosomal proteolysis of intrinsic factor, and is bound to transcobalamin II, a vitamin B12 binding protein synthesized in the enterocytes. Transcobalamin II is in vesicles destined for export from the enterocytes, and it is assumed that vitamin B12 binds to the apoprotein in these vesicles rather than in the lysosomes, because otherwise newly synthesized transcobalamin would be hydrolyzed by lysosomal proteases (Seetharam, 1999).

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