Digestion and Absorption of Vitamin

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 conformational change, resulting in dimerization and greatly enhanced resistance to proteolysis. 

Vitamin B12 is absorbed from the distal third of the Ueum 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 Bi2 binding sites in the ileal mucosa, so that not more than about 0.7 to 1.1 nmol (1 to 1.5 /xg) 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. 

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