Vitamin B12 Absorption Requires Two Binding Proteins

Vitamin B12 is absorbed bound to intrinsic factor, a small glycoprotein secreted by the parietal cells of the

CH2CONH2

ch2ch2conh2

h2ncoch2ch2 h2ncoch2

H3C'

H2NCOCH

h2ncoch2ch2 h2ncoch2

H3C'

H2NCOCH

CH,

ch2ch2conh2

CH, ch2ch2conh2

Figure 45-13. Vitamin B12 (cobalamin). R may be varied to give the various forms of the vitamin, eg, R = CN- in cyanocobalamin; R = OH- in hydroxocobal-amin; R = 5'-deoxyadenosyl in 5'-deoxyadenosylcobal-amin; R = H2O in aquocobalamin; and R = CH3 in methylcobalamin.

gastric mucosa. Gastric acid and pepsin release the vitamin from protein binding in food and make it available to bind to cobalophilin, a binding protein secreted in the saliva. In the duodenum, cobalophilin is hy-drolyzed, releasing the vitamin for binding to intrinsic factor. Pancreatic insufficiency can therefore be a factor in the development of vitamin B12 deficiency, resulting in the excretion of cobalophilin-bound vitamin B12. Intrinsic factor binds the various vitamin B12 vitamers, but not other corrinoids. Vitamin B12 is absorbed from the distal third of the ileum via receptors that bind the intrinsic factor-vitamin B12 complex but not free intrinsic factor or free vitamin.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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