Cobalamin binding proteins

Cobalamin binding protein Intrinsic factor of Castle... 

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. 

Seetharam B and Alpers DH (1994) Cobalamin binding proteins and their receptors. In Vitamin Receptors Vitamins as Ligands in Cell Communication, K Dakshinamurti (ed.), pp. 78-105. Cambridge, UK Cambridge University Press. 

Lee, E. ., Seetharam, B Alpers, D. H., and DeSchryver-Kecskemeti, K. (1989). Jmmuno-histochemical survey of cobalamin-binding proteins. G strrJtTrtO oJogy 97, 1171-1180. 

Intestinal Receptor for IF-B 2 Complex. Although this receptor is not, strictly speaking, a cobalamin-binding protein, it is essential for normal absorption of dietary cobalamin. It is present on the membrane of microvilli of ileal but not jejunal or duodenal cells, with the highest concentration in the distal 60-cm portion of the small intestine. The purified receptor is composed of two subunits (M.W. 90,(X)0 and 140,000) and binds free IF and IF-B12 complex, although free IF binds more slowly. Subsequent transport of cobalamin into enterocytes is accomplished by an active process. 

Cobalamins extracted from the sample matrix (see Section 26.3.1) are allowed to bind to a cobalamin binding protein in competition with labelled cobalamin. 

Critical points are the choice of the cobalamin binding protein and the choice of method to separate the non-bound cobalamin from the protein-bound fraction. 

The extracted cobalamins are mixed with excess cobalamin-binding protein of unknown origin and are allowed to bind. The reaction mixture is applied to a cobalamin-precoated microchip. The amount of unsaturated cobalamin-binding protein that binds to the chip is detected and is inversely proportional to the cobalamin concentration in the sample. The specificity of the method depends on the binding protein employed. If intrinsic factor is used as binding protein, only cobalamin will be quantified. If a haptocorrin-like protein is employed, the sum of cobalamin and analogues is measured. 

An alternative strategy has been applied in our laboratory (Nexo et al. 2002). The prineiple is to initially remove all unsaturated cobalamin binding proteins in the sample by absorption to a solid-phase cobalamin surface. Measurement of transcobalamin in this pre-treated sample by ELISA reflects the amount of holoTC in the sample. 

Non-intrinsic factor cobalamin-binding protein Ovalbumin... 

Greibe, E., Fedosov, S., Nexo, E. 2012. The cobalamin-binding protein in zebrafish is an intermediate between the three cobalamin-binding proteins in human. PLoS One, 1, c35660. 

Nexo, E. 1998. Cobalamin Binding Proteins. In Krautler, B., Arigoni, D., Golding, B. T. (eds.) Vitamin B12 and B12 Proteins. Weinheim WUey-VCH. 

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