Cobalamin-binding transport proteins

Transport. Transcobalamin II dehvers the absorbed vitamin 3 2 to cells and is the primary plasma vitamin B22-binding transport protein. It is found in plasma, spinal fluid, semen, and extracellular fluid. Many cells, including the bone marrow, reticulocytes, and the placenta, contain surface receptor sites for the transcobalamin II—cobalamin complex. 

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. 

After the stomach s acidic environment facilitates the breakdown of vitamin B12 bound to food, the vitamin B12 binds to the intrinsic factor released by the stomach s parietal cells. The secretion of intrinsic factor generally corresponds to the release of hydrochloric acid and serves as a cell-directed carrier protein similar to transferrin for iron. This complex, resistant to degradation, forms in the duodenum and allows for subsequent absorption of vitamin B12 in the terminal ileum. The cobalamin-intrinsic factor complex is taken up into the ileal mucosal cell, the intrinsic factor is discarded, and the cobalamin is transferred to transcobalamin It, which serves as a transport protein. This complex is secreted into the circulation and is taken up by the fiver, bone marrow, and other cells. Transcobalamin 11 has a short half-fife of 1 hour and is rapidly cleared from the blood. Consequently, most circulating cobalamin is bound to serum haptocorrins (formerly transcobalamin I and transcobalamin IB) whose function is unknown. However, it should be noted that an alternate pathway for vitamin B12 absorption independent of intrinsic factor or an intact ter-... 

Dietary cobalamin is absorbed from animal food sources by a multistage process shown in Figure 42-2. Cobalamin absorption requires the presence of a protein (the intrinsic factor, IF) secreted from the parietal cells of the stomach to bind cobalamin and aid in its absorption in the ileum. The protein is released into the ileum while the cobalamin is transported to the blood stream where it binds specialized serum proteins, the transcobalamins (TC), which transport it to other tissues such as liver where cobalamin can be stored (usually several milligrams are present in liver). In the absence of the intrinsic factor... 

Vitamin B,. Vitamin Bi2 the colloquial name for cobalamin (Cbl), is a large polar molecule that must be bound to specialized transport proteins to gain entry into cells. After oral administration it is bound to intrinsic factor (IF), a protein released from the parietal cells in the stomach and proximal cells in the duodenum. The Cbl-IF complex binds to an IF-receptor located on the surface of the ileum, which triggers a yet undefined endocy-totic process. After internalization, the fate of the IF-Cbl complex has yet to be clarified. It vvas reported that IF-Cbl complex dissociates at acidic pH, and Cbl is transferred to transco-balamin II by Ramasamy and coworkers (94), whereas Dan and Cutler (95) found evidence of free Cbl in endosomes and the basolateral side of the membrane after administration to the apical surface of Caco-2 cell monolayers. It is clear, however, that Cbl is transported into all other cells only when bound to transcobal-amin II. 

Fedosov, S. N., Fedosova, N. U., Krautler, B., Nexo, E., Petersen, T. E. 2007. Mechanisms of discrimination between cobalamins and their natural analogues during their binding to the specific B 12-transporting proteins. Biochemistry, 46, 6446-58. 

The hydrophilic cobalamin molecule has to be transported from the intestine to the blood by an elaborate transfer system. Cobalamin is first released from binding substances in the food by peptic activity at low pH in the stomach and becomes bound to so-called R-binders present in saliva and gastric juice. In the ileum pancreatic proteolytic enzymes dissociate the cobalamins from the R-binders and in this way facilitate their binding to Intrinsic Factor, a glycoprotein that in man is synthesized and released by the gastric parietal cells. The cobalamin-Intrinsic Factor complex becomes attached to specific receptors on the mucosal surface of the distal part of the ileum (45). Through an incompletely defined mechanism cobalamin enters the mucosal cell and is passed on to the plasma transport protein transcobalamin II (46). This 38,000-MW polypeptide carries cobalamin through the portal circulation first to the liver and distributes it from there to the other tissues. 

The human Bu-binder intrinsic factor is a glycoprotein of ca. 44kDa, with a high binding constant (in 1 1 complexes) for vitamin B12 (1) and other cobalamins. The intrinsic factor is secreted by cells of the gastric mucosa and specifically binds cobalamins and carries them to the ileum. There the ileum receptor protein accepts the corrinoid from the intrinsic factor complex and transports it further across the intestinal epithelial absorptive cell. The cobalamins then appear to be bound to transcobalamin II and transported in the blood in this way to membrane-bound transcobalamin/corrin receptor proteins of the specific cells. ... 

On the ingestion of a physiologic quantity of cobalamin, a series of specific events occur in a sequential fashion to ensure appropriate and adequate absorption. First, cobalamin is released from dietary proteins, whereon the salivary R proteins ensure intragastric binding of the substance. Thereafter, intraduodenal digestion of the R protein cobalamin complexes occurs, particularly by pancreatic proteases, which are responsible for the release of cobalamin. Once cobalamin is free within the duodenum or upper small bowel, binding of the vitamin by intrinsic factor occurs before its transport to the lower ileum. At this point, the intrinsic factor-bound cobalamin is attached to the apical membrane of the ileum cells, and absorption via specific receptor-mediated pathways occurs. The transcellular transport of cobalamin in the ileal mucosa is a slow process and presumably involves endocytosis. The vitamin is thereafter released into the portal circulation and passes to the liver for processing. 

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