title: The role of sialic acid in the binding of calcium ions to intrinsic factor and its intestinal receptor.
abstract: The binding of vitamin B-12-intrinsic factor (cobalamin-intrinsic factor) to its intestinal receptor requires Ca2+. Only low concentrations of Ca were shown to be needed; therefore apparently high-affinity Ca-binding sites, such as neuraminic acids, are involved. Accordingly, the Ca-binding properties of purified human intrinsic factor and purified hog intrinsic factor receptor were studied by gel filtration and by isoelectric focusing. Cobalamin-intrinsic factor binds 45Ca2+ as a stable complex and in electrofocusing the 45Ca activity resolves into several peaks corresponding to the acidic isoproteins of cobalamin-intrinsic factor. Digestion with neuraminidase abolishes the Ca-binding capacity of cobalamin-intrinsic factor. This intrinsic factor binds 45Ca2+ via sialic acids. The purified receptor also binds 45Ca2+ but the binding is not lost after neuraminidase treatment. Thus the receptor does not bind Ca2+ with sialic acid residues sterically accessible to neuraminidase. The desialylated receptor still binds both cobalamin-intrinsic factor and desialocobalamin-intrinsic factor.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003016:Clostridium perfringens / Q000201:enzymology; D005750:Gastric Juice; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D007437:Intrinsic Factor / Q000378:metabolism*; D009439:Neuraminidase / Q000494:pharmacology*; D011485:Protein Binding; D011956:Receptors, Cell Surface / Q000302:isolation & purification / Q000378:metabolism*; D012794:Sialic Acids / Q000378:metabolism*; D013552:Swine
chemical_list: D011956:Receptors, Cell Surface; D012794:Sialic Acids; C029486:intrinsic factor-cobalamin receptor; D007437:Intrinsic Factor; D009439:Neuraminidase; D002118:Calcium