The role of sialic acid in the binding of calcium ions to intrinsic factor and its intestinal receptor.

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.