Effect of divalent cations and pH on intrinsic factor-mediated attachment of vitamin B12 to intestinal microvillous membranes

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Abstract
Calcium, but not other divalent cations, is required for optimal uptake of intrinsic factor-bound 57Co-labeled cyanocobalamin (IFB12) by microvillous membranes isolated from hamster ileal-absorptive cells. Chelation of divalent cations by disodium ethylenediaminetetraacetate (EDTA) promptly removes IFB12 previously attached to microvillous membranes. High concentrations of CaCl2 or MgCl2 also markedly inhibit membrane uptake of IFB12 and rapidly remove previously attached IFB12. Similarly, reduction of pH to below 5.4 prevents membrane attachment of IFB12 and removes virtually all IFB12 already bound to microvillous membranes. The effects of calcium depletion, increased salt concentrations, and acidification on membrane uptake of IFB12 were completely reversible. These findings are consistent with the concept that the formation of calcium salt bridges is essential for attachment of IFB12 to the ileal-absorptive surface.