Effect of divalent cations and pH on intrinsic factor-mediated attachment of vitamin B 12 to intestinal microvillous membranes.

Calcium, but not other divalent cations, is required for optimal uptake of intrinsic factor-bound (57)Co-labeled cyanocobalamin (IFB(12)) by microvillous membranes isolated from hamster ileal-absorptive cells. Chelation of divalent cations by disodium ethylenediaminetetraacetate (EDTA) promptly removes IFB(12) previously attached to microvillous membranes. High concentrations of CaCl(2) or MgCl(2) also markedly inhibit membrane uptake of IFB(12) and rapidly remove previously attached IFB(12). Similarly, reduction of pH to below 5.4 prevents membrane attachment of IFB(12) and removes virtually all IFB(12) already bound to microvillous membranes. The effects of calcium depletion, increased salt concentrations, and acidification on membrane uptake of IFB(12) were completely reversible. These findings are consistent with the concept that the formation of calcium salt bridges is essential for attachment of IFB(12) to the ileal-absorptive surface.