Binding of intrinsic factor to ileal brush border membrane in the rat

The rat ileal brush border membrane binds both free ¹²⁵I]-intrinsic factor (IF) and the IF-⁵⁷Co]cobalamin (cbl) complex. This binding is observed with IF isolated from rat stomach, but not from IF isolated from hog, canine and human stomachs. The binding of rat-IF⁵⁷Co]cbl can be blocked with free rat IF but not with hog IF. The IF-cbl complex binds at a higher affinity (Ka=0.15 × 10⁹ M^?1) compared to that of free IF (Ka=0.9 × 10⁹ M^?1). Rat IF-cbl also binds efficiently to human and canine ileal membranes. While antibody to the canine ileal receptor blocks the binding of rat, human or hog IF-⁵⁷Co]cbl to human and canine ileal membranes, it does not affect the binding of rat IF-⁵⁷Co]cbl to rat ileal membranes. These findings demostrate that the rat ileal receptor is different from canine and human ileal receptors.