Intrinsic factor-mediated intestinal absorption of cobalamin in the dog

The purpose of these studies was to determine whether gastric intrinsic factor and the ileal intrinsic factor receptor participate in the process of cobalamin absorption in the dog. Physicochemical analysis of gastrointestinal fluids and mucosal extracts obtained 3-5 h after cyano[57Co]-cobalamin was fed to dogs demonstrated that 1) all cyano-[57Co]cobalamin became bound to proteins during intraluminal transport; and 2) mucosal cyano[57Co]cobalamin in the extract of the ileal mucosa was bound to intrinsic factor, to intrinsic factor coupled to receptor protein, and to proteins with properties similar to R protein and transcobalamin II. A significant fraction of the cyano[57Co]cobalamin in the mucosal extract was membrane bound and, upon solubilization with Triton X-100, was found to contain immunoreactive intrinsic factor that, however, could no longer couple to the isolated receptor. The formation of the complex of cobalamin with intrinsic factor and the receptor protein and the selective accumulation of cobalamin in the ileum indicate that the intrinsic factor-mediated mechanism for absorption of this vitamin is active in the dog.