| Abstract: | The vitamin B-12-intrinsic factor receptor was shown to be present in pig ileum and localized on the brush borders of the enterocytes, and to be solubilisable with Triton X-100. At neutral pH and in the presence of Ca2+ it bound the vitamin B-12-intrinsic factor but not the vitamin B-12-cobalophilin complex. The solubilised vitamin B-12-intrinsic factor-receptor complex consisted of two molecular species with clear Stokes radii (13.11 and 33.53 nm), sedimentation coefficients (15.1 and 45.1 S) and molecular weights (1 600 000 and 12 000 000). A third smaller macromolecule possibly also represented the receptor. Some receptor activity was present in extracts prepared with buffers lacking detergent. There was evidence that the receptor is a membrane lipoprotein from which the lipids reversibly dissociate. Free intrinsic factor also bound to the solubilized receptor and its vitamin B-12-binding site seemed not to be involved in the attachment to the receptor. A small portion of the vitamin B-12-intrinsic factor spontaneously dissociated from the receptor and nearly all dissociated in the presence of Na2-EDTA. TheStokes radius of the dissociated vitamin B-12-intrinsic factor complex was 0.17 nm smaller than before binding to the receptor. Intrinsic factor and cobalophilin were present in ileal extract and observations were made on their molecular characteristics. These proteins, polymers of the vitamin B-12-intrinsic factor complex and binding of vitamin B-12 and its protein complexes to detergent micelles may give spurious receptor-like effects which must be properly controlled. |
