Studies on the solubilized porcine ileal intrinsic factor receptor and on a 340 000-dalton component binding vitamin B-12.
A 340 000-dalton component "C-III" was found when Triton X-100-containing extracts of ileal mucosa were incubated with human or porcine intrinsic factor vitamin B-12 preparations. It was not formed when abnormal human intrinsic factor, unable to attach to the intrinsic factor receptor, was used. Prolonged storage promoted the trnasfer of vitamin B-12 to it from the vitamin B-12-intrinsic factor recptor species C-I and C-II. The component was also present in ileal extracts prepared with or without detergent and it bound vitamin B-12 directly. Immunologically and by electrofocusing it could be classified as a cobalophilin but its molecular dimensions were larger than described for cobalophilin. It thus represents a novel vitamin B-12 binding protein, possibly a macromolecular acceptor of vitamin B-12 which accepts vitamin B-12 bound via intrinsic factor to the ileal intrinsic factor recptor. In the presence of EDTA or at low pH, vitamin B-12-intrinsic factor did not bind to any of the receptor species and under the same conditions it could all be dissociated from the receptor complexes but not from C-III. The dissociated receptor was able to recombine with vitamin B-12-intrinsic factor and it appeared to bind free and vitamin B-12-bound intrinsic factor in vivo.
D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D003035:Cobalt; D006801:Humans; D007082:Ileum / Q000378:metabolism*; D007413:Intestinal Mucosa / Q000378:metabolism; D007437:Intrinsic Factor / Q000378:metabolism*; D046911:Macromolecular Substances; D008667:Metalloproteins / Q000378:metabolism; D011955:Receptors, Drug / Q000378:metabolism*; D013552:Swine; D014805:Vitamin B 12 / Q000378:metabolism*
D002352:Carrier Proteins; D046911:Macromolecular Substances; D008667:Metalloproteins; D011955:Receptors, Drug; D003035:Cobalt; D007437:Intrinsic Factor; D014805:Vitamin B 12