Solubilized Receptor for Intrinsic Factor-Vitamin B12 Complex from Guinea Pig Intestinal Mucosa

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Abstract
The absorption of vitamin B12 in many animals requires its prior association with intrinsic factor (IF) and attachment to a specific receptor in the intestine. Employing Triton X-100, we have solubilized from guinea pig ileum a factor that binds intrinsic factor-vitamin B12 complex (IF-B12). This binding factor was soluble to the extent that it was not sedimented by centrifugation at 100,000 g for 1 h and was small enough to enter the included volume of a Sepharose 4-B column. Furthermore, the ileal extract contained no microfine particles of membrane upon electron microscopic search. When a portion of the extract was incubated with a mixture of gastric juice and 57Co-labeled vitamin B12, a portion of the radioactivity was excluded from a Sephadex G-200 column. When gastric juice from a patient with a congenital abnormality of IF that prevented its binding to intestine was substituted for normal human gastric juice, radioactivity was not excluded from the gel, indicating failure of this abnormal IF-B12 to bind to the intestinal extract. These data suggested the presence of a specific binder of IF-B12 in the ileal mucosal extract. The reactions of normal IF-B12 with the solubilized binding factor and with the membrane-bound “receptor” had several characteristics in common, including calcium dependence, temperature independence, and pH optimum near neutral. Extracts from the distal intestine showed more activity than did those from the proximal. The solubilized binding facter seemed specific for IF-B12 in that it was not blocked by prior incubation with excesses of either free vitamin B12 or IF. Binding activity of the extract was decreased by incubation at pH 2.0, by heating to 56°C, and by incubation with chymotrypsin and dithiothretiol. Incubation with trypsin, neuraminidase, and sulphydryl blockers did not affect it. The Triton X-100 extract of guinea pig ileal mucosa contains a specific binding factor that probably is the receptor for IF-B12. This appears to be a protein with function dependent on peptide and disulphide linkages.