Intrinsic Factor-mediated Absorption of Cobalamin by Guinea Pig Ileal Cells

Abstract

A B S T R A C T 30 min, we instilled into tied-off ileal loops of intact guinea pigs radiolabeled intrinsic factor-cobalamin complex and extracted nondissociable radioactivity 2-4.5 h later. The proportion of extracted 57Co eluting as free cobalamin increased to 39-46%, that eluting as intrinsic factor-cobalamin complex declined to 22-45%, and 9-34% now eluted as a macromolecule that reacted with antitranscobalamin II antibody but not antiintrinsic factor antibody. Extracted 35S radioactivity eluted in several peaks in addition to the intrinsic factor peak. These findings suggest that (a) after reversible attachment of intrinsic factor-cobalamin complex to its ileal surface receptor, an energy-dependent process prevents removal of the complex from the cell Parts of this research were reported at the Annual Meetings of the American

Assessing the site of increased intestinal permeability in coeliac and inflammatory bowel disease.

“…There was no discrepancy between the rate of appearance of 3-0-methyl-Dglucose and 51CrEDTA after duodenal instillation in normal subjects and previous studies have also shown that sulphapyridine appears within six minutes of caecal instillation.9 Our results, however, show an inconsistency between the time of appearance of the 'ileal' and 'caecal' markers in serum and in six cases sulphapyridine appeared before the 'ileal' marker. These findings are consistent with reports of a 30-240 minute delay for the appearance of vitamin B12 after ileal instillation, 8 12 which is probably due to intracellular processing of the vitamin B12 intrinsic factor complex.16 17 The 'first appearance' of 57CoVitBI2 in serum is therefore likely to overestimate the time it takes for the 'head' of the test solution to reach the ileum. In both normal subjects and patients with ulcerative colitis there is a significant lag time between the serum detection of 3-0-methyl-Dglucose and 51CrEDTA whereas the appearance of both markers in the serum after direct instillation into the duodenum is both rapid and simultaneous.…”

Receptor‐mediated endocytosis of the intrinsic factor—cobalamin complex in HT 29, a human colon carcinoma cell line

“…3). Endocytosis of IF-Cbl has been reported in viva in guinea pig ileum by electron microscope radioautography [15] and in vitro, in ileal isolated cells [16] although an immunocytochemical electron microscopic study failed to demonstrate it [17]. Endoeytosis of IF-Cbl was inhibited by pretreating the cells for l h with either EDTA, vinblastine, or monensin.…”

Disorders of Cobalamin (Vitamin B12) Absorption and Transport

“…Attachment is neither energy nor temperature dependent, but requires extracellular Ca and a pH greater than 5.4 (22). Subsequent entry of cobalamin into the absorptive cell clearly requires cellular energy (23), but how the vitamin enters the ileal cell and then reaches the portal circulation remains poorly understood. It is still uncertain, for example, whether the entire IF -cobalamin complex enters the absorptive cell (23)(24)(25) or whether IF remains at the cell surface (26).…”

“…Unquestionably, however, once cobalamin reaches portal blood it is no longer bound to IF but rather to TCII. In fact, there is increasing evidence (23,(27)(28)(29) that cobalamin binds to TCII either within the enterocyte or at its plasma membrane surface.…”