Intestinal uptake and release of cobalamin complexed with rat intrinsic factor

The mechanism of uptake of intrinsic factor (IF) and cobalamin (Cbl) by enterocytes and their subsequent fate have been uncertain. To examine this problem double-labeled IF X Cbl was added to small intestinal organ cultures. When 125I-IF X [57Co]Cbl was added to rabbit ileal explants, binding and internalization increased linearly for 24 h. After an 18-h chase with nonlabeled IF X Cbl, no 125I-IF returned to the cell surface. An amount of 35-45% of the internalized Cbl was found free, not bound to IF or any other protein. About 60% of both internalized ligands was bound to membranes but by a non-Ca2+-dependent bond, suggesting binding to a protein other than the brush-border receptor. Cobalamin was released from IF at pH 5.0 to the same degree (30%) as free Cbl was found inside the cell (35-45%). Neither pancreatic proteases nor ileal homogenates effected release of Cbl from IF. When cathepsins were added, the Cbl released was no greater than could be attributed to pH 5.0 alone. Chloroquine added to tissue explants did not alter the percentage of free intracellular Cbl. From these results we suggest that IF X Cbl is internalized and detached from the receptor within the enterocyte. The mechanism of release is not known but seems to require an acid pH (5.0). The Cbl is released in the mucosa, perhaps when the IF X Cbl complex enters a nonlysosomal cellular compartment with an acidic environment. There is no substantial recycling of IF to the brush-border membrane.