Nearly all binding of cobalamin (vitamin B12) to intrinsic factor occurs in which of the following organs?
ExplanationAlthough intrinsic factor is secreted by the oxyntic cells of the gastric mucosa, binding to cobalamin occurs in the duodenum. Cobalamin, also known as vitamin B12, is synthesized only by microorganisms and, in the human diet, is provided almost entirely from animal products. Gastric digestion of food liberates cobalamin where, at low pH, it binds primarily to the R protein-type binder, haptocorrin, derived primarily from salivary secretions. In the duodenum, pancreatic proteases release cobalamin from the haptocorrin but have no effect on intrinsic factor derived from parietal cells. Cobalamin rapidly complexes with intrinsic factor and is transported along the gut to the distal 60 cm of the ileum, where specific receptors located on the villus tip cells bind the cobalamin-intrinsic factor complex.
