Intestinal Absorption Of Macromolecules In The Adult *
This chapter discusses macromolecules whose absorption is initiated by binding to the surface of enterocytes. According to knowledge, amino acids and peptides are produced by proteases in the lumen of the gut. The digestion of peptides is continued by luminal carboxypeptidases, membrane-bound aminopeptidases of the brush border, and cytosolic peptidases. Cyanocobalamin functions as a coenzyme, and its dietary deficiency results in pernicious anemia. Minot and Murphy975 demonstrated the dramatic curative effect of a liver diet in pernicious anemia. The “extrinsic factor” of the liver was later purified and identified as cyanocobalamin. However, the intestinal absorption of the vitamin requires an additional compound, the “intrinsic factor” (IF), which is secreted by the stomach. Dietary cyanocobalamin is bound to proteins, but after a gradual release by gastrointestinal proteases it forms a complex with IF. The ingested food stimulates the secretion of both IF and proteases.