Intestinal transport of amino acids, peptides and proteins

In animals, the nutritional needs for amino acids are met by the assimilation of dietary proteins. Digestion of proteins in the lumen of the gastrointestinal tract generates products which are absorbable by enterocytes. These products are mainly amino acids and small peptides. In addition to amino acids and peptides originating from protein digestion, non-proteinogenic free amino acids present in the food such as taurine are also absorbed at the intestinal epithelium. The paracellular pathway for absorption of amino acids and larger molecules is restricted by tight cell junctions. The transcellular route requires transport across two morphologic and functional different membranes of the enterocyte: The apical (luminal) brush border membrane and the basolateral (abluminal, serosal) membrane. At both membranes, multiple transport systems are expressed to handle the different digestion end products. So far, seven different amino acid transport systems in the apical membrane and seven in the basolateral membrane of enterocytes are known on a functional and molecular level. The carriers take up amino acids either against a concentration gradient by using inwardly directed Na+- or H+-gradients or in amino acid exchange or they catalyse facilitated diffusion. In addition to the amino acid transporters, enterocytes express in their apical membrane an active transport system for di- and tripeptides. In the basolateral membrane, a peptide transport system has been characterized functionally but not yet on a molecular level. Intestinal amino acid and peptide transport is under regulatory control of intracellular and extracellular signals. Under certain circumstances large proteins can be absorbed intact at the intestinal epithelium by endocytotic mechanisms.