Protein Digestion and Absorption

Dietary proteins undergo luminal digestion by gastric and pancreatic proteases and then membrane digestion by peptidases associated with the brush border membrane of enterocytes. The end products of this digestion are predominantly dipeptides and tripeptides and, to a lesser extent, free amino acids. Peptides are absorbed across the brush-border membrane via a proton-coupled peptide transporter whereas free amino acids are absorbed via multiple amino acid transporters. Once inside the cell, dipeptides and tripeptides are hydrolyzed into free amino acids by cytosolic peptidases. Amino acids then exit across the basolateral membrane via specific amino acid transporters. There are several genetic diseases related to amino acid transport defects in the intestine; clinical consequences of these defects are dependent on specific amino acids whose absorption is impaired in each of these defects. To date, there are no genetic diseases identified in peptide transport in the intestine that result in significant clinical consequences.