Most protein digestion occurs principally in the small intestine under the influence of the proteolytic enzymes of the pancreatic secretion. When the proteins leave the stomach they are mainly in the form of large polypeptides. Immediately upon entering the small intestine, the partial breakdown products are attacked by the pancreatic enzymes. Trypsin and chymotrypsin split protein molecules into small polypeptides, carboxypolypeptidase then cleaves individual amino acids from the carboxyl ends of the polypeptides. The brush border of the small intestine contains several different enzymes for hydrolysing the remaining small peptides. The constituent amino acids are then absorbed. Most naturally occurring amino acids are L-isomers which are transported against concentration gradients by sodium-dependent carrier mechanisms. There are four carrier systems for amino acids: for neutral amino acids (histidine), for basic amino acids (lysine), for dicarboxylic acids (glutamic acid) and a fourth transports the amino acids proline, hydroxyproline and glycine.
Enterocytes do not have transporters to carry proteins across the plasma membrane and proteins cannot permeate tight junctions. However, studies suggest that very small amounts of proteins may be absorbed intact. In most instances, the extent of this absorption is small and nutritionally not significant, however it can result in immune reactions, hormonal or toxic effects. This is most clearly seen in neonates. This enhanced ability, which is rapidly lost, is of immense importance because it allows the newborn babies to acquire passive immunity by absorbing immunoglobulins from colostral milk.
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