The hydrolysis of proteins in the gastro-intestinal tract is accomplished by proteases secreted in gastric juice and pancreatic juice and also by proteases present in the intestinal mucosa.
Gastric Digestion: The proteolytic enzyme present in gastric juice is pepsin. Its optimum PH is about 2.0. Pepsin is an endopeptidase and it hydrolyses peptide bonds in the interior of the protein molecule. Pepsin hydrolyses dietary proteins mainly into a mixture of polypeptides.
If gastric HCL production is low and not adequate to maintain the PH of the stomach contents between 2 and 3, protein digestion in the stomach may be negligible. Pepsin has a strong clotting action on milk. This is of considerable importance in the digestion of milk proteins in infants.
Proteolysis in the Intestines: The digestion of polypeptides produced in the stomach takes place in the intestines. The proteases involved in the digestion are trypsin, chymotrypsin and carboxy peptidases A and B secreted in pancreatic juice and amino peptidases present in the intestinal mucosa. Trypsin and chymotrypsin act at PH 7.4 to 8.0. Trypsin hydrolyses mainly peptide linkages containing arginine or lysine and chymotrypsin hydrolyses peptide linkages containing tyrosine or phenylalanine.
Carboxy peptidase A hydrolyses the end group in peptides containing aromatic or aliphatic amino acid, thus releasing free amino acids. Carboxy peptides B hydrolyses peptides containing arginine and lysine residues. The intestinal mucosa contains a group of amino peptides which complete the hydrolysis of peptides to amino acids. The intestinal mucosa also contains tripeptidase, dipeptidase which hydrolyse tri and dipeptides.
The ultimate products of digestion of proteins are amino acids and these are absorbed.
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