Protein and amino acid metabolism in the intestine of the pig : from digestion to appearance in the portal vein

Abstract

The digestion of dietary proteins is achievedby several proteolytic enzymes, released by the pancreas and peptidases from the brush border. The products of digestion are composed of free amino acids and peptides. Amino acids and peptides are transported into the enterocytes where the latter are hydrolysed. The profile of amino acids in the portal vein is different from dietary amino acids. Indeed, intestinal metabolism is very active. Both circulating and luminal amino acids are taken up by the intestine in order to ensure the synthesis of constitutive and secreted endogenous proteins. Intestinal protein turnover rate exceeds 50% per day. The synthesis of endogenous proteins (mucins for example) with a specific amino acid composition induces high requirements for some amino acids like threonine. The gastrointestinal tract is the main organ utilising circulating glutamine and dietary glutamate. Catabolism of these amino acids produces alanine, aspartic acid, proline and, via the enzymes of urea cycle, ornithine, citrulline and arginine. Essential amino acids seem to be involved in catabolism in the intestine as well. Finally, the role of the intestine is not limited to protein digestion and amino acid absorption but intestinal metabolism changes dietary amino acid availability to the other extraintestinal tissues.