Uptake and fate of absorbed amino acids and peptides in the mammalian intestine.

Intraluminal and brush-border digestion of proteins results in a mixture of amino acids and small peptides. Thirteen brush-border peptidases have been described. Despite all of these enzymes, some peptides escape digestion and are absorbed intact. The assimilated products of protein digestion can follow multiple paths: absorption into the blood as amino acids or small peptides, metabolism within the enterocyte, incorporation into proteins of the enterocyte, and incorporation into proteins to be secreted into plasma. Unlike other tissues, the intestinal mucosa is not very responsive to metabolic regulation as regards amino acid uptake or regulation of protein synthesis. Most effects after dietary manipulation or drug or hormonal stimulation are modes (two-to fivefold increases). This constitutive metabolism of amino acids in the intestinal mucosa is consistent with its essential role in absorption. The mucosa also is a major contributor to apolipoproteins, which are probably the quantitatively most important proteins secreted from the intestine. Alterations in apoprotein secretion have been noted after fat feeding, and are both transcriptionally and translationally regulated. Although the fractional renewal rate of protein in the intestine is the highest of any tissue in the body, the quantitative importance of alterations in protein synthesis or secretion to the fate of intracellular amino acids is not known.