1Absorption of amino acids and peptides from the intestine

Abstract

Until recently it was generally believed that proteins underwent complete hydrolysis within the lumen of the gut, and that only free amino acids were taken up by the mucosa of the small intestine. It has now been shown that this picture of protein absorption was incomplete, and that though, with a few exceptions, intact peptides do not enter the blood, there is another important mode of protein absorption involving mucosal uptake of small peptides followed by cellular hydrolysis. A great deal is known about the mechanisms of absorption of free amino acids, and this subject has beeia covered in a number of recent reviews (Wiseman, 1968; Gray and Cooper, 1971; Matthews, 1971). This review will deal mainly with the differences between the characteristics of peptide absorption and those of amino acid absorption, the mechanisms of peptide absorption, and the clinical and nutritional implications of absorption of peptides. Where references are not given, they will be found in more detailed accounts (Matthews, 1971, 1972, 1974). Standard three-letter abbreviations for free and peptide-bound amino acids will be used throughout. Amino acids should be taken to be the Lforms unless otherwise indicated.