Intestinal absorption of amino acids and peptides

The aim of this account is to summarize current views on the absorption of the products of protein digestion, amino acids and peptides.Longer reviews of protein absorption (Wiseman, 1968; Matthew, 1969 Matthew, , 1971;; Gray & Cooper, 1971) will provide references not given here.Hormonal effects on absorption are reviewed by Levin (1969).Protein digestion is initiated in the stomach by pepsins and continued in the small intestine by the pancreatic proteases.Though much digestion occurs within the lumen of the gut, the action of proteases adsorbed at the brush border may also play an important part.After a protein meal, the intestinal lumen contains a complex mixture of small peptides and free amino acids, in which peptides predominate.Further breakdown of oligopeptides to dipeptides and free amino acids may occur through the action of brush-border oligopeptidases.It is now established that though, in general, only free amino acids enter the portal blood during protein absorption, protein digestion products leave the in- testinal lumen in two forms ( I ) as free amino acids and (2) as small peptides (Fig. I).Since the components of proteins are transported within the body almost entirely as free amino acids, the mucosal epithelium of the small intestine is distinguished from other epithelia by its ability to handle small peptides on a large scale.Comparatively little is known at the present time about the mechanisms of absorption of peptides. Absorption of amino acidsAmino acids are absorbed by carrier-mediated transport, and for glycine and most L-amino acids, transport has been shown to be active.I n adult small intestine, energy for transport is derived mainly, but perhaps not exclusively (De la Noiie, I 970), from oxidative metabolism.T h e active transport mechanisms appear to he situated in the plasma membrane of the mucosal poles of the absorptive cells, since amino acids are concentrated in the cells while transport is going on.For some years it was thought that the transport mechanisms were stereochemically specific for L-amino acids, but it has now been shown that several D-amino acids are actively