Amino Acids - Chap 03

Assimilation of dietary or microbial (ruminants) protein involves the interaction of a series of steps beginning in the stomach (non-ruminants), abomasum (ruminants), or proventriculus (poultry) and ending with the transport of amino acids and peptides from the basolateral membrane of the small intestine. In the glandular stomach, hydrochloric acid (HCl) denatures dietary protein and promotes proteolysis protein to large polypeptides via the action of pepsin. On entering the small intestine, pancreatic proteases principally hydrolyse large polypeptides and proteins into oligopeptides of six or less amino acid residues as well as free amino acids. Degradation of dietary protein continues by hydrolytic enzymes of the small intestine epithelia that are present in the luminal surface (apical membrane or brush border) of absorptive epithelial cells (enterocytes). Brush border peptidases split oligopeptides of six or less amino acids in length. Many of the resulting diand tripeptides are transported into the enterocyte intact by a single H+-coupled transporter and then hydrolysed to free amino acids by cytosolic peptidases (primary) or transported across the basolateral membrane. In contrast, free amino acids are absorbed by a variety of iron-dependent and -independent transporters. The fate of absorbed peptides is principally further hydrolysis to free amino acids by a variety of cytosolic peptidases, whether absorbed as free or peptide-bound amino acids, cytosolic amino acids are available as energy substrates, incorporation into constitutive protein, or transport across the basolateral membrane into blood. Ultimately, digested protein enters the hepatic portal circulation in the form of free amino acid and peptides. The working hypothesis for assimilation of luminal proteins by enterocytes is illustrated in Fig. 3.1. The model identifies gastric, luminal, glycocalyx/apical membrane, and intracellular hydrolytic digestion events, in addition to apical and basolateral membrane-mediated absorption events of peptidebound and free amino acids by specific transport proteins. Each component of the model is discussed in this chapter. Although poorly understood, and in contrast to the specificity of digestion and transport events, it is also important to note that the potential contribution to the absorption of amino acids by relatively non-specific transmembrane simple diffusion and paracellular flow events may be of nutritional significance. Despite anatomical differences in the digestive tracts among farm animal species,