Introduction

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 (HQ) 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 di- and tripep-tides 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

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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 peptide-bound 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,

© CAB International 2003. Amino Acids in Animal Nutrition, 2nd edition (ed. J.P.F. D'Mello)

Protein

Protein

BLOOD

Fig. 3.1. The current model for the role of peptide uptake in protein assimilation, as adapted from Ganapathy etal. (1994). The relative contributi on of peptide (di-, tri-) versus free amino acid (AA) to total protein assimilation through hydro lytic and transport events is indicated by the relative thickness of the lines. After hydrolysis by gastric proteases and luminal peptidases, oligopeptides are hydrolysed to small peptides and free amino acids by apical membrane-bound (1) peptidases. Peptides are absorbed across brush border membrane by PepT1 (2). Whereas a small proportion of absorbed peptides are then absorbed intact across the basolateral membrane by a H+-independent transport activity (3), the majority are hydrolysed by intracellular peptidases (4). The resulting free AA, plus those absorbed across the apical membrane by a complement of Na+-dependent and -independent amino acid transporters (5) are then transported across the basolateral membrane by a complement of Na+-independent and amino acid exchanger transport proteins (6). The extracellular-intracellular H+ gradient that drives PepT1 activity, is re-established by the combined function of the apical Na+/H+ exchanger (7) and the basolateral Na+/K+ ATPase (8), which re-establishes the extracellular-intracellular Na+ gradients diminished by both Na+/H+ exchanger and Na+-coupled free amino acid transport. The contribution to total protein assimilation by free AA uptake from the lumen is represented by a composite transporter model (7), representing AA transport by Na+-coupled, AA counterexchange, and/or facilitated transport proteins. The transepithelial passage of intact proteins is also indicated (dashed line). The mechanisms responsible for this relatively minor, but immunologically important process, have been reviewed by Gardner (1994).

enzyme and transporter expression and activity of the associated tissues is fundamentally similar. Although differences do exist, enzymes and transport proteins responsible for digestion and absorption have probably adapted to the nature of the food more than the type of animal (Lassiter and Edwards, 1982). Therefore, processes involved in mammalian protein digestion and peptide and amino acid absorption are generally common to all species. We will attempt to point out differences where known. Our goal in this chapter is to review some of the more recent findings regarding peptide and amino acid net flux and transport processes. We begin with a brief review of processes involved in protein digestion in the glandular stomach. Pregastric digestion processes of ruminants and birds are topics of the other reviews.

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