Mammalian and avian protein digestion is initiated in the stomach (non-ruminants), abo-
masum (ruminants) or proventriculus (poultry) in the presence of HQ and pepsin. As discussed by Atasoglu and Wallace in Chapter 15, ruminant animals derive their amino acid supply from a mixture of feed protein that escapes ruminai degradation and microbial protein that is formed as a result of microbial fermentation in the reticulorumen. Microbial protein is readily digested by the host animal and constitutes a well-balanced array of essential amino acids for ruminants in many production systems. Gastric digestion involves the secretion of HC1 by gastric parietal cells. Hydrochloric acid is required to initiate the conversion of pepsinogens into pepsins, and also to maintain pepsin activity. Pepsins are secreted as inactive precursors (i.e. pepsinogens) by chief cells in the stomach. In chickens, both gastric acid and pepsinogen are secreted by oxynticopeptic cells of the proventriculus (true stomach). Although HCI production is relatively high, little digestion occurs in the proventriculus as there is little storage capacity, and digesta transit rate is rapid. The synthesis and secretion of inactive precursors, known as zymogens or proenzymes, allow vertebrates to digest exogenous protein without destroying constituent protein in the stomach and pancreas. Once secreted and activated (pH<5.0), pepsin increases the susceptibility of native proteins to attack by pancreatic proteases by opening the tertiary and quaternary structure of the protein and exposing amino acid residues to the pancreatic endopeptidases (Guan and Green, 1996). For some dietary proteins, gastric predigestion is also important for stimulation of pancreatic exocrine secretion and cholecystokinin (CCK) release in the intestinal phase of digestion in the rat (Guan and Green, 1996) and dog (Meyer and Kelly, 1976). As mentioned, there are a number of pepsinogens secreted and these are converted into the analogous pepsins when the pH is less than 5.0. Once pepsin is present in the lumen of the stomach, the reaction becomes autocatalytic, which involves the splitting off of a peptide chain and peptide fragments. The pepsins are most active at pH less than 4.0 and become inactive at pH greater than 6.0, although the optimum pH for pepsin activity varies from species to species (Crevieu-Gabriel et al., 1999).
Pepsins are most active at peptide bonds that include phenylalanine, tyrosine, leucine, valine and glutamic acid (Ulshen, 1987). Its most pronounced effect is between leucine and valine, tyrosine and leucine, or between the aromatic amino acids such as phenylalanine-phenylalanine or phenylalanine-tyrosine. The principal function of digestion in the stomach is the transformation of protein into large polypeptides, although peptic digestion in acid pH can produce free or peptide-bound amino acids. As suggested above, both amino acids and peptides are good stimuli for release of hormones that stimulate pancreatic enzyme secretion, e.g. CCK. Ultimately, the object of stomach proteolysis is to make peptide molecules available that are susceptible to further hydrolysis by proteolytic enzymes in the small intestine.
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