Intestine protein digestion

Delcroix, M., Sajid, M., Caffrey, C.R., Lim, K.-C., Dvorak, J., Hsieh, I., Bahgat, M., Dissous, C., and McKerrow, J.H. (2006) A multienzyme network functions in intestinal protein digestion by a platy-helminth parasite./. Biol. Chem. 281, 39316-39329. 

Digestion of fats occurs only in the small intestine. Protein digestion begins in the stomach and is completed in the small intestine. During digestion, fats are broken down into fatty acids and glycerol. Proteins are broken down into amino acids. 

Lysine availability tests are useful to determine the degree of heat damage in proteins, but they do not necessarily determine potential decreases in intestinal protein digestion or information on individual AA digestibility. Although the technique may be appropriate for eomparing different samples within the same feeds, it is not recommended for use among various feeds. 

FIGURE 2.16 pH versus enzymatic activity. The activity of enzymes is very sensitive to pH. The pH optimum of an enzyme is one of its most important characteristics. Pepsin is a protein-digesting enzyme active in the gastric fluid. Trypsin is also a proteolytic enzyme, but it acts in the more alkaline milieu of the small intestine. Lysozyme digests the cell walls of bacteria it is found in tears. 

M Yoshioka, RH Erikson, JF Woodly, R Gulb, D Guam, YS Kim. Role of rat intestinal brush-border membrane angiotensin-converting enzyme in dietary protein digestion. Am J Physiol 253 G-781-G-786, 1987. 

Digestion and absorption in the small intestine. Most digestion and absorption of carbohydrates, proteins, and lipids occurs in the small intestine. A summary of the digestive enzymes involved in these processes is found in Table 18.3. 

Protein digestion occurs in two stages endopeptidases catalyse the hydrolysis of peptide bonds within the protein molecule to form peptides, and the peptides are hydrolysed to form the amino acids by exopeptidases and dipeptidases. Enteropeptidase initiates pro-enzyme activation in the small intestine by catalysing the conversion of trypsinogen into trypsin. Trypsin is able to achieve further activation of trypsinogen, i.e. an autocatalytic process, and also activates chymotrypsinogen and pro-elastase, by the selective hydro-... 

Although glutamine is present in protein and therefore made available in the lumen of the intestine by digestion, most of this is metabolised after absorption by the enterocytes in the intestine. Glutamine, therefore, has to be synthesised in the body, the precursors for which are glucose and branched-chain amino acids. It is synthesised in muscle, adipose tissue and the lung (see Figure 8.23). Furthermore, muscle... 

Protein digestion starts in the mouth and continues in your stomach and small intestines. This is due to pepsin, which is secreted in the saliva and obviously the gastric juice, followed by pancreatic enzymes, then absorbed by the mucosal cells in the small intestines. In short, the digestive system breaks down protein into its peptide amino acid structures so they can be absorbed in the small intestine via the... 

Enzyme assays As shown previously the LMW fraction had a repressing effect on the protein digestion in the in vivo experi-ment. Accordingly, it was of interest to study in vitro the effect of this fraction on the kinetics of reactions catalyzed by proteases and peptidases present in the gastro-intestinal tract. 

The effects of premelanoidins on nitrogen retention is explained by a reduction in protein digestibility due to (a) the inactivation of the proteolytic enzymes, (b) the formation of indigestible peptides (20,38), (c) the inhibitory effects on amino acid absorption (33) and (d) by a decrease in the efficiency of the protein synthesis. The Maillard products also have an inhibitory effect on the intestine saccharidases (39). 

---