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Updated: 08/21/07 08:12 AM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Authors: D.R. Saunders, C.E. Rubin, and J.D. Ostrow
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Download Chapter ♦ Download Lecture Slides & Notes
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M. Normal Protein Absorption
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Proteins are amino acid polymers (polypeptides). Protein contains four calories per gram. A 70 kg person should eat at least 45 grams of good quality protein (containing all of the essential amino acids) daily in order to maintain a balance between intake and body needs (nitrogen balance).
Many amino acids can be synthesized by the body from simple precursors, but certain ones cannot. These are the essential amino acids which must be provided by an adequate intake of proteins of suitable composition. |
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Amino acids are absorbed efficiently as dipeptides About 20 different membrane peptidases are present |
Proteins are partially broken down to large oligopeptides in the stomach by pepsins at optimal pHs of 2 and 3.5. The peptidases of the pancreas further hydrolyze the proteins and large oligopeptides at an optimal pH of 7-8. After the combined action of gastric and pancreatic proteolytic enzymes, dietary protein is largely reduced to a mixture of small oligopeptides (2 6 amino acid residues) and free amino acids (15 25%). The final digestion of the small oligopeptides to three absorbable forms (amino acids, di- and tripeptides) is catalyzed by oligopeptidases attached to the microvillous membrane. The di- and tripeptides are absorbed more easily than amino acids. Within the cytoplasm, most are quickly hydrolyzed to their constituent amino acids. The absorbed amino acids and a small fraction of the dipeptides find their way to the portal venules in the lamina propria and reach the liver via the portal vein.
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Multiple membrane carriers for amino acids and peptides |
There is a membrane carrier for the transport of di- and tripeptides which is the main way that amino acids from dietary protein enter the absorptive cell. There is one membrane carrier for neutral amino acids, another for basic amino acids and a third system for proline and hydroxyproline. Dipeptides and neutral amino acid transport systems require Na+ in the lumen and probably involve a common carrier in a coupled transport system. Absorbed amino acids, particularly glutamine, are major fuels for small intestinal mucosa.
Approximately two-thirds of protein within the lumen is dietary in origin. The rest is endogenous: 10 g/d of protein enzymes, 10 g/d of protein in exfoliated epithelial cells and 1 4 g/d of plasma proteins normally leaked into the gastrointestinal lumen. |
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| Fecal protein is mainly bacterial protein |
Amino acids, di- and tripeptides are absorbed most efficiently in the jejunum. By the time the meal enters the ileum, 80% of the food protein together with the endogenous protein has been digested and absorbed. Ten percent is absorbed in the ileum. The protein content of the stools equals about 10% of protein eaten but its source is bacteria and cellular debris. Colonic bacteria can deaminate some of this protein to liberate ammonia which diffuses into the portal blood and reaches the liver, where it is made into urea, or into amino acids.
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Next Section (N): Protein Malabsorption » |
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