Digestion and Absorption

Protein digestion begins in the stomach, initiated by pepsins. Pepsins split peptide bonds involving Phe or Tyr R-groups. These endopeptidases (split polypeptides inside the molecule) are secreted by the gastric lining as inactive precursors, pepsinogens, which are activated at acidic pH. Acids further facilitate the enzymes' catalytic attack by denaturing the proteins. The activity of the gastric enzymes normally produces large fragments (poly-, oligopep-tides). After stomach resections or in case of pharmacological blockage of gastric acid production, subsequent breakdown by pancreatic enzymes can compensate for that step. In the duodenum, pepsins are deactivated quickly by rising pH. Pancreatic endo- and carboxypeptidases (the latter split polypeptides from the carboxyl end) hydrolyze long-chain peptides into shorter fragments or single AA (A). As in the stomach, these pancreatic enzymes are secreted as inactive precursors: trypsinogen is activated to trypsin by the brush border enzyme enteropepti-dase. Trypsin increases its own concentration autocatalytically and activates chymotrypsinogen and carboxypepti-dases in turn. Trypsin hydrolyzes peptide bonds involving Arg and Lys, while chymotrypsin attacks aromatic AA. Finally, activated elastase attacks preferentially neutral aliphatic AA. As in the case of carbohydrates, the final digestive step is achieved by enzymes of the brush border membrane. Preferentially, these are aminopeptidases (split from the amino end) and dipeptidases that produce absorbable free AA, di-, and tripeptides.

Cellular uptake is mediated by various carrier proteins that are specific to particular AA groups (B). With regard to the uptake of neutral di- and tripep-tides, two mechanisms are being discussed: absorption into the mucosa cell through nonspecific carriers with subsequent intracellular hydrolysis and transport-coupled hydrolysis at the cell membrane. The carriers involved in this uptake may be driven by a Na+ gradient or may be independent thereof. Ba-solateral removal occurs passively through AA accumulation inside mucosa cells.

Approximately 25 % of the AA released into the portal vein leave the mucosa cells as di- and tripeptides and approximately 5 % as proteins endogenously synthesized by the cells. Since a significant share of the absorbed AA are used for energy or as building materials by the intestinal mucosa cells, only a fraction of the absorbed AA actually end up in the bloodstream.

Entire proteins are sometimes absorbed, too, albeit in very small amounts. Since their sequences do not correspond to any of the bodies' own proteins, they are recognized as foreign by immune-competent cells. Physiologically, the purpose of this process may be to stimulate intestinal IgA and IgG secretion, thereby maintaining an important defense mechanism. There is also discussion, however, whether increased intestinal permeability (e. g., "leaky gut syndrome" in neonates) might be responsible for food allergies and autoimmune diseases.

Digestion and Absorption

- A. Digestion and Absorption

Long-chain peptides

Brush border

Long-chain peptides

Brush border

Lipid Digestion And Absorption

Amino-oligopeptidases

Aminopeptidases

Dipeptidases

Dipeptidyl-aminopeptidase Glutamyl transpeptidase

Tripeptides

Dipeptides

Free amino acids

Mucosa cell

Amino-oligopeptidases

Aminopeptidases

Dipeptidases

Dipeptidyl-aminopeptidase Glutamyl transpeptidase

Tripeptides

Q ] Aminopeptidases

Dipeptides

Free amino acids

I Dipeptidases, Ì Proline dipeptida.

Protein secretion

Portal vein

25 % di- and tripeptides

70 % free amino acids

5 % protein es

I- B. Mechanisms of Absorption -

Protein Digestion Dipeptide Tripeptide
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