Protein Digestion and Peptide / Amino Acid Absorption

3. Protein Digestion and Peptide / Amino Acid Absorption For the body to assimilate nutritional protein, it must first be broken down into small peptide fragments and free amino acids. - this occurs to a limited extent in the stomach - the majority hydrolysis and absorption occurs in the small intestine Digestion and absorption supplies circulating blood with a small amount of bioactive peptides and a “pool” of amino acids. In this absorptive state, amino acids are trans ported via portal blood to the liver then on to subsequent organs and tissues.

4. Recommended Daily Allowances 31-50 y.o. Adult RDA Typical Ingestion 57 kg Female 46 g/day 65 g/day 70 kg Male 57 g/day 100 g/day

5. Endogenous proteins The body digests an additional 50 – 100 g of endogenous protein / day. - Secreted or sloughed into the lumen of the gastrointestinal tract. - Saliva - Gastric Juice - Pancreatic Enzymes and other Secretions - Sloughed Intestinal Cells - Proteins that leak into the intestinal lumen from the blood

6. Exogenous and endogenous protein Approximately 115 – 200 g protein / day. Only about 1.6 g Nitrogen (approx. 10 g protein) lost in fecal mass - endogenous protein or dietary protein not absorbed in the intestine - represented in the microflora mass in the large intestine

7. Central Nervous System Dietary Protein HCl Pepsin Pancreatic Release of Zymogens Limited Gastric Hydrolysis and Denaturation CCK Digestion within Intestinal lumen Activated Pancreatic Carboxypeptidases Endogenous Proteins Absorptive Enterocyte Apical Membrane Transport Enterocyte Apical Membrane Aminopeptidases Selected Metabolism within Enterocytes AA Pool in Enterocyte Cytosol Basolateral Membrane Transport Portal Blood Circulation

8. Protein Digestion in Phases Gastric hydrolysis of peptide linkages in the protein Digestion of protein to smaller peptides by action of pancreatic proteases, which are secreted as zymogens and activated in the lumen of the small intestine Hydrolysis of peptide linkages in oligopeptides by brush-border (apical) membrane peptidases and transport of amino acids and di- and tripeptides across the brush-border membrane of absorptive enterocytes

9. Protein Digestion in Phases Further digestion of di- and tripeptides by cytoplasmic peptidases in the enterocyte Metabolism of amino acids within the enterocyte Transport of amino acids across the basolateral membrane of the enterocyte into the interstitial fluid from which the amino acids enter the venous capillaries and hence the portal blood

10. Vagus Nerve Pancreatic Acinar Cell Intestinal Epithelial Endocrine Cell Duodenal Enterocyte Enteropeptidases Zymogens released into Intestine Trypsinogen Chymotrypsinogen Proelastase Procarboxypeptidase A Procarboxypeptidase B (+) Active enzymes in Intestine Trypsin Chymotrypsin Elastase Carboxypeptidase A Carboxypeptidase B (+)

11. 1. brush-border membrane peptidases2. brush-border membrane amino acid transporters 3. brush-border membrane di- and tripeptide transporters 4. intracellular peptidases 5.basolateral-membrane amino acid carriers 6. basolateral membrane di- and tripeptide carriers

12. Monomeric Amino Acid Transporters and Transport Systems in the Small Intestine System Common Ion Name Name Substrate Dependency Location A SNAT2 Ala, Asn, Cys, Na+ Basolateral Glu, Gly, His Met, Pro, Ser B or Bo BoAT1 neutral AAs Na+ Apical Glu Bo+ATBo+ neutral and Na+,Cl- Apical dibasic Aas, Arg, D-Ser Y+ CAT-1 Arg, dibasic AAs none Basolateral X-ag EAAT3 Glu, Arg H+, Na+,K+ Apical IMINO SIT1 Pro, pipecolate Na+ Apical Iminoacid PAT1 Pro, Gly, Ala, H+ Apical GABA, Tau, Ser GLY GLYT1 Glycine Na+, Cl- Basolateral T TAT1 aromatic AAs, none Basolateral Creatine CRTR Creatine Na+, Cl- Apical Pept1 PEPT1di, tripeptides, H+ with NHE3 Apical Car, B-lactam, antibiotics, Angiotensin converting enzyme inhibitors

13. Heterodimeric Amino Acid Transporters and Transport Systems in the Small Intestine System Common Ion Name Name Substrate Dependency Location asc asc-1 plus neutral D,L-Aas, none Basolateral Ala, D-Ser y+Ly+LAT plus Arg, dibasic and none (or Na+ Basolateral neutral AAs for large Aas) x-c xCT plus Cys/Glu exchange none Basolateral L LAT2 plus BCAA, neutral AAs none Basolateral bo+ boA+T plus dibasic AAs, Arg, none Apical Cys, large neutral Aas, exchanges extracellular dibasics with intracellular neutral AAS.

14. Selected Bioactive Peptides Derived from Food Source Name Physiological Activity Meat L-carnosineantiinflammatory, antioxident, prevents glycation Wheat GliadorphinOpioid agonist Wheat Gluten exorphin-AS inhibition of stress induced pain, CNS opioid agonist Milk B-lactorphin ACE inhibitor Milk B-casokinin -7 ACE inhibitor Milk Caseoplatelin inhibits platelet aggregation Milk Lacottransferrin inhibits platelet aggregation thrombic inhibitory peptide Milk Lactoferricin antimicrobial Milk Phosphopeptide calcium/phosphate stabilizing to enhance absorption

15. Paracellular uptake Paracellular uptake occurs by movement Of peptides between the mucosal cells instead of through mucosal cells. This occurs when tight junctions between epithelial cells are damaged and become “leaky”. Leaky junctions increase non-specific permeability of the intestinal epithelium to all macromolecules. Polypeptide uptake and transport In the presence of healthy mucosa, milligram amounts of of intact poly peptides can be absorbed via endocytotic uptake across the brush-border membrane and followed by exocytosis across the basolateral membrane.

16. Intact large peptides IgA/M-cell presentation to Peyer’s Patches Enterocyteendocytosis/lysosome/exocytosis Paracellular movement across leaky junctions Crypt of Lieberkuhn Lymphoid tissue Immune response