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.

Recommended Daily Allowances y.o. Adult RDATypical Ingestion 57 kg Female46 g/day 65 g/day 70 kg Male57 g/day100 g/day

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

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

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

Protein Digestion in Phases 1.Gastric hydrolysis of peptide linkages in the protein 2.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 3.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

Protein Digestion in Phases 4.Further digestion of di- and tripeptides by cytoplasmic peptidases in the enterocyte 5.Metabolism of amino acids within the enterocyte 6.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

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

1. brush-border membrane peptidases 2. 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

Monomeric Amino Acid Transporters and Transport Systems in the Small Intestine SystemCommonIon Name NameSubstrateDependencyLocation ASNAT2Ala, Asn, Cys,Na+Basolateral Glu, Gly, His Met, Pro, Ser B or B o B o AT1neutral AAsNa+Apical Glu B o+ ATBo+neutral andNa+,Cl-Apical dibasic Aas, Arg, D-Ser Y+CAT-1Arg, dibasic AAsnoneBasolateral X - agEAAT3Glu, ArgH+, Na+,K+Apical IMINOSIT1Pro, pipecolateNa+Apical IminoacidPAT1Pro, Gly, Ala,H+Apical GABA, Tau, Ser GLYGLYT1GlycineNa+, Cl-Basolateral TTAT1aromatic AAs, noneBasolateral CreatineCRTRCreatineNa+, Cl-Apical Pept1PEPT1di, tripeptides,H+ with NHE3Apical Car, B-lactam, antibiotics, Angiotensin converting enzyme inhibitors

Heterodimeric Amino Acid Transporters and Transport Systems in the Small Intestine SystemCommonIon Name NameSubstrateDependencyLocation ascasc-1 plusneutral D,L-Aas,noneBasolateral Ala, D-Ser y+Ly+LAT plusArg, dibasic andnone (or Na+Basolateral neutral AAsfor large Aas) x - cxCT plusCys/Glu exchangenoneBasolateral LLAT2 plusBCAA, neutral AAsnoneBasolateral b o +b o A+T plusdibasic AAs, Arg,noneApical Cys, large neutral Aas, exchanges extracellular dibasics with intracellular neutral AAS.

Selected Bioactive Peptides Derived from Food SourceNamePhysiological Activity MeatL-carnosineantiinflammatory, antioxident, prevents glycation WheatGliadorphinOpioid agonist WheatGluten exorphin-ASinhibition of stress induced pain, CNS opioid agonist MilkB-lactorphinACE inhibitor MilkB-casokinin -7ACE inhibitor MilkCaseoplatelininhibits platelet aggregation MilkLacottransferrin inhibits platelet aggregation thrombic inhibitory peptide MilkLactoferricinantimicrobial MilkPhosphopeptidecalcium/phosphate stabilizing to enhance absorption

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.

Intact large peptides 1.IgA/M-cell presentation to Peyer’s Patches 2.Enterocyte endocytosis/lysosome/exocytosis 3.Paracellular movement across leaky junctions Crypt of Lieberkuhn Lymphoid tissue Immune response