Protein Digestion and Peptide / Amino Acid Absorption

For the body to assimilate nutritional protein, it must first be broken downinto 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 liverthen on to subsequent organs and tissues.

Recommended Daily Allowances

31-50 y.o. AdultRDA Typical Ingestion

57 kg Female 46 g/day 65 g/day70 kg Male 57 g/day 100 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 withinIntestinal lumen

Absorptive EnterocyteApical Membrane Transport

Selected Metabolismwithin Enterocytes

AA Pool in EnterocyteCytosol

Basolateral MembraneTransport

Portal Blood Circulation

Activated PancreaticCarboxypeptidases

Pancreatic Release ofZymogens

Central Nervous System

HClPepsin

Enterocyte ApicalMembraneAminopeptidases

EndogenousProteins

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

DuodenalEnterocyte

IntestinalEpithelialEndocrine Cell

EnteropeptidasesZymogens released intoIntestine

Trypsinogen

ChymotrypsinogenProelastase

Procarboxypeptidase AProcarboxypeptidase B

Trypsin

ChymotrypsinElastaseCarboxypeptidase ACarboxypeptidase B

(+)

(+)

Active enzymes inIntestine

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

Monomeric Amino Acid Transporters and Transport Systems in the Small Intestine

System Common IonName Name Substrate Dependency LocationA SNAT2 Ala, Asn, Cys, Na+ Basolateral

Glu, Gly, HisMet, Pro, Ser

B or Bo BoAT1 neutral AAs Na+ ApicalGlu

Bo+ ATBo+ neutral and Na+,Cl- Apicaldibasic Aas, Arg, D-Ser

Y+ CAT-1 Arg, dibasic AAs none BasolateralX-ag EAAT3 Glu, Arg H+, Na+,K+ ApicalIMINO SIT1 Pro, pipecolate Na+ ApicalIminoacid PAT1 Pro, Gly, Ala, H+ Apical

GABA, Tau, SerGLY GLYT1 Glycine Na+, Cl- BasolateralT TAT1 aromatic AAs, none BasolateralCreatine CRTR Creatine Na+, Cl- ApicalPept1 PEPT1 di, tripeptides, H+ with NHE3 Apical

Car, B-lactam,antibiotics, Angiotensinconverting enzyme inhibitors

Heterodimeric Amino Acid Transporters and Transport Systems in the Small Intestine

System Common IonName Name Substrate Dependency Locationasc asc-1 plus neutral D,L-Aas, none Basolateral

Ala, D-Ser

y+L y+LAT plus Arg, dibasic and none (or Na+ Basolateralneutral 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 ApicalCys, large neutral Aas, exchanges extracellular dibasicswith intracellularneutral AAS.

Selected Bioactive Peptides Derived from Food

Source Name Physiological Activity

Meat L-carnosine antiinflammatory, antioxident,prevents glycation

Wheat Gliadorphin Opioid agonistWheat Gluten exorphin-AS inhibition of stress induced pain,

CNS opioid agonistMilk B-lactorphin ACE inhibitorMilk B-casokinin -7 ACE inhibitorMilk Caseoplatelin inhibits platelet aggregationMilk Lacottransferrin inhibits platelet aggregation

thrombic inhibitorypeptide

Milk Lactoferricin antimicrobialMilk Phosphopeptide calcium/phosphate stabilizing

to enhance absorption

Paracellular uptakeParacellular uptake occurs by movementOf 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 transportIn 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 ofLieberkuhn

Lymphoid tissueImmune response