Topic: absorption of protein

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protein digestion absorption intestine amino acid transporters

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Absorption of Protein

Overview

Proteins provide ~10–15% of energy intake in a typical Western diet (~70–100 g/day). In addition to dietary protein, the intestinal tract receives ~30 g/day of endogenous protein (from salivary, gastric, pancreatic, biliary secretions, and desquamated cells). Nearly all protein is absorbed in the small intestine under normal conditions, with virtually none entering the large intestine.

A critical conceptual distinction: unlike carbohydrates (which must be fully hydrolyzed to monosaccharides before absorption), dietary proteins are only partially digested in the lumen — yielding a mixture of small peptides and free amino acids (AAs). Final digestion to free AAs is completed inside the enterocyte.

Step 1: Luminal Digestion

Stomach — Pepsin

Chief cells secrete pepsinogen (a zymogen). At acidic pH (~3), pepsinogen auto-activates to pepsin via an autocatalytic mechanism (acid-base catalysis using two aspartate residues).
Pepsin is an aspartate protease that hydrolyzes proteins into smaller polypeptides and oligopeptides.
The stomach's role is primarily preparatory — it denatures proteins, mixes them into chyme, and slowly releases them into the duodenum.

Small Intestine — Pancreatic Proteases

When chyme enters the duodenum, CCK (released from I cells in response to partially digested proteins) stimulates pancreatic enzyme secretion. Secretin (released from S cells in response to acid) triggers bicarbonate secretion to neutralize acid and bring pH to neutral — essential because pancreatic enzymes only function optimally at neutral pH.

Pancreatic zymogens and their activators:

Zymogen	Active Enzyme	Type	Products
Trypsinogen	Trypsin (activated by enterokinase/enteropeptidase)	Endopeptidase	Oligopeptides
Chymotrypsinogen	Chymotrypsin (activated by trypsin)	Endopeptidase	Oligopeptides
Proelastase	Elastase (activated by trypsin)	Endopeptidase	Oligopeptides
Procarboxypeptidase A/B	Carboxypeptidases A/B (activated by trypsin)	Exopeptidases	Free AAs from C-terminus

Trypsin is the master activator — it activates all other pancreatic proteolytic enzymes (including itself, via autocatalysis). Absence of trypsin alone (e.g., in cystic fibrosis, chronic pancreatitis) effectively abolishes all pancreatic proteolysis.

The result of luminal digestion is a mixture of oligopeptides (2–8 AAs), small peptides (di- and tripeptides), and free AAs.

Step 2: Brush-Border (Membrane) Digestion

Oligopeptides are further hydrolyzed by brush-border membrane (BBM) peptidases on the luminal surface of enterocytes:

Aminopeptidases — cleave from the N-terminus, releasing free AAs and smaller peptides
Dipeptidyl aminopeptidase IV — cleaves dipeptides from the N-terminus
Carboxypeptidases — cleave from the C-terminus

The final products delivered to the BBM transporters are predominantly di- and tripeptides and free amino acids.

Step 3: Absorption Across the Enterocyte

A. Peptide Absorption via PepT1 (SLC15A1)

The dominant route for nitrogen absorption is via the H⁺-coupled peptide transporter PepT1 located in the apical/brush-border membrane:

Transports all di- and tripeptides (potentially 400 dipeptides + 8,000 tripeptides from dietary proteins)
Driven by a H⁺ electrochemical gradient (generated by Na⁺/H⁺ exchange in the BBM, itself dependent on the Na⁺ gradient maintained by the basolateral Na⁺/K⁺-ATPase)
Electrogenic — each transport cycle moves one net positive charge into the cell
Promiscuous — accepts substrates regardless of constituent AA charge, size, or configuration; also transports β-lactam antibiotics and prodrugs (valacyclovir, valganciclovir)
Advantages over free AA transport: higher efficiency (2–3 AAs per cycle), lower osmolality in the lumen, better stability of otherwise unstable AAs (glutamine, cysteine, tyrosine)

Once di/tripeptides enter the enterocyte, cytoplasmic peptidases hydrolyze them to free AAs.

B. Free Amino Acid Transport Systems (Brush-Border Membrane)

Multiple Na⁺-coupled and H⁺-coupled cotransporters handle free AAs at the apical membrane:

System	Transport Mechanism	Substrates
B⁰,AT1 (SLC6A19)	Na⁺-dependent	Neutral AAs (broad)
B⁰,AT2 (SLC6A15)	Na⁺-dependent	Neutral AAs (restrictive)
b⁰,+AT (SLC7A9)	Na⁺-independent	Cationic + cystine
EAAT3 (SLC1A1)	Na⁺/K⁺-dependent	Anionic AAs (Glu, Asp)
PAT1 (SLC36A1)	H⁺-coupled	Small neutral AAs (Gly, Ala, Pro)
TAUT (SLC6A6)	Na⁺/Cl⁻-coupled	Taurine, β-alanine
ATB⁰,+ (SLC6A14)	Na⁺/Cl⁻-coupled	Neutral + cationic AAs

Step 4: Exit Across the Basolateral Membrane

Free AAs released within the enterocyte exit into the portal circulation via basolateral membrane (BLM) transporters:

System L (LAT2/SLC7A8 + CD98 chaperone) — primary Na⁺-independent system for neutral AAs; functions as an obligatory exchanger, releasing AAs into portal blood in exchange for influx of other AAs
System T (TAT1/SLC16A10) — Na⁺-independent efflux of aromatic AAs (Phe, Tyr, Trp); functionally coupled to LAT2
System y⁺L — transports cationic AAs (Arg, Lys) in Na⁺-independent manner; driven by the inside-negative membrane potential, which favors efflux of cationic AAs

Portal blood then carries absorbed AAs to the liver for first-pass metabolism.

Summary Diagram of the Process

Dietary Protein
      ↓
[STOMACH] Pepsin → Polypeptides + Oligopeptides
      ↓
[PANCREATIC ENZYMES] Trypsin/Chymotrypsin/Elastase/Carboxypeptidases
      → Di- & Tripeptides + Free AAs
      ↓
[BRUSH BORDER PEPTIDASES] Aminopeptidases, Dipeptidyl peptidases
      → Di- & Tripeptides + Free AAs
      ↓
[ENTEROCYTE UPTAKE]
  • PepT1 (H⁺-coupled) → Di/Tripeptides → cleaved by cytoplasmic peptidases → Free AAs
  • Multiple Na⁺/H⁺-coupled transporters → Free AAs directly
      ↓
[BASOLATERAL EXIT]
  • LAT2, TAT1, y⁺L → Portal bloodstream

Special Situations

Neonates

Brush-border and microvillar peptidases are present at adult levels in fetal intestine.
Pinocytosis (macromolecular transport) is highly active in the first 2 weeks of life, enabling absorption of intact maternal immunoglobulins from breast milk.
High intracellular lysosomal proteases (cathepsins) compensate for initially low pancreatic enzyme output.
Gastric pH is neutral at birth, dropping to ~2.2 within the first day of life.

PepT1 and Drug Delivery

PepT1's promiscuity is pharmacologically exploited: β-lactam antibiotics (amoxicillin, cephalexin), ACE inhibitor prodrugs, and antivirals (valacyclovir, valganciclovir) are absorbed via PepT1, dramatically improving oral bioavailability compared to free drug forms.

Clinical Correlates & Disorders

Condition	Defect	Consequence
Cystic fibrosis / Chronic pancreatitis	Absent pancreatic proteases (trypsin absent → all zymogens unactivated)	Protein malabsorption, steatorrhea, failure to thrive
Cystinuria	Absent dibasic AA transporter (cystine, Lys, Arg, Orn) in intestine & kidney	Failure to absorb dibasic AAs; cystine kidney stones
Hartnup disease	Absent neutral AA transporter (B⁰AT1) in intestine & kidney	Pellagra-like rash, cerebellar ataxia (Trp malabsorption → ↓ niacin)
Kwashiorkor	Dietary protein deficiency → ↓ essential AAs	Muscle wasting, hypoalbuminemia, edema, immune compromise

Sources:

Sleisenger and Fordtran's Gastrointestinal and Liver Disease, Ch. 102
Basic Medical Biochemistry: A Clinical Approach, 6e, Ch. 1
Costanzo Physiology, 7th Edition, Ch. 8
Yamada's Textbook of Gastroenterology, 7e

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