The Final Stages of Protein Digestion
Before single amino acids can be absorbed, dietary protein must first be broken down by a series of digestive enzymes. This process starts in the stomach with pepsin and continues in the small intestine with pancreatic proteases. Finally, brush border enzymes break peptides into single amino acids, dipeptides, and tripeptides, the absorbable forms.
Mechanisms for Amino Acid Absorption into Enterocytes
Amino acid absorption primarily takes place in the duodenum and jejunum and involves different transport systems for movement across the apical (lumen-facing) and basolateral (blood-facing) membranes.
The Sodium-Dependent System: Secondary Active Transport
A major mechanism for free amino acid absorption is sodium-dependent secondary active transport. This moves amino acids into the enterocyte against their concentration gradient, driven by an electrochemical gradient created by the $\text{Na}^{+}/\text{K}^{+}$ ATPase pump on the basolateral membrane. A carrier protein on the apical membrane binds both a sodium ion and an amino acid, allowing the amino acid to be co-transported into the cell as sodium moves down its gradient. The specific transporter type depends on the amino acid's properties.
Specialized Transporter Systems for Different Amino Acid Groups
Enterocytes have specialized transport systems for different amino acid groups based on their size and charge. Examples include:
- Neutral Amino Acid Transporters (System $\text{B}^0$): For amino acids like alanine, serine, and threonine, often sodium-dependent.
- Basic Amino Acid Transporters (System $\text{b}^{0,+}$): For basic amino acids like lysine and arginine, sometimes sodium-independent.
- Acidic Amino Acid Transporters (System $\text{X}^{-}_{AG}$): For acidic amino acids such as glutamate and aspartate.
- Imino Acid Transporters (IMINO System): For imino acids like proline.
Peptide Absorption and Intracellular Hydrolysis
Dipeptides and tripeptides are also significantly absorbed via the proton-dependent transporter PEPT1. Inside the enterocyte, these peptides are quickly broken down into free amino acids by cytoplasmic enzymes before entering the bloodstream.
Facilitated Diffusion for Amino Acids
Amino acids exit the enterocyte across the basolateral membrane into the bloodstream via facilitated diffusion. This passive process moves them down their concentration gradient from inside the cell to the blood.
Comparing Amino Acid and Peptide Absorption
The body absorbs both free amino acids and small peptides efficiently. The table below compares these two pathways.
| Feature | Single Amino Acid Absorption | Dipeptide/Tripeptide Absorption |
|---|---|---|
| Primary Transport Mechanism (Apical) | Secondary active transport (sodium-dependent) | Secondary active transport (proton-dependent, via PEPT1) |
| Driving Force | Sodium electrochemical gradient | Proton electrochemical gradient |
| Energy Requirement | Indirectly requires ATP | Indirectly requires energy |
| Inside the Enterocyte | Remains as a free amino acid | Hydrolyzed into free amino acids |
| Transport into Blood | Facilitated diffusion | Free amino acids move via facilitated diffusion |
| Efficiency | Highly efficient, with multiple specialized transporters | Often absorbed more rapidly than free amino acids due to high capacity of PEPT1 |
Conclusion: The Integrated System of Amino Acid Absorption
The absorption of single amino acids is a complex, integrated process in the small intestine involving specialized transporters. It relies heavily on electrochemical gradients, particularly the sodium gradient maintained by ion pumps. The ability to absorb both free amino acids and small peptides ensures efficient protein assimilation necessary for health and growth. For detailed information on transport mechanisms, refer to the IUPHAR/BPS Guide to PHARMACOLOGY on Amino acid transporters.
