The absorption of dietary proteins in the small intestine largely depends on breaking down proteins into smaller peptides—primarily di- and tripeptides—which are then transported into the enterocytes (intestinal epithelial cells). This transport is mediated by a specialized protein known as peptide transporter 1 (PepT1).

Mechanism of PepT1-Mediated Peptide Transport

PepT1 is a proton-coupled oligopeptide transporter located primarily on the apical membrane of enterocytes in the small intestine. Its key features include:

• Proton-Coupled Transport: PepT1 functions as a cotransporter with hydrogen ions (H+). The transporter simultaneously moves di- or tripeptides along with H+ ions into the cell. This process is driven by the inward proton gradient, where H+ concentration is higher in the intestinal lumen compared to inside the epithelial cell.

• Electrogenic Transport: The movement is electrogenic, meaning it results in an electrical charge transfer due to the cotransport of positively charged protons and peptides.

• Secondary Active Transport: PepT1 exploits the proton gradient maintained by other cellular mechanisms, such as the sodium-hydrogen exchanger and the Na+/K+ ATPase, making it a form of secondary active transport.

Physiological Role and Importance

• Dietary Protein Absorption: PepT1 is responsible for the majority of di- and tripeptide uptake from the gut lumen, supplying amino acids for metabolic needs after further intracellular hydrolysis or direct release into the bloodstream.

• Transport of Peptidomimetic Drugs: Some pharmaceuticals mimicking peptide structures utilize PepT1 for absorption, improving their oral bioavailability.

• Location: PepT1 expression is highest in the proximal small intestine (duodenum and jejunum), tapering off towards the ileum.

Answer to the Question

Among the options provided regarding the ion coupled with peptide transporter 1 during peptide absorption:

• PepT1 transports di- and tripeptides along with hydrogen ions (H+).

Therefore, the correct choice is:

(3) H+