Dipeptide transport across rat alveolar epithelial cell monolayers

Area of Science:

• Cell Biology
• Biochemistry
• Pulmonary Physiology

Background:

• Primary cultured rat alveolar epithelial cells mimic type I pneumocytes.
• These cells form tight monolayers crucial for lung barrier function.

Purpose of the Study:

• To investigate the transepithelial transport and metabolism of glycyl-D-phenylalanine (Gly-D-Phe) and glycyl-L-phenylalanine (Gly-L-Phe).
• To characterize the differences in transport and metabolism between these two model dipeptides across alveolar epithelial cells.

Main Methods:

• Primary cultured rat alveolar epithelial cells were used to create tight monolayers.
• Reverse-phase high-performance liquid chromatography (HPLC) quantified parent dipeptides and metabolites (D- or L-phenylalanine).
• Apparent permeability coefficients (Papp) were determined in apical-to-basolateral (AB) and basolateral-to-apical (BA) directions.

Main Results:

• Gly-D-Phe showed consistent Papp (1.6 x 10(-7) cm/sec) in both directions and concentrations, with no detectable metabolism.
• Gly-L-Phe transport varied: Papp was higher at 1 mM than 10 mM in the AB direction.
• Significant metabolism of Gly-L-Phe to L-phenylalanine was observed, particularly in the AB direction (77-83%) and BA direction (19-40%), which was reduced by actinonin (an aminopeptidase inhibitor).

Conclusions:

• Alveolar epithelial cells exhibit differential transport and metabolism of stereoisomeric dipeptides.
• Gly-L-Phe undergoes significant intracellular or membrane-associated metabolism, unlike Gly-D-Phe.
• The directionality and concentration dependence of Gly-L-Phe transport suggest active transport and enzymatic processing by aminopeptidases.