DIPEPTIDE TRANSPORT CHARACTERISTICS OF THE APICAL MEMBRANE OF RAT LUNG TYPE-II PNEUMOCYTES

The transport of a hydrolysis-resistant dipeptide, D-phenylalanyl-L-al anine (D-Phe-L-Ala), has been studied by high-performance liquid chrom atography in rat lung epithelial cells and apical membrane vesicles. T ime-dependent uptake of D-Phe-L-Ala into isolated type II pneumocytes was shown. Uptake was saturable, and Michaelis-Menten kinetics were fi tted to the data and gave an apparent Michaelis constant (K-m) of 3.4 mM and a maximum velocity (V-max) of 7.0 nmol . mg protein(-1). min(-1 ). However, known peptide transport inhibitors unexpectedly increased intracellular D-Phe-L-Ala concentration when initial rates of peptide uptake were studied. Apical (brush-border) membrane vesicles prepared from rat lung also showed time- and concentration-dependent influx of D-Phe-L-Ala (apparent K-m 2.0 mM, V-max 0.53 nmol . mg protein(-1). mi n(-1)). Influx of this neutral dipeptide into the vesicles was shown t o be both electrogenic and stimulated by an inwardly directed proton g radient. Influx was inhibitable by mercuric chloride and by the amino acid residue modifying compounds N-acetylimidazole and diethylpyrocarb onate. These findings strongly suggest the presence of a proton-couple d peptide transport protein in the apical surface of the type II cell. This transporter may play a role in lung homeostasis.