AJP - Lung Cellular and Molecular Physiology, Vol 269, Issue 2 137-L143, Copyright © 1995 by American Physiological Society
Dipeptide transport characteristics of the apical membrane of rat lung type II pneumocytes
D. Meredith and C. A. Boyd
Department of Human Anatomy, University of Oxford, United Kingdom.
The transport of a hydrolysis-resistant dipeptide, D-phenylalanyl-L-alanine
(D-Phe-L-Ala), has been studied by high-performance liquid chromatography
in rat lung epithelial cells and apical membrane vesicles. Time-dependent
uptake of D-Phe-L-Ala into isolated type II pneumocytes was shown. Uptake
was saturable, and Michaelis-Menten kinetics were fitted to the data and
gave an apparent Michaelis constant (Km) of 3.4 mM and a maximum velocity
(Vmax) 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 Km 2.0 mM, Vmax
0.53 nmol.mg protein-1.min-1). Influx of this neutral dipeptide into the
vesicles was shown to be both electrogenic and stimulated by an inwardly
directed proton gradient. Influx was inhibitable by mercuric chloride and
by the amino acid residue modifying compounds N-acetylimidazole and
diethylpyrocarbonate. These findings strongly suggest the presence of a
proton-coupled peptide transport protein in the apical surface of the type
II cell. This transporter may play a role in lung homeostasis.
