Temporal dependence of ectopeptidase expression in alveolar epithelial cell culture: implications for study of peptide absorption

Abstract

There is little data available regarding the extent of peptide metabolism encountered following inhalation to the lung. We have studied the activity of five ectopeptidases in primary rat alveolar epithelial cells, A549 cells and pulmonary macrophages. Peptidase activity was assayed in the plasma membrane fractions (PMF) of primary type II alveolar epithelial cells (ATII cells) after 2 days in culture and after 7 days in culture when they had formed monolayers of type I-like cells (ATI-like cells). Dipeptidyl peptidase IV (DPP) activity fell from 36.65 mU/mg protein to 16.29 mU/mg protein between day 2 and day 7 in culture, aminopeptidase N (AMN) activity increased from 16.16 mU/mg protein to 23.99 mU/mg protein, angiotensin-converting enzyme (ACE) activity was lost (4.29 mU/mg protein at day 2, not detected at day 7), and carboxypeptidase M (CPM) activity was acquired (not detected at day 2, 5.20 mU/mg protein at day 7). The profile of exopeptidase expression in A549 cells was similar to that of primary rat alveolar cells at day 7 in culture (DPP 24.24 mU/mg protein, AMN 47.74 mU/mg protein, CPM 4.28 mU/mg protein, ACE not detected). Macrophages expressed high levels of aminopeptidases (DPP 46.85 mU/mg protein, AMN 28.28 mU/mg protein) but carboxypeptidase activity was not detected. Low neutral endopeptidase 24.11 (NEP) activity was found in all cell types studied (0.96–2.41 mU/mg protein). The qualitative and quantitative changes in the peptidase activity of primary cultured rat alveolar cells between day 2 and day 7 in culture has implications for the use of alveolar cell monolayers as drug absorption models to investigate peptide absorption from the lung. Ectopeptidase activity in cultured alveolar cells can be used to infer the peptide-metabolising capacity of the surface of the alveolar epithelium. The aminopeptidase activity in particular, if representative of enzyme activity in vivo, would offer a significant metabolic barrier to systemic delivery of peptides via the lung.