Ectopeptidases of alveolar epithelium: candidates for roles in alveolar regulatory mechanisms.

This commentary discusses the implications of recent observations indicating that at least one, and probably three, ectopeptidases are exhibited on the surface of alveolar epithelial cells. The unique biological feature of the ectopeptidases is that they are expressed on the surface of only selected cell types and at specific stages in the differentiation of those cells. Consequently they are positioned to exert their enzymatic activity in a very restricted locale. Drawing from information derived from studies of this activity on other cells, we develop the proposition that the ectopeptidases on alveolar epithelial cells regulate the biological activity of peptides encountered at this site. These may be peptides produced by the alveolar epithelial cells or cytokines that have the potential of influencing the alveolar cells. Awareness that these ectopeptidases are expressed on the luminal surface of these cells thus opens heretofore unrecognized avenues of investigation.