Localization of a candidate surfactant convertase to type II cells, macrophages, and surfactant subfractions

Pulmonary surfactant exists in the alveolus in several distinct subtypes th at differ in their morphology, composition, and surface activity. Experimen ts by others have implicated a serine hydrolase in the production of the in active small vesicular subtype of surfactant (N. J. Gross and R. M. Schultz . Biochim. Biophys. Acta 1044: 222-230, 1990). Our laboratory recently iden tified this enzyme in the rat as the serine carboxylesterase ES-2 [F. Barr, H. Clark, and S. Hawgood. Am. J. Physiol. 274 (Lung Cell. Mel. Physiol. 18 ): L404-L410, 1998]. In the present study we determined the cellular sites of expression of ES-2 in rat lung using a digoxygenin-labeled ES-2 riboprob e. ES-2 mRNA was localized to type II cells and alveolar macrophages but no t to Clara cells. Using a specific ES-2 antibody, we determined the protein distribution of ES-2 in the lung by immunohistochemistry, and it was found to be consistent with the sites of mRNA expression. Most of the ES-2 in ra t bronchoalveolar lavage is in the surfactant-depleted supernatant, but ES- 2 was also consistently localized to the small vesicular surfactant subfrac tion presumed to form as a consequence of conversion activity. These result s are consistent with a role for endogenous lung ES-2 in surfactant metabol ism.