Phospholipase A2 in alveolar type II epithelial cells: biochemical and immunologic characterization

The enzyme phospholipase A2 (PLA2) catalyzes phospholipid hydrolysis and is thought to play important roles in surfactant synthesis and in the generation of lipid mediators, including eicosanoids and platelet-activating factor. This study sought to characterize PLA2 in rat type II pneumocytes by biochemical and immunologic means. Type II cells were found to contain an alkaline-active, Ca(2+)-dependent PLA2 activity predominantly localized to cytosol fraction. This activity preferred phospholipids containing arachidonate in the sn-2 position and phosphatidylethanolamine (PE) over phosphatidylcholine (PC); it was active at physiologically relevant Ca2+ concentrations and was resistant to dithiothreitol. These biochemical features were suggestive of a high-molecular-weight form of PLA2, and immunoblot analysis of type II cell extracts indeed detected a 97-kDa protein corresponding to a high-molecular-weight PLA2, but no low-molecular-weight pancreatic-type enzyme. Human A549 cells, which share certain features with type II cells, contained a similar PLA2 activity and immunoreactive protein. By contrast, whole rat lung cytosol contained both a 97-kDa PLA2 and a 14-kDa pancreatic-type PLA2, but its activity was typical of the latter in that it lacked sn-2 specificity for arachidonate and was inhibited by dithiothreitol. These results indicate that rat type II as well as human A549 epithelial cells contain a high-molecular-weight PLA2, for which mounting evidence suggests an important role in physiological and pathophysiological situations.