Role of the palmitoylation of surfactant-associated protein C in surfactant film formation and stability.

The effect of palmitoylation of pulmonary surfactant-associated protein C (SP-C) on the surface activity of phospholipid mixtures of dipalmitoylphosphatidylcholine and phosphatidylglycerol was studied. Phospholipids reconstituted with palmitoylated or depalmitoylated bovine SP-C were examined at neutral and acidic pH using a captive bubble surfactometer. At low pH, effective lipid adsorption and near zero surface tensions upon compression were obtained even with protein-free samples. At physiological pH, only SP-C-containing samples achieved such properties. Lipid adsorption was decreased by prior SP-C depalmitoylation. Bubbles with palmitoylated SP-C were more mechanically stable and required less compression to reach low surface tensions. Subphase depletion experiments showed that dynamically cycled surface layers containing palmitoylated SP-C maintained their surface activity after subphase lipid depletion. In contrast, surface activity was rapidly lost where depalmitoylated SP-C or SP-B was included. Our results indicate that although SP-C palmitoylation has little effect on its ability to enhance lipid adsorption and surface tension reduction, it greatly enhances lipid respreading and film stability and is therefore important for surfactant function.