The role of pulmonary surfactant protein C during the breathing cycle

A model pulmonary surfactant consisting of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, and the surfactant associated protein C (SP-C) was investigated by film balance measurements. The surface of the film balance should mimic the air-liquid interface of the lung. In order to visualize their chemical and topographical structure surface films formed from the model surfactant were investigated by fluorescence light microscopy (FLM) and, after transfer to a solid substrate, by time-of-flight secondary ion mass spectrometry (TOF-SIMS) and scanning force microscopy (SFM). The FLM images of the system revealed that in an early stage of compression phase separation into a protein-rich phase and a pure lipid phase occurred. In the region of physiological surface tensions the course of the isotherm showed a plateau with extremely high compressibility. By SFM the formation of stacks of lipid bilayers which was reversible on expansion was detected in the plateau region. From FLM and TOF-SIMS we revealed that these structures are enriched in SP-C. We developed a molecular model of the multilayer structure. According to that, SP-C spans a single bilayer with its long axes in parallel to the membrane normal. It stabilizes, or destabilizes, respectively, the bilayer stacks according to the phase of the breathing cycle. (C) 1998 Elsevier Science S.A. All rights reserved.