THE ROLE OF PULMONARY SURFACTANT PROTEIN-C DURING THE BREATHING CYCLE

A model pulmonary surfactant consisting of dipalmitoylphosphatidylchol ine, dipalmitoylphosphatidylglycerol, and the surfactant associated pr otein C (SP-C) was investigated by film balance measurements. The surf ace of the film balance should mimic the air-liquid interface of the l ung. In order to visualize their chemical and topographical structure surface films formed from the model surfactant were investigated by fl uorescence light microscopy (FLM) and, after transfer to a solid subst rate, by time-of-flight secondary ion mass spectrometry (TOF-SIMS) and scanning force microscopy (SFM). The FLM images of the system reveale d that in an early stage of compression phase separation into a protei n-rich phase and a pure lipid phase occurred. In the region of physiol ogical surface tensions the course of the isotherm showed a plateau wi th extremely high compressibility. By SFM the formation of stacks of l ipid bilayers which was reversible on expansion was detected in the pl ateau 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 lo ng axes in parallel to the membrane normal. It stabilizes, or destabil izes, respectively, the bilayer stacks according to the phase of the b reathing cycle. (C) 1998 Elsevier Science S.A. All rights reserved.