EFFECTS OF LUNG SURFACTANT SPECIFIC PROTEIN SP-B AND MODEL SP-B PEPTIDE ON LIPID MONOLAYERS AT THE AIR-WATER-INTERFACE

Lung surfactant, a mixture of dipalmitoylphosphatidylcholine, phosphat idylglycerols, fatty acids and four lung surfactant specific proteins, forms tightly packed monolayers at the alveolar interface which are c apable of lowering the normal air-water surface tension to almost zero . Isolating the roles of the individual components of lung surfactant is essential to understanding the biophysics of surfactant functioning and to developing new low-cost replacement surfactants. We have used fluorescence microscopy to show that both the full length lung surfact ant specific protein SP-B1-78 and a shorter model peptide SP-B1-25 alt er the phase behavior and surface morphology of palmitic acid (PA) mon olayers. PA is an important component of both natural and replacement lung surfactants, Both the protein and the peptide inhibit the formati on of condensed phase in monolayers of PA, resulting in a new fluid PA -protein phase. This fluid phase forms a network that separates conden sed phase domains at coexistence. The network persists to high surface pressures, altering the nucleation, growth and morphology of monolaye r collapse structures, leading to lower surface tensions on compressio n and more reversible respreading on expansion, factors essential to t he in vivo performance of both natural and replacement lung surfactant s. The network is stabilized by the low line tension between the fluid phase and the condensed phase as confirmed by the formation of extend ed linear domains or ''stripe'' phases. Similar stripes are found in m onolayers of fluorescein-labeled SP-B1-25,that the reduction in line t ension is due to the protein. The peptide retains many of the essentia l functions of the full length protein and may be a lower cost substit ute in replacement surfactants. (C) 1997 Elsevier Science B.V.