Palmitoylation of lung surfactant protein SP-C alters surface thermodynamics, but not protein secondary structure or orientation in 1,2-dipalmitoylphosphatidylcholine Langmuir films

Pulmonary surfactant-specific protein, SP-C, isolated from porcine lung lavage, has been deacylated to investigate the role of the two thioester linked palmitoyl chains located near the N-terminus. Surface thermodynamic properties, secondary structure, and orientation of native and deacylated SP-C in 1,2-dipalmitoylphosphatidylcholine (DPPC) monolayers has been characterized by combined surface pressure-molecular area (pi-A) isotherms and infrared reflection-absorption spectroscopy (IRRAS) measurements. The isotherms indicate that deacylation of SP-C produces more fluid monolayers at pressures less than 30 mN m(-1). The helical secondary structure and tilt angle (70-80 degrees relative to the surface normal) of SP-C remained essentially unchanged upon deacylation in DPPC monolayers at a surface pressure similar to 30 mN m(-1). The results are consistent with a model that acylation of SP-C may influence the rapid protein-aided spreading of a surface-associated surfactant reservoir, but not the structure of DPPC or SP-C in the monolayer at higher surface pressures. (C) 1999 Elsevier Science B.V. All rights reserved.