Effect of Hydrophobic Surfactant Proteins SP-B and SP-C on the Phase and Morphology of Protein Deficient Native Surfactant Films

Abstract

We present alterations in the phase, morphology and mechanical properties of a native replacement surfactant film induced either by the presence or absence of surfactant specific proteins SP-B and SP-C. Using Langmuir isotherms and fluorescence microscopy, the individual lipid-protein interactions in a complicated native surfactant system are explored. The surface tension lowering property of SurvantaTM, a native surfactant, is significantly compromised in the absence of the proteins, as is the ability of the film to undergo reversible collapse. A lack of proteins also causes the characteristic shoulder, prevalent at ∼ 40 mN/m in most lung surfactant mixtures, to disappear. A lack of this characteristic shoulder illustrates the inability of the film to undergo reversible squeeze out by forming “surface associated surfactant reservoirs”. Addition of SP-B causes an increase in the amount of surfactant material adsorbed from the sub-phase. Further it increases the monolayer stability and the compressibility modulus of the protein deficient film. SP-B is therefore responsible for helping the film achieve a high enough surface pressure during compression, as well as quick re-absorption of material during expansion. SP-C plays a dominant role in the formation of bilayer patches containing unsaturated lipids. SP-C also changes the mechanisms of monolayer collapse, and the film collapses via the formation of reversible collapse cracks. However, it is only in the presence of both SP-B and SP-C that the monolayer films are able to perform all the biophysical functions necessary for the proper working of the lung surfactant. These observations provide conclusive evidence showing that both SP-B and SP-C have distinct biophysical functions in the lung surfactant system, making them equally necessary for the long term survival of air-breathing mammals.