Lipid-Protein Interactions with the Hydrophobic SP-B and SP-C Lung Surfactant Proteins in Dipalmitoylphosphatidylcholine Bilayers

Abstract

Pulmonary surfactant is a lipid/protein system which has the essential role of stabilizing the lung airway spaces during successive respiratory cycles. It mainly consists of 90% lipids and 10% of some specific proteins. The biophysical activity of surfactant is assumed to reside in the behaviour of the main phospholipid component, dipalmitoylphosphatidylcholine (DPPC), as a monomolecular film in the alveolar air-water interface. However, the presence of several specific proteins has been shown to be essential in the development of the complex surfactant cycle in vivo. Surfactant is secreted by epithelial type II cells and must be transported through the aqueous lining layer until it reaches the air-liquid interface where it must spread as the monolayer form. Two proteins (SP-B and SP-C) having unusual structural properties -including an extreme hydrophobicity- have been demonstrated to promote the adsorption of surfactant lipids from the hypophase to the interface. In this context, exogeneous surfactant materials consisting of suspensions of lipids plus these hydrophobic surfactant proteins are already being used with success in treating neonatal respiratory distress syndrome.