Interfacial properties of surfactant proteins

Pulmonary surfactant proteins play a key role in the modulating the interfacial properties of surfactant phospholipids, leading to a complex and dynamic cycle of material at the air-liquid interface of alveoli. Hydrophobic surfactant proteins, SP-B and SP-C, modulate arrangements of surfactant phospholipids in bilayers and monolayers, and promote transfer of tensoactive molecules between different surfactant structural assemblies. These properties are essential for rapid adsorption of surfactant into the air-liquid interface, and respreading of surfactant from collapse phases during expansion, and, consequently, the maintenance of an interfacial surfactant complex that is stable during respiratory dynamics. SP-C may impart special physical properties to surfactant films. Compressed SP-C-containing monolayers seem to be at the same time solid enough to stabilize the respiratory surface, but flexible enough to elastically recuperate when expanded. SP-A appears to have a cooperative participation in the interfacial properties of surfactant containing SP-B and SP-C. Although the essential role of SP-A in the direct tensoactive properties of surfactant remains unresolved, it is essential to form TM, leading to the view that it can have an important role in monolayer formation also. SP-D, like SP-A, may have more important roles in host-defense mechanisms, but under some conditions, such as in surfactant containing high amounts of phosphotidylinositol, it may play a role in the tensoactive function of surfactant. While lipids, especially DPPC, are the main components of surfactant responsible for lowering of surface tension in the lung, the surfactant proteins, particularly SP-B and SP-C, are crucial in providing it with full physiological and physical activity.