[Biochemical and clinical aspects of pulmonary surfactant proteins]

Pulmonary surfactant is a complex mixture of phospholipids and proteins which is synthesized and secreted by alveolar type II cells. Its presence is essential to prevent the collapse of alveoli at the end of expiration. Recently, it has been demonstrated that in addition to its reduction of surface tension of alveolar surfaces, pulmonary surfactant exhibits several other functions in the alveolar lining layer, and surfactant proteins are definitely involved in the expression of these functions. The present study first focused on the recent advances in basic research of hydrophilic surfactant apoproteins, SP-A and SP-D. Both are glycoproteins with C-type lectin structure at the C-terminal region and collagenous structure at the N-terminal half of the proteins. We revealed that SP-A binds specifically to dipalmitoylphosphatidylcholine and galactose-ceramide and asialo GM2, while SP-D binds specifically to phosphatidylinositol and glucose-ceramide. We discuss the physiologic and metabolic roles of the specific lipid binding with surfactant proteins of the surfactant system. We next studied changes in pulmonary surfactant in respiratory diseases using anti-human SP-A monoclonal antibodies. We demonstrated SP-A immunoglobulin complex in the sera of patients with idiopathic pulmonary fibrosis and pulmonary alveolar proteinosis.