Characterization of lung surfactant: factors promoting formation of artifactual lipid-protein complexes

Because uncertainty exists as to whether dipalmitoyl phosphatidylcholine, the major component of lung surfactant, is part of a lipoprotein molecule, a study was designed to investigate the relationships between the phospholipids and proteins of rabbit lung washings obtained by lavage with aqueous solutions. Surface-active sediments contained phospholipid and protein in ratios directly dependent upon the ratios in the washings from which they were obtained. Comparison of negatively stained lung washings and sediments revealed that sedimentation caused extensive aggregation of surfactant "liposomes". Analytical ultracentrifugation revealed that both cell-free washings from lungs and suspension of pure phosphatidylcholine contained components with flotation rates ranging from 25 to 400. Density gradient centrifugation of washings, without prior sedimentation, resulted in the appearance of a phospholipid band associated with only a small amount of protein. The density of the band varied depending upon temperature. No qualitative differences in the protein compositions of the phospholipid band and other gradient fractions were found by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Albumin, IgG, IgM, and several nonplasma proteins were present. These results indicate that little, if any, protein is specifically attached to the phospholipids of lung surfactant; rather, the lipid-protein complexes of lung washings are the result of a nonspecific association caused by removal of water-soluble surfactant from the lung and promoted by sedimentation.