Journal of Lipid Research, Vol 16, 224-234, Copyright © 1975 by Lipid Research, Inc.
Characterization of lung surfactant: factors promoting formation of artifactual lipid-protein complexes
SA Shelley, MV L'Heureux and JU Balis
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.
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