Isolation and Partial Characterization of Human Low Molecular Weight Protein Associated with Pulmonary Surfactant

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Abstract
A low molecular weight (MW) protein was isoalted from the bronchoalveolar lavage fluid of a patient with alveolar proteinosis. The protein was isolated on DEAE-cellulose and CM-cellulose columns by a cross-reaction with the monoclonal antibody against pig low MW protein (15 kDa) used as marker. Acidic ethanol-soluble proteins obtained from the fractions eluted by 0.09 to 0.16 M NaCl concentration from the CM-cellulose column migrated mainly as a 15-kDa band in the SDS-PAGE system without urea but mainly as a 5-kDa band in a system with 8 M urea. The isoelectric point of the protein was pH 10 to 11, and it contained a large proportion of hydrophobic amino acids (72%), especially leucine (17%). The arginine content was also high (9%). Two monoclonal antibodies were raised against this low MW protein, and immunohistochemical studies revealed that the antigen was located in the inclusions of alveolar wall cells in normal lungs and in lungs from a patient with alveolar proteinosis. These results indicate that the low MW protein originates from lamellar inclusions of alveolar wall cells (possibly type II epithelial cells) and is secreted into alveolar spaces.