Composition of human airway mucins and effects after inhalation of acid aerosol

Abstract

Characterization of normal airway mucus is required to elucidate mechanisms protecting the airways and to understand changes associated with disease and environmental insult. Toward this goal, we collected bronchial washes (10 ml saline) from healthy human subjects to 1) evaluate the yield of high-density material (delta > or = 1.35 g/ml), and 2) characterize glycoconjugates associated with collected secretions. Samples were lipid extracted followed by CsCl density gradient centrifugation. The yield of high-density material from individual subjects was variable but sufficient to demonstrate that mucin glycoproteins are a major constituent of mucus from healthy airways and that proteoglycans are absent. Next, we investigated whether inhalation of H2SO4 aerosol (1,000 microgram/m3), an environmental insult associated with alterations in mucociliary clearance, changes the composition of high-density glycoproteins in airway secretions. In a paired, double-blinded study, high-density fractions of bronchial secretions from 12 subjects were collected 18 h after exposures of 2 h to aerosolized NaCl and H2SO4. In all cases the high-density material displayed characteristics of mucin glycoproteins. In addition, a unique 150-kDa glycoprotein was detected in most but not all samples and may represent a small mucin glycoprotein differentially expressed in humans. No differences were noted between the two exposure conditions in the profiles of the glycoproteins or proteins after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Statistically, large changes with acid exposure in the composition of carbohydrates and amino acids were absent. Thus no substantial systematic changes in airway mucin glycoproteins or closely associated proteins and glycoproteins were correlated with H2SO4 exposure. Alternatively, statistical analysis of the differences between exposures in glycoprotein constituents among subjects denoted greater variability in carbohydrates compared with amino acids with repeated sampling, suggesting normal daily variations in the mucin composition of individual airway mucus.