Stabilization of lung surfactant particles against conversion by a cycling interface

The large active particles of pulmonary surfactant are depleted in patients with acute respiratory distress syndrome and in animal models of this disorder. We studied in vitro conversion of large to small particles, separated by differential sedimentation, to determine how factors lavaged from rabbits injured by intravenous oleic acid would affect conversion. In half-filled test tubes rotated end over end, samples from injured animals increased the recovery of large particles from 40 +/- 6% of uncycled samples for controls to 62 +/- 21%. We hypothesized that proteins in the injured samples, and perhaps also the proteinase inhibitors used previously to block conversion (N. J. Gross and R. M. Schultz. Biochim. Biophys. Acta 1044: 222-230, 1990), stabilized surfactant particles by limiting access to the cycling interface. Hemoglobin, neutrophil elastase, and alpha1-antiproteinase (alpha1-PI) oxidized to eliminate its antiproteinase activity all stabilized large particles against conversion. Hemoglobin was most effective, increasing recovery from 18 +/- 5% for controls to 86 +/- 5% with 0.4 mg/ml hemoglobin. Native alpha1-PI had no effect on conversion. Our results suggest that acceleration of normal conversion is unlikely to explain the depletion of large particles in injured lungs. They also suggest that conversion of surfactant particles separated by differential sedimentation requires no proteinase susceptible to inhibition by alpha1-PI. They provide an alternate hypothesis related to interfacial effects rather than proteinase inhibition for the previously reported effect of alpha1-PI on conversion of particles separated according to density.