Degradation of surfactant protein B by alveolar type II cells

Surfactant protein B (SP-B) metabolism was studied in primary cultures of alveolar type II cells. Iodinated SP-B reconstituted with surfactant was incorporated rapidly into lung pneumocytes and degraded to trichloroacetic acid (TCA)-soluble products after a lag period of 1 h. Cellular degradation of SP-B occurred whether or not phospholipid liposomes or surfactant was added to the phospholipid-poor SP-B. Uptake and degradation of SP-B at 37 degrees C showed a linear increase up to 3 micrograms SP-B/ml after which the slope of the curve became less steep with increasing concentrations of SP-B in the media. After 4 h of incubation with SP-B, 35% of the SP-B processed was recovered as degradation products. Ninety-six percent of the degradation products were in the media and only 4% were recovered in the cell. The bulk of the breakdown of SP-B occurred inside the type II cells since degradation did not occur at 4 degrees C, showed a 1-h lag period, was proportional to the SP-B protein internalized by the cells, was inhibited 47% by ammonium chloride, was unaffected by the addition of protease inhibitors to the medium, and cell-conditioned medium produced only limited SP-B degradation. Alveolar macrophages also degraded SP-B, whereas other cell types degraded SP-B to a lesser extent. Thus the specificity of the metabolism of SP-B may be through the capability of lung cells to degrade SP-B.(ABSTRACT TRUNCATED AT 250 WORDS)