Low-molecular-weight hydrophobic proteins from bovine pulmonary surfactant

Pulmonary surfactant stabilizes the lung by reducing the surface tension in the terminal air spaces. Lipid extract surfactant contains approx. 1% (w/w) low-molecular-weight hydrophobic proteins SP-B (15 kDa: nonreduced) and SP-C (3.5 kDa) and with the remainder being mainly phospholipids. The hydrophobic proteins were purified from bovine lipid extract surfactant using delipidation by phospholipase C digestion followed by hydroxyapatite chromatography. The phospholipase C step removed most of phosphatidylcholine resulting in a 10-fold enrichment of hydrophobic proteins relative to phospholipid. Chromatography of this preparation on a hydroxyapatite column resulted in the elution of phospholipids followed by SP-C and then SP-B. The column chromatography was repeated to remove residual phospholipids and yield purified SP-B and SP-C. The final recovery of SP-B from the lipid extracts was about 15-20% and that of SP-C was 5-10%. The bovine surfactant proteins were reconstituted with phospholipids and examined for their ability to lower the surface tension with a pulsating bubble surfactometer. Reconstituted surfactant preparations containing SP-B and dipalmitoylphosphatidylcholine plus dioleoylphosphatidylglycerol were capable of reducing the surface tension to near zero values at minimum bubble radius while the reconstitutes with SP-C only lowered the surface tension to approx. 20 mN/m. A more rapid decrease in surface tension was observed with reconstituted samples containing both hydrophobic proteins. These results indicate that both SP-B and SP-C can promote the adsorption and spreading of surfactant lipids at the air/liquid interface. In addition, SP-B appears to facilitate the squeeze-out of unsaturated phospholipids leading to an enrichment of dipalmitoylphosphatidylcholine in the monolayer.