Roles of different hydrophobic constituents in the adsorption of pulmonary surfactant

Surface tension-time adsorption isotherms were measured at 37°C for calf lung surfactant extract (CLSE) and subfractions of its constituents: the complete mix of surfactant phospholipids (PPL), phospholipids depleted in anionic phospholipids (mPPL), hydrophobic surfactant proteins plus phospholipids (SP and PL, SP and mPL), and neutral lipids plus phospholipids (N and PL). Adsorption experiments were done using a static bubble surfactometer where diffusion resistance was present, and in a Teflon dish where diffusion was minimized by subphase stirring. The contribution of diffusion to bubble adsorption measurements decreased as phospholipid concentration increased, and was small at 0.25 mM phospholipid. At this phospholipid concentration, PPL, mPPL, and N and PL all adsorbed more rapidly and to lower final surface tensions than dipalmitoyl phosphatidylcholine (DPPC) on the bubble. However, none of these phospholipid mixtures adsorbed to surface tensions below 46 mN/m after 20 min, behavior that was significantly worse than CLSE, SP and PL, and SP and mPL which additionally contained hydrophobic SP. Both CLSE and SP and PL rapidly adsorbed to surface tensions below 25 mN/m at 0.25 mM phospholipid concentration on the bubble, as did SP and mPL at a somewhat reduced rate. Further experiments defining the influence of hydrophobic apoprotein content showed that addition of even 0.13% SP (by wt) to PPL improved adsorption substantially, and that mixtures of PPL combined with 1% SP had adsorption very similar to CLSE. Mixtures of SP combined with mPPL had taster adsorption than corresponding mixtures of SP:DPPC, and neither fully matched the adsorption rates of CLSE and SP and PL even at high SP levels (4% in SP:mPPL and 5.2% in SP:DPPC). These results demonstrate that although the secondary zwitterionic and anionic phospholipids and neutral lipids in lung surfactant enhance adsorption relative to DPPC, the hydrophobic SP have a much more pronounced effect in promoting the rapid entry of pulmonary surfactant into the air-water interface.