ROLES OF DIFFERENT HYDROPHOBIC CONSTITUENTS IN THE ADSORPTION OF PULMONARY SURFACTANT

Surface tension-time adsorption isotherms were measured at 37 degrees C for calf lung surfactant extract (CLSE) and subfractions of its cons tituents: the complete mix of surfactant phospholipids (PPL), phosphol ipids depleted in anionic phospholipids (mPPL), hydrophobic surfactant proteins plus phospholipids (SP&PL, SP&mPL), and neutral lipids plus phospholipids (N&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 co ntribution of diffusion to bubble adsorption measurements decreased as phospholipid concentration increased, and was small at 0.25 mM phosph olipid. At this phospholipid concentration, PPL, mPPL, and N&PL all ad sorbed more rapidly and to lower final surface tensions than dipalmito yl phosphatidylcholine (DPPC) on the bubble. However, none of these ph ospholipid mixtures adsorbed to surface tensions below 46 mN/m after 2 0 min, behavior that was significantly worse than CLSE, SP&PL, and SP& mPL which additionally contained hydrophobic SP. Both CLSE and SP&PL r apidly adsorbed to surface tensions below 25 mN/m at 0.25 mM phospholi pid concentration on the bubble, as did SP&mPL at a somewhat reduced r ate. Further experiments defining the influence of hydrophobic apoprot ein content showed that addition of even 0.13% SP (by wt) to PPL impro ved adsorption substantially, and that mixtures of PPL combined with 1 % SP had adsorption very similar to CLSE. Mixtures of SP combined with mPPL had faster adsorption than corresponding mixtures of SP:DPPC, an d neither fully matched the adsorption rates of CLSE and SP&PL even at high SP levels (4% in SP:mPPL and 5.2% in SP:DPPC). These results dem onstrate that although the secondary zwitterionic and anionic phosphol ipids and neutral lipids in lung surfactant enhance adsorption relativ e to DPPC, the hydrophobic SP have a much more pronounced effect in pr omoting the rapid entry of pulmonary surfactant into the air-water int erface.-Wang, Z., S. B. Hall, and R. H. Netter. Roles of different hyd rophobic constituents in the adsorption of pulmonary surfactant.