ADSORPTION OF PULMONARY SURFACTANT PROTEIN-D TO PHOSPHOLIPID MONOLAYERS AT THE AIR-WATER-INTERFACE

The intrinsic surface activity of recombinant rat surfactant-associate d protein D (SP-P) expressed in CHO-K1 cells has been determined from measurements of surface tension of its aqueous solutions. The interact ions of recombinant SP-D with monolayers of phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylinositol (PI) spread at the air-water interface have been characterized. Injection of SP-D beneat h preformed lipid monolayers at surface pressures less than 30 mN/m pr oduced an increase in surface pressure, consistent with SP-D penetrati ng the lipid films. The adsorption of SP-D to the lipid monolayers did not display significant head group dependency, suggesting that the ch anges in surface pressure produced by the protein were likely due prim arily to hydrophobic interactions with the lipid layers. In the presen ce of calcium ions in the subphase, SP-D displayed lower surface activ ity by itself and a reduced ability to generate surface pressure chang es during adsorption to lipid monolayers compared to these properties of the protein in the absence of 2 mM Ca2+. Circular dichroism measure ments on SP-D solutions with or without Ca2+ suggested that the cation s altered the conformation of the protein and this possibly led to the calcium dependency of the surface activity of the protein in the pres ence or absence of lipid monolayers, Compressional isotherms of surfac e pressure versus area for SP-D/(DPPC-PI) and SP-D/(DPPC-PG) films for med by adsorption of the protein to preformed Lipid monolayers were co nsistent with incorporation of some or all of the SP-D molecules into the lipid layers. The isotherms obtained on compression of the SP-D/li pid films to a maximum surface pressure of about 70 mN/m were consiste nt with the interpretation that any SP-D which was incorporated by ads orption was apparently not squeezed out, nor was lipid removed by the protein. The work suggests that when soluble recombinant SP-D is allow ed to interact with phospholipid monolayers under the selected conditi ons of this experiment, it does so to a limited extent driven by prima rily hydrophobic forces, and apparently without a high selectivity for phospholipid head groups.