Adsorption of pulmonary surfactant protein SP-A to monolayers of phospholipids containing hydrophobic surfactant protein SP-B or SP-C: Potential differential role for tertiary interaction of lipids, hydrophobic proteins, and SP-A

Surface balance techniques were used to study the interactions of surfactant protein SP-A with monolayers of surfactant components preformed at the air-water interface. SP-A adsorption into the monolayers was followed by monitoring the increase in the surface pressure Delta pi after injection of SP-A beneath the films. Monolayers of dipalmitoylphosphatidylcholine (DPPC):egg phosphatidylglycerol (PG) (8:2, mol/mol) spread at initial surface pressure pi(i) = 5 mN/m did not promote the adsorption of SP-A at a subphase concentration of 0.68 mu g/mL as compared to its adsorption to the monolayer-free surface. Surfactant proteins, SP-B or SP-C, when present in the films of DPPC:PG spread at pi(i) = 5 mN/m, enhanced the incorporation of SP-A in the monolayers to a similar extent; the Delta pi values being dependent on the levels of SP-B or SP-C, 3-17 wt %, in the lipid films. Calcium in the subphase did not affect the intrinsic surface activity of SP-A but reduced the Delta pi values produced by the adsorption of the protein to all the preformed films independently of their compositions and charges. The divalent ions likely modified the interaction of SP-A with the monolayers through their effects on the conformation, self-association, and charge state of SP-A. Values of Delta pi produced by adsorption of SP-A to the films of DPPC:PG with or without SP-B or SP-C were a function of the initial surface pressure of the films, pi(i). In the range of pressures 5 less than or equal to pi(i) less than or equal to 35 mN/m, where the monolayers existed in the liquid expanded (LE)/liquid condensed (LC) coexistence region, both the composition and the proportion of the LC phase in the films controlled the magnitude of Delta pi. Monolayers of DPPC:PG plus 17 wt % SP-B or SP-C, which had similar phase properties with LC phase occupying a maximum 25% of the total monolayer area, displayed different abilities to enhance the adsorption of SP-A to the surface. Results revealed that SP-B and SP-C in their pure monolayers had similar abilities in promoting the adsorption of SP-A, whereas SP-B, when present into the lipid films in the LE/LC coexistence state, displayed a higher capacity than SP-C to attract SP-A from the subphase. Lipid-induced changes in the conformations of the proteins might have modulated the interactions of SP-A with SP-B and SP-C incorporated into the phospholipid monolayers.