Effect of pulmonary surfactant protein A and neutral lipid on accretion and organization of dipalmitoylphosphatidylcholine in surface films.

The effects of surfactant-associated protein A (SP-A) on lipid adsorption to the air-water interface and accumulation of dipalmitoylphosphatidylcholine (DPPC) in the surface region were investigated at 37 degrees C. Dispersions used were bovine pulmonary lipid extract surfactant with or without neutral lipid (NL). Lipid adsorption was examined with the Wilhelmy plate technique and DPPC accumulation by monitoring surface radioactivity from [14C]DPPC with a scintillation probe. SP-A enhanced the rate of lipid adsorption, while both SP-A and NL increased the extent of DPPC accumulation. At the specific radioactivity used [14C]DPPC monolayers were undetectable: the surface radioactivity arose from surface-associated DPPC beneath the monolayer. At the highest concentration studied (0.3 mg lipid/ml), NL greatly enhanced DPPC accumulation and SP-A attenuated this effect. Langmuir-Blodgett (L-B) films were prepared from [14C]DPPC-labeled dispersions (0.3 mg lipid/ml) at equilibrium surface tension. Autoradiographs of L-B films from lipid extract surfactant exhibited a diffuse misty appearance, while NL promoted formation of heterogeneous DPPC aggregates. Addition of SP-A to lipid extracts without NL generated DPPC aggregates; more uniform larger aggregates appeared in the presence of SP-A and NL. Radiation measurements confirmed that the L-B films were composed of more than monolayers. SP-A did not increase DPPC levels in films deposited from lipid extracts unless NL was present. These results indicate that neutral lipid cooperates with surfactant-associated protein A to organize dipalmitoylphosphatidylcholine in the surface films and enhance formation of a DPPC-rich reservoir below the air-water interface.