Original Article

Interaction of pulmonary surfactant protein A with dipalmitoylphosphatidylcholine and cholesterol at the air/water interface

Correspondence to: Shou-Hwa Yu.

Interaction of pulmonary surfactant protein A (SP-A) with pure and binary mixed dipalmitoylphosphatidylcholine (DPPC) and cholesterol (3.5 wt%) at the air/saline, 1.5 mM CaCl₂ interface was investigated using a rhomboid surface balance at 37°C. Surface tension–area isotherms were measured to access the surface active properties of the monolayers. The organization of DPPC and cholesterol in DPPC and DPPC/cholesterol mixed monolayers with or without SP-A at equilibrium surface tension (23 mN/N) was revealed by autoradiographs of Langmuir-Blodgett (L-B) films deposited from [¹⁴C]DPPC or [¹⁴C]cholesterol-labeled monolayers. The results showed that SP-A can interact with the polar head groups of DPPC monolayers and aggregate DPPC molecules. SP-A decreased the surface area reduction required for DPPC monolayers to achieve near zero surface tension from 30 to 25% of the area at equilibrium. SP-A also reduced the collapse surface tension of pure cholesterol from 27 to 23 mN/m. DPPC and cholesterol formed homogeneous mixed monolayers when both were dissolved in the spreading solvent prior to spreading, while separate cholesterol-rich domains appeared when DPPC and cholesterol were spread successively. Cholesterol resisted squeeze-out from either mixed monolayer through compression. Although SP-A could not promote the squeeze-out of cholesterol from homogeneous mixed monolayers, it facilitated that of cholesterol domains especially when SP-A had first interacted with DPPC.

These results indicate that pulmonary surfactant protein A facilitates the squeeze-out of cholesterol domains from mixed monolayers by condensing DPPC and limiting lateral interactions of DPPC with cholesterol domains.—Yu, S-H., and F. Possmayer. Interaction of pulmonary surfactant protein A with dipalmitoylphosphatidylcholine and cholesterol at the air/water interface. J. Lipid Res. 1998. 39: 555–568.

Supplementary key words: air/water interface, dipalmitoylphosphatidylcholine, monolayer, cholesterol, L-B film, pulmonary surfactant, surface tension, surfactant-associated protein A

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