Solubility vs. electrostatics: What determines the lipid/protein interaction in the lung surfactant

¹ Institute of Biochemistry, Westfaelische Wilhelms-Universitaet Muenster
² Tascon GmbH

^* To whom correspondence should be addressed. E-mail: gallah{at}uni-muenster.de.

Submitted on February 14, 2007
Revised on March 26, 2007
Accepted on 11 April 2007

	Abstract

Mammalian lung surfactant is a complex lipid/protein mixture covering the alveolar interface and holds the crucial function of reducing the surface tension at this boundary to minimal values. Surfactant protein SP-B plays an important role for this purpose and was in focus of many recent studies. However, the specificity of lipid/SP-B interactions is discussed controversially. Since these investigations were accomplished at varying pH conditions (pH 5.5 and 7.0), we studied the specificity of these interactions in a dipalmitoylphosphatidyl-choline (DPPC)/dipalmitoylphosphatidylglycerol (DPPG)/SP-B (4:1:0.2 mol%) model system at either pH. Mainly fluorescence microscopy and laterally resolved time-of-flight secondary ion mass spectrometry were used to reveal information about the phase behavior of the lipids and the molecular distribution of SP-B in the lipid mixture. DPPG forms separated condensed domains due to a strong hydrogen bond network, from which the protein is mainly excluded. Considering the protein as an impurity of the lipid mixture leads to the principle of the zone melting process: An impurity is highly more soluble in a liquid phase than in a solid phase. The phase behavior effect of the lipids mainly outperforms the electrostatic interactions between DPPG and SP-B leading to a more passively achieved colocalization of DPPC and SP-B.

Key Words: Lung surfactant, Surfactant protein (SP)-B, Time-of-flight secondary ion mass spectrometry, lipid/protein interaction, monolayer

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