Electrochemical and immunoelectron microscopy evidence of lipid-protein interaction in Langmuir-Blodgett films of the human lung surfactant

Abstract

The extracellular lung surfactant surface film (ELSSF) which lines the mammalian lung alveoli at the alveolar air-aqueous cell surface interface is vital in both the breathing and the pulmonary defence processes. The molecular composition of, the structure of and the interaction in the ELSSF was studied, after the ELSSF of human lung lavages could be separated from the subphase and reassembled from its components by using the multicompartment Fromherz-type Langmuir-Blodgett trough. Transmission electron microscopy images of immunogold-labelled and negatively stained isolated film specimens were seen in a continuous layer of mostly phospholipid head groups surfactant-specific protein SpA molecules. Electrical double-layer capacitance and oxygen reduction potential measurements carried out by transferring the surface film from the air-water to a mercury-saline interface of a hanging mercury drop electrode revealed a strong lipid-protein SpA interaction. SpA molecules were partly squeezed out from the film by compression; a proteinless lipid film proved to be a condensed multilayer. Contact with SpA transformed the multilayer into a loose monomolecular film. It is suggested that SpA molecules play a lipid-transporting .role, removing lipids in excess from the air-water interface into the aqueous subphase and vice versa. Lipidprotein interaction can be of importance in vivo. An explanation of how the surfactant film works during the two phases of breathing is proposed.

Impact of Strong and Weak Lipid-Protein Interactions on the Structure of a Lipid Bilayer on a Gold Electrode Surface

“…[13,14,16,17] A strong interaction often indicates the penetration of a protein inside a lipid membrane and is usually connected with an increase of the capacitance of the lipid film adsorbed on an electrode surface. [14,17] An increase in the capacitance is explained by the formation of defects in the membrane and higher dielectric constant of a protein than lipid molecules. MAG binds to glycolipids at the surface of a lipid membrane and does not penetrate inside the lipid bilayer.…”

“…[13][14][15][16] Electrochemical studies provide valuable information concerning the effect of the electric field on the structure and stability of a lipid membrane. However, the molecular-scale structural changes that accompany lipid-protein interactions in the presence of electric fields are not described in the literature.…”

“…[13][14][15][16] Electrochemistry was used to study the stability and structure of model lipid membranes due to a lipid-protein interaction in the presence of changing electric fields. [15][16][17][18] To date, electrochemically studied lipid-protein or lipid-peptide interactions were strong and involved either insertion, penetration of a protein into a lipid assembly [14][15][16][19][20][21] or irreversible attachment of a protein to the membrane surface. [13] In the studies listed above the capacitance minimum due to a lipid monolayer or bilayer adsorbed directly on an electrode surface increased upon a lipid-protein interaction.…”

Interaction of siglec protein with glycolipids in a lipid bilayer deposited on a gold electrode surface

“…It was concluded that measured time delay originates from slow reorientation of the protein incorporated into the lipid monolayer allowing the acceptance of the cysteine group to the mercury surface [29]. Ladanyi et al used electrochemistry to study the interaction between lipid extracts of lung lavages films and the surfactant specific protein A (SpA) [32]. On the mercury electrode a pure lipid film showed a minimum capacitance of 1.1 lF cm À2 , indicating a formation of lipid multilayers.…”

“…After addition of the SpA the minimum capacitance increased to 3.4 lF cm À2 , which is much higher than the capacitance of the lipid monolayer. The strong lipid-protein interaction is connected with a penetration of SpA into the lipid assembly and reorientation of the lipid molecules resulting in a formation of a loosely packed monolayer [32]. Brosseau has shown that the capacitance minimum of the three component lipid bilayer is close to $1 lF cm À2 and $10 lF cm À2 , when the lipid films were LB-LS transferred in the absence and in presence of the B subunit of the cholera toxin in the subphase [11].…”