Differential Effects of Lyso-Phosphatidylcholine on the Adsorption of Phospholipids to an Air/Water Interface

¹ Oregon Health & Science University
² Portland State University

^* To whom correspondence should be addressed. E-mail: sbh{at}ohsu.edu.

Submitted on May 26, 2006
Revised on July 3, 2006
Accepted on 27 September 2006

	Abstract

To determine how the hydrophobic surfactant proteins promote insertion of the surfactant lipids into an air/water interface, we measured how lysophosphatidylcholine (LPC) affects adsorption. Existing models contend that the proteins function either by disordering the lipids or by stabilizing a negatively curved structure located between the adsorbing vesicle and the interface. Because LPC produces greater disorder but positive curvature, the models predict opposite effects. With vesicles containing either dioleoyl phosphatidylcholine (DOPC) or the neutral and phospholipids (N&PL) isolated from calf surfactant, LPC increased the initial rate at which surface tension fell. The final surface tension, however, remained well above the value of ~25 mN/m expected for a saturated surface. With two preparations, dioleoyl phosphatidylethanolamine (DOPE) and gramicidin A (GrA)-DOPC, that form the negatively curved hexagonal-II (H_II) phase and adsorb rapidly, LPC instead had little effect on initial adsorption, but delayed the fall of surface tension below ~30 mN/m. LPC produced a similar inhibition of the late adsorption for calf lung surfactant extract (CLSE). Unlike DOPE and GrA-DOPC, small angle X-ray scattering and ³¹P-nuclear magnetic resonance for CLSE detected no evidence for the H_II phase. Our results indicate that although LPC can promote the initial adsorption of vesicles containing only lamellar lipids, it inhibits the facilitation by the hydrophobic proteins of late adsorption. Our findings support a model in which the surfactant proteins accelerate adsorption by producing a focal tendency to stabilize a negatively curved kinetic intermediate without a general shift to the H_II phase.

Key Words: hydrophobic surfactant proteins, lipid polymorphism, membrane fusion, negative curvature, pulmonary surfactants, stalk intermediate

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