INTERACTION OF A SUBSTANCE-P AGONIST AND OF SUBSTANCE-P ANTAGONISTS WITH LIPID-MEMBRANES - A THERMODYNAMIC ANALYSIS

The molecular characteristics of the neuropeptide substance P (SP), its agonist [Sar9,Met-(O2)11]SP, and three of its antagonists [D-Arg1,D-Pro2,D-Trp7,9,Leu11]SP, [D-Arg1,D-Trp7,9,Leu11]SP, and [D-Pro2,D-Trp7,9]SP were investigated at the air/water interface and when bound to lipid monolayers and bilayers. Measurement of the Gibbs adsorption isotherm showed that the surface areas of SP and its agonist (240 +/- 5 angstrom2 at biologically relevant concentrations) were distinctly larger than those of the antagonists (138 +/- 5 angstrom2) [Seelig, A. (1990) Biochim. Biophys. Acta 1030, 111-118]. The surface activity of the peptides increased in the order [Sar9,Met(O2)11]SP < SP < [D-Pro2,D-Trp7,9]SP < [D-Arg1,D-Trp7,9,Leu11]SP = [D-Arg1,D-Pro2,D-Trp7,9,Leu11]SP and correlated with the respective binding affinities to lipid membranes. The agonist did not insert into neutral and negatively charged bilayers or into densely packed lipid monolayers (at surface pressures > 31 mN/m). In contrast, the three antagonists gave rise to a strong binding both to neutral and to charged lipid monolayers and bilayers. The degree of binding was evaluated from the area increase of lipid monolayers upon peptide insertion, and the binding isotherms were analyzed in terms of the Gouy-Chapman theory. At the monolayer-bilayer equivalence pressure of approximately 32 mN/m, the binding can be described by a surface partition equilibrium with binding constants of (4.5 +/- 0.1) x 10(3) M-1 for [D-Pro2,D-Trp7,9]SP and (1.3 +/- 0.1) x 10(4) M-1 for both [D-Arg1,D-Trp7,9,Leu11]SP and [D-Arg1,D-Pro2,D-Trp7,9,Leu11]SP for pure palmitoyloleoylphosphatidylcholine (POPC) membranes. For mixed POPC/palmitoyloleoylphosphatidylglycerol (POPG) or pure POPG bilayers the Gouy-Chapman analysis yields rather similar binding constants with small variations depending on the membrane composition. A comparison of the intrinsic binding constants demonstrates that the biological activity of SP and its analogues is inversely proportional to their hydrophobic lipid affinity.