ELECTROSTATIC AND NONPOLAR PEPTIDE-MEMBRANE INTERACTIONS - LIPID-BINDING AND FUNCTIONAL-PROPERTIES OF SOMATOSTATIN ANALOGS OF CHARGE Z = +1 TO Z = +3

The interaction of four structurally related somatostatin analogues (effective electric charge +0.4 less-than-or-equal-to z less-than-or-equal-to +3) with lipid membranes was studied with titration calorimetry and was compared with the functional activity of the peptides. Surface activity measurements provided average cross-sections of 70 or 135 angstrom2, indicating that the cyclic molecules orient at the air-water interface with their ring system either parallel (z = +3) or perpendicular (z = +1) to the surface or switching between the two orientations according to the surface density (z = +2). The nonspecific binding of the peptides to sonified lipid vesicles was enthalpy-driven with a DELTAH of -4 to -7.5 kcal/mol. A consistent quantitative analysis of the binding isotherms was achieved by combining electrostatic attractions, calculated via the Gouy-Chapman theory, with a nonspecific surface partition equilibrium for the nonpolar interactions. The electrostatic attraction of the cationic peptides varied strongly according to the peptide charge. Due to the flat ring structure of the cyclic peptides, their true physical charge was sensed at the membrane surface, and no ''charge screening'' was observed. Peptide binding to the negative charged membrane was accompanied by a proton-uptake of the N-terminal amino group of 0.23-0.38 H+/peptide. Deviations from the theoretical prediction of 0.39 H+/peptide can be explained by a preferential binding of the nonprotonated species. The nonpolar interactions, as described by the surface partition coefficients of the four peptides, fell into a narrow range of K congruent-to 50-230 M-1 whereas the apparent overall binding constants were between 200 and 5000 M-1. The binding affinities to the somatostatin receptors varied considerably with dissociation constants from 2.8 x 10(-10) to 3 x 10(-6) M and paralleled the inhibitory effect of the peptides on growth hormone release. Functional activity appeared to correlate inversely with lipid binding, suggesting that those peptides which were bound too tightly to the lipid phase had a decreased probability to react with the active site on the protein receptor.