Interaction of glucagon with artificial lipid bilayer membranes

The enhancement of fluorescence emission from the tryptophan residue of glucagon, the quenching of that emission with acrylamide and with 5-doxyl and 16-doxyl stearic acid, circular dichroism spectra, the release of 6-carboxyfluorescein, and polarized infrared attenuated total reflection (IR-ATR) spectra were used to study the interaction of glucagon with intact lipid vesicles and flat bilayers. Dimyristoylphosphatidylcholine bound the peptide only below the main transition temperature, thus confirming earlier results of Epand et al. (1977). However, the peptide is also bound by vesicles of unsaturated lipids above their transition temperature, suggesting an influence of lipid area on the binding process. Circular dichroism showed that binding to such vesicles also increases the helix content of glucagon. The IR-ATR study and a comparison with dynorphin-A-(1-13)-tridecapeptide revealed profound differences in orientation of the two peptides. The dichroic ratios and the derived order parameters indicated an isotropic orientation of the helical segments of glucagon, but did not exclude a principal orientation of the molecules lying flat on the membrane surface. In contrast, the axis of the dynorphin helix is clearly oriented normal to the interface. The two peptides also differ in their rates of 6-carboxyfluorescein release, suggesting a deeper penetration of the primary amphiphilic helix of dynorphin A-(1-13) than of the secondary amphiphilic helix of glucagon.

Conformation induction in melanotropic peptides by trifluoroethanol: fluorescence and circular dichroism study

Conformation induction in the two related peptides, alpha-melanocyte stimulating hormone (alpha-MSH) and delta-melanocyte stimulating hormone (delta-MSH), have been studied in solvent media containing varying percentages of the membrane-mimetic solvent 2,2,2-trifluoroethanol (TFE) using fluorescence and circular dichroism (CD) spectroscopy. Singular value decomposition (SVD) analysis of the CD spectra at different TFE concentrations showed that these spectra can be described as linear combinations of only two distinct basis spectra, corresponding to the peptides in the random-coil and 'folded' conformations. For alpha-MSH the spectrum of the folded state is very similar to the standard spectrum of the alpha-helix, while that for delta-MSH has partial resemblance to the helical spectrum. Fitting the data on ellipticity (at 222 nm) as a function of TFE volume fraction to an equation based on a two-state model describing TFE-induced conformation induction in the peptides gave values of (1.1 +/- 0.4) and (4.2 +/- 0.5) kcal mol-1 for alpha-MSH and delta-MSH, respectively, for the free energy of equilibrium between the helix and coil forms in water. Measurement of fluorescence emission parameters (emission maximum, quantum yield, steady-state anisotropy and mean excited-state lifetime) indicated that the microenvironment around the single tryptophan residues of both peptides changes in like manner with increasing concentration of TFE in the solvent. The similarity of fluorescence behaviour of the peptides suggests that their Trp fluorophores do not participate in secondary structure formation in TFE.

Lipid-induced secondary structures and orientations of (Leu5)-enkephalin: helical and crystallographic double-bend conformers revealed by IRATR and molecular modelling

Lipid-induced secondary structures and orientations of the two enantiomeric [Leu5]-enkephalins, L-Tyr-Gly-Gly-L-Phe-L-Leu, and D-Tyr-Gly-Gly-D-Phe-D-Leu, on flat multi-bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were examined with polarized attenuated total reflection IR (IRATR) spectroscopy and molecular mechanics procedures. The membrane-bound peptides showed identical IR spectra in the amide I and II band regions that indicated membrane-induced secondary structures and specific orientations of the non-zwitterionic molecules. A Lorentzian band shape analysis based on second derivatives of the original curves and observed band polarizations suggested the presence of helical structures (beta III- and alpha-turns), oriented more or less perpendicular to the membrane surface. Other folded structures, e.g. beta I- and gamma turns, were not excluded. Molecular modelling of non-zwitterionic (Leu5)-enkephalin with two beta III-turns or an alpha-turn resulted in essentially four low-energy conformers containing (i) two beta III-turns, (ii) one alpha-turn, (iii) a beta III-turn fused to an alpha-turn, and (iv) a beta III-turn fused to a beta I-turn as in the crystallographic molecular conformation described by Aubry et al. [Biopolymers 28, 27-40 (1989)]. Zwitterionic [Leu5]-enkephalin with two beta III-turns collapsed to a C13 turn (a distorted alpha-turn) bridged by a gamma I-turn (v). The alignment of the amide I oscillators within the helical structures, (i), (ii) and (iii), and the double-bend structures, (iv) and (v), explained the observed amide I and II polarizations. Differences between these and other lipid-induced [Leu5]-enkephalin conformers reported in the literature may be caused by the lipid polymorphism of the model membranes used. Possible implications of the new conformers for the molecular mechanism of opioid receptor selection are discussed in terms of the membrane compartments theory.

Fluorescence study of melanocyte stimulating hormones in AOT reverse micelles

Fluorescence emission from the single tryptophan residues of two melanocyte stimulating hormones, alpha-MSH and delta-MSH, and their quenching kinetics were studied in aqueous solution and in reverse micelles of AOT/water/isooctane. Incorporation into micelles caused blue shifted and narrower emission peaks, altered quantum yields and considerably enhanced anisotropies for both peptides when compared to emission from bulk water. The variation of emission parameters with micellar water content was interpreted to suggest that while the tryptophan in alpha-MSH lies in close vicinity of the water-AOT molecular interface, that in delta-MSH is solubilized in the central water pool. Total emission intensity decays followed complex (biexponential) kinetics in both aqueous and micellar media. Although the mean lifetimes for both peptides were always nearly the same, the average rotational correlation times in micelles for alpha-MSH were three times as much as those for delta-MSH. Stern-Volmer plots obtained using acrylamide and CCl4 as quenchers localized in the micellar and organic pseudophases, respectively, were non-linear and dependent on emission wavelength. Quenching by acrylamide was more efficient for delta-MSH than for alpha-MSH, while the opposite was true for quenching by CCl4. The implication of this result for localization of the peptides in micelles was consistent with the earlier one emerging from these studies.

Orientation change of glycopeptide in lipid bilayer membrane induced by lectin binding

A lectin-induced orientation change of a helical glycopeptide in lipid bilayer membranes was studied. Glycopeptides composed of hydrophobic nona-(G8) and pentapeptide (G4) with a fluorescent probe at the N-terminal and a lactose unit at the C-terminal were synthesized. The glycopeptides were incorporated into lipid bilayer membranes with the lactose unit exposed to the aqueous phase and the peptide chain buried in the membrane. G8 takes a partially helical structure in the membrane, while G4 an irregular structure. Upon binding of lectin to G8 held in the membrane of DPPC liposome, enhancement of fluorescence intensity of the N-terminal anthryl group, reduction of fluorescence quenching of the anthryl group with acrylamide, and increase of CF-leakage from the DPPC liposome were observed. G8', which lacks the O-anthryrlmethylserine residue from G8, formed a voltage-dependent ion channel in BLM experiments. The frequency of single current fluctuations induced by G8' incorporation increased with addition of lectin. These results indicate that the peptide segment of G8 prefers taking a more perpendicular orientation to the membrane upon association with lectin.

Interaction of melittin derivatives with lipid bilayer membrane. Role of basic residues at the C-terminal and their replacement with lactose

Melittin possesses an amphiphilic property in the primary sequence in which hydrophilic residues are located at the C-terminal region from Lys-21 to Gln-26. A part of the hydrophilic sequence was cleaved off by endopeptidase Arg-C to obtain melittin 1-22. The affinity of melittin 1-22 for neutral phospholipid membrane was reduced to 1/3 that of melittin, indicating that the basic residues, Lys-23 and Arg-24, are important in binding of melittin to the membrane. The melittin 1-22 was extended toward the C-terminal end by connection of lactose (melittin-lac), the membrane affinity of which was slightly higher than the melittin 1-22, but lower than melittin. The leakage experiment of 5,6-carboxyfluorescein encapsulated in DPPC liposomes showed that the activities of melittin 1-22 and melittin-lac in membrane lysis were much lower than melittin. However, the melittin 1-22 formed a voltage-dependent ion-channel in an azolectin bilayer membrane. It is thus considered that Lys-23 and Arg-24 residues of melittin play an important role in binding to the polar region of membrane for lysis, but not for ion-channel formation.

The effect of a membrane potential on the interaction of mastoparan X, a mitochondrial presequence, and several regulatory peptides with phospholipid vesicles

Recently the pH gradient evoked by a K+ diffusion potential was shown to translocate a synthetic monobasic amphipathic hexapeptide across the bilayer of lipid vesicles (De Kroon, A.I.P.M., Vogt, B., Van 't Hof, R., De Kruijff, B. and De Gier, J. (1991) Biophys. J. 60, in press). Here this observation is extended by studying the effect of a membrane potential on a set of bioactive peptides. The panel of peptides comprises the toxin mastoparan X, a tryptophan-containing analogue of the presequence of the mitochondrial protein cytochrome oxidase subunit IV (preCoxIV(1-25)W18), and the regulatory peptides ACTH(1-24), alpha-MSH, ACTH(1-10), dynorphin A, bombesin, and LHRH. The interaction of these peptides with phospholipid vesicles has been measured using the intrinsic tryptophan residue as fluorescent probe. In the absence of a K+ diffusion potential only mastoparan X and the presequence show considerable binding to vesicles consisting of phosphatidylcholine (PC). In contrast, under these conditions all peptides display affinity for vesicles consisting of the acidic phospholipid cardiolipin (CL), the extent of which depends on the net positive charge of the peptide. Application of a K+ diffusion potential to large unilamellar vesicles (LUV) consisting of PC results in a time dependent tryptophan fluorescence increase for mastoparan X, which is accelerated upon incorporating increasing amounts of CL into the LUV. A similar fluorescence increase in response to a K+ diffusion potential was observed for the above model peptide. Yet the mechanism resulting in the fluorescence increase of mastoparan X is completely different from that of the hexapeptide. Binding experiments indicate that a membrane potential-induced enhanced binding of the peptide to the outer surface of the vesicles contributes to the fluorescence increase. PreCoxIV(1-25)W18, dynorphin A, and ACTH(1-24) show fluorescence responses upon applying a membrane potential that are consistent with that of mastoparan X, whereas the other peptides tested do not respond up to a LUV CL content of 50%. The results tentatively suggest that the membrane potential only affects a peptide when it has the ability to adopt a stable membrane bound conformation.

New principle in QSAR: membrane requirements

In this review I discuss our recent work on the possible role of the lipid phase of the target cell membrane in mediating receptor subtype selectivity of peptides.

Estimated membrane structure and receptor subtype selection of an opioid alkaloid-peptide hybrid

Preferred conformation, orientation, and accumulation of dynorphin A-(1-8)-octapeptide, naltrexone, and N beta-(D-Leu-D-Arg-D-Arg-D-Leu-D-Phe)-naltrexamine (Lipkowsky et al., 1988) were estimated according to established procedures. Opioid binding site affinities and selectivities available from the literature were correlated with the estimated parameters of lipid membrane interaction. The results agreed with the molecular mechanism of opioid receptor subtype selection proposed earlier.

Membrane structure of bombesin studied by infrared spectroscopy. Prediction of membrane interactions of gastrin-releasing peptide, neuromedin B, and neuromedin C

Bombesin, in contact with flat phospholipid bilayer membranes, was shown to adopt a membrane structure similar to that of substance P, dynorphin-(1-13)-tridecapeptide, and adrenocorticotropin-(1-24)-tetracosapeptide. The C-terminal message segment, comprising 8-10 amino acid residues, is inserted into a relatively hydrophobic membrane compartment as an alpha-helical domain oriented perpendicularly on the membrane surface. The N-terminal, hydrophilic tetrapeptide segment remains in the aqueous compartment as a random coil. This was shown with IR and IR attenuated total reflection spectroscopy. Equilibrium thermodynamic estimations confirmed the observed membrane structure with respect to helix length, strength of hydrophobic membrane association, and orientation (caused by favorably oriented molecular amphiphilic and helix electric dipole moments). The membrane structure may explain why Trp-8 and His-12 are essential for biologic activity. Neuromedin B is predicted to be able to adopt a membrane structure similar to that of bombesin. However, gastrin-releasing peptide and neuromedin C are predicted not to behave in the same manner. The molecular mechanism of receptor subtype selection by bombesin-like peptides may prove to be similar to that observed earlier for opioid peptides and the neurokinins.