The effect of counterions on melittin aggregation

Conformational flexibility determines selectivity and antibacterial, antiplasmodial, and anticancer potency of cationic α-helical peptides

We used a combination of fluorescence, circular dichroism (CD), and NMR spectroscopies in conjunction with size exclusion chromatography to help rationalize the relative antibacterial, antiplasmodial, and cytotoxic activities of a series of proline-free and proline-containing model antimicrobial peptides (AMPs) in terms of their structural properties. When compared with proline-free analogs, proline-containing peptides had greater activity against Gram-negative bacteria, two mammalian cancer cell lines, and intraerythrocytic Plasmodium falciparum, which they were capable of killing without causing hemolysis. In contrast, incorporation of proline did not have a consistent effect on peptide activity against Mycobacterium tuberculosis. In membrane-mimicking environments, structures with high α-helix content were adopted by both proline-free and proline-containing peptides. In solution, AMPs generally adopted disordered structures unless their sequences comprised more hydrophobic amino acids or until coordinating phosphate ions were added. Proline-containing peptides resisted ordering induced by either method. The roles of the angle subtended by positively charged amino acids and the positioning of the proline residues were also investigated. Careful positioning of proline residues in AMP sequences is required to enable the peptide to resist ordering and maintain optimal antibacterial activity, whereas varying the angle subtended by positively charged amino acids can attenuate hemolytic potential albeit with a modest reduction in potency. Maintaining conformational flexibility improves AMP potency and selectivity toward bacterial, plasmodial, and cancerous cells while enabling the targeting of intracellular pathogens.

Does dimeric melittin occur in aqueous solutions?

Melittin, a peptide from bee venom, is known to undergo a monomer / tetramer conversion in aqueous solutions. We have studied the possible participation of dimers in the association equilibrium of melittin by sedimentation equilibrium experiments in the analytical ultracentrifuge and subsequent mathematical analysis of the concentration distributions obtained. It was found that the dimeric state is not significantly populated, the contribution of dimer to the total peptide weight probably being below 0.5%.

Toxins and antimicrobial peptides: Interactions with membranes

The innate immunity to pathogenic invasion of organisms in the plant and animal kingdoms relies upon cationic antimicrobial peptides (AMPs) as the first line of defense. In addition to these natural peptide antibiotics, similar cationic peptides, such as the bee venom toxin melittin, act as nonspecific toxins. Molecular details of AMP and peptide toxin action are not known, but the universal function of these peptides to disrupt cell membranes of pathogenic bacteria (AMPs) or a diverse set of eukaryotes and prokaryotes (melittin) is widely accepted. Here, we have utilized spectroscopic techniques to elucidate peptide-membrane interactions of alpha-helical human and mouse AMPs of the cathelicidin family as well as the peptide toxin melittin. The activity of these natural peptides and their engineered analogs was studied on eukaryotic and prokaryotic membrane mimics consisting of <200-nm bilayer vesicles composed of anionic and neutral lipids as well as cholesterol. Vesicle disruption, or peptide potency, was monitored with a sensitive fluorescence leakage assay. Detailed molecular information on peptide-membrane interactions and peptide structure was further gained through vibrational spectroscopy combined with circular dichroism. Finally, steady-state fluorescence experiments yielded insight into the local environment of native or engineered tryptophan residues in melittin and human cathelicidin embedded in bilayer vesicles. Collectively, our results provide clues to the functional structures of the engineered and toxic peptides and may impact the design of synthetic antibiotic peptides that can be used against the growing number of antibiotic-resistant pathogens.

Conolysin-Mt: a conus peptide that disrupts cellular membranes

Conus venoms are estimated to comprise over 100,000 distinct pharmacologically active peptides, the majority probably targeting ion channels. Through the characterization of a cytolytic peptide from the venom of Conus mustelinus, conolysin-Mt, we expand the known conopeptide mechanisms to include association with and destruction of cellular membranes. A new 23AA conopeptide, conolysin-Mt has potent hemolytic activity when tested on human erythrocytes. At a concentration of 0.25 microM, the peptide permeabilized both negatively charged prokaryotic (PE:PG) and zwitterionic eukaryotic (PC:cholesterol) model membranes. The affinity constants (KA) of conolysin-Mt for PE:PG and PC:cholesterol model membranes were 0.9 +/- 0.3 x 10(7) and 3 +/- 1 x 10(7) M-1, respectively. In contrast, conolysin-Mt exhibited low antimicrobial activity (MIC > 50 microM) against two Escherichia coli strains, with an MIC for the Gram-positive S. aureus of 25-50 microM. The specificity of conolysin-Mt for native eukaryotic membranes is a novel feature of the peptide compared to other well-characterized cytolytic peptides such as melittin.

Dissection of Antibacterial and Toxic Activity of Melittin

Melittin, a naturally occurring antimicrobial peptide, exhibits strong lytic activity against both eukaryotic and prokaryotic cells. Despite a tremendous amount of work done, very little is known about the amino acid sequence, which regulates its toxic activity. With the goal of understanding the basis of toxic activity and poor cell selectivity in melittin, a leucine zipper motif has been identified. To evaluate the possible structural and functional roles of this motif, melittin and its two analogs, after substituting the heptadic leucine by alanine, were synthesized and characterized. Functional studies indicated that alanine substitution in the leucine zipper motif resulted in a drastic reduction of the hemolytic activity of melittin. However, interestingly, both the designed analogs exhibited antibacterial activity comparable to melittin. Mutations caused a significant decrease in the membrane permeability of melittin in zwitterionic but not in negatively charged lipid vesicles. Although both the analogs exhibited similar secondary structures in the presence of negatively charged lipid vesicles as melittin, they failed to adopt a significant helical structure in the presence of zwitterionic lipid vesicles. Results suggest that the substitution of heptadic leucine by alanine impaired the assembly of melittin in an aqueous environment and its localization only in zwitterionic but not in negatively charged membrane. Altogether, the results suggest the identification of a structural element in melittin, which probably plays a prominent role in regulating its toxicity but not antibacterial activity. The results indicate that cell selectivity in some antimicrobial peptides can probably be introduced by modulating their assembly in an aqueous environment.

Conformational analysis of melittin in solution phase: Vibrational circular dichroism study

The vibrational absorption and vibrational circular dichroism (VCD) spectra of melittin in D(2)O solutions at different pH values, different salt concentrations, or different 2,2,2-trifluoroethanol (TFE) concentrations are recorded in the amide I' (1850-1600 cm(-1)) region. Two models are used to simulate this peptide in different conditions, and a coupled oscillator program is used to obtain the calculated absorption and VCD spectra. This study indicates that melittin adopts a mixed structure in D(2)O solution at low pH, low salt concentration, or low TFE concentration. With an increase in pH, salt concentration, or TFE concentration, the structure changes to alpha-helix and further increases lead to aggregation. These results demonstrate the versatility of VCD in probing the conformations of peptides under different environmental perturbations.

Protein in sugar films and in glycerol/water as examined by infrared spectroscopy and by the fluorescence and phosphorescence of tryptophan

Sugars are known to stabilize proteins. This study addresses questions of the nature of sugar and proteins incorporated in solid sugar films. Infrared (IR) and Raman spectroscopy was used to examine trehalose and sucrose films and glycerol/water solvent. Proteins and indole-containing compounds that are imbedded in the sugar films were studied by IR and optical (absorption, fluorescence, and phosphorescence) spectroscopy. Water is able to move in the sugar films in the temperature range of 20-300 K as suggested by IR absorption bands of HOH bending and OH stretching modes that shift continuously with temperature. In glycerol/water these bands reflect the glass transition at approximately 160 K. The fluorescence of N-acetyl-L-tryptophanamide and tryptophan of melittin, Ca-free parvalbumin, and staphylococcal nuclease in dry trehalose/sucrose films remains broad and red-shifted over a temperature excursion of 20-300 K. In contrast, the fluorescence of these compounds in glycerol/water solvent shift to the blue as temperature decreases. The fluorescence of the buried tryptophan in Ca-bound parvalbumin in either sugar film or glycerol/water remains blue-shifted and has vibronic resolution over the entire temperature range. The red shift for fluorescence of indole groups exposed to solvent in the sugars is consistent with the motion of water molecules around the excited-state molecule that occurs even at low temperature, although the possibility of static complex formation between the excited-state molecule and water or other factors is discussed. The phosphorescence yield for protein and model indole compounds is sensitive to the matrix glass transition. Phosphorescence emission spectra are resolved and shift little in different solvents or temperature, as predicted by the small dipole moment of the excited triplet state molecule. The conclusion is that the sugar film maintains the environment present at the glass formation temperature for surface Trp and amide groups over a wide temperature excursion. In glycerol/water these groups reflect local changes in the environment as temperature changes.

The monolayer technique: a potent tool for studying the interfacial properties of antimicrobial and membrane-lytic peptides and their interactions with lipid membranes

Erudites of the antiquity already knew the calming effect of oil films on the sea waves. But one had to wait until 1774 to read the first scientific report on oil films from B. Franklin and again 1878 to learn the thermodynamic analysis on adsorption developed by J. Gibbs. Then, in 1891, Agnes Pockels described a technique to manipulate oil films by using barriers. Finally, in 1917, I. Langmuir introduced the experimental and theoretical modern concepts on insoluble monolayers. Since that time, and because it has been found to provide invaluable information at the molecular scale, the monolayer technique has been more and more extensively used, and, during the past decade, an explosive increase in the number of publications has occurred. Over the same period, considerable and ever-increasing interest in the antimicrobial peptides of various plants, bacteria, insects, amphibians and mammals has grown. Because many of these antimicrobial peptides act at the cell membrane level, the monolayer technique is entirely suitable for studying their physicochemical and biological properties. This review describes monolayer experiments performed with some of these antimicrobial peptides, especially gramicidin A, melittin, cardiotoxins and defensin A. After giving a few basic notions of surface chemistry, the surface-active properties of these peptides and their behavior when they are arranged in monomolecular films are reported and discussed in relation to their tridimensional structure and their amphipathic character. The penetration of these antimicrobial peptides into phospholipid monolayer model membranes, as well as their interactions with lipids in mixed films, are also emphasized.

Fluoroalcohols as structure modifiers in peptides and proteins: hexafluoroacetone hydrate stabilizes a helical conformation of melittin at low pH

The effect of hexafluoroacetone hydrate (HFA) on the structure of the honey bee venom peptide melittin has been investigated. In aqueous solution at low pH melittin is predominantly unstructured. Addition of HFA at pH approximately 2.0 induces a structural transition from the unstructured state to a predominantly helical conformation as suggested by intense diagnostic far UV CD bands. The structural transition is highly cooperative and complete at 3.6 M (50% v/v) HFA. A similar structural transition is also observed in 2,2,2 trifluoroethanol which is complete only at a cosolvent concentration of approximately 8 M. Temperature dependent CD experiments support a 'cold denaturation' of melittin at low concentrations of HFA, suggesting that selective solvation of peptide by HFA is mediated by hydrophobic interactions. NMR studies in 3.6 M HFA establish a well-defined helical structure of melittin at low pH, as suggested by the presence of strong NH/NHi+1 NOEs throughout the sequence, along with many medium range helical NOEs. Structure calculations using NOE-driven distance constraints reveal a well-ordered helical fold with a relatively flexible segment around residues T10-G11-T12. The helical structure of melittin obtained at 3.6 M HFA at low pH is similar to those determined in methanolic solution and perdeuterated dodecylphosphocholine micelles. HFA as a cosolvent facilitates helix formation even in the highly charged C-terminal segment.

Anions and the anomalous gel filtration behavior of notexin and scutoxin

Based on their mol. wts, notexin and scutoxin elute later than expected from gel filtration columns in multiple peaks [Francis et al. (1991) Toxicon 29, 85-96]. Notexins present in these peaks have identical amino acid sequences and unmodified amino acid side-chains. Scutoxin is an isoform of notexin which contains arginine at position 16 and glutamate at position 82. Like notexin, it also elutes in different fractions on a gel filtration column, yet the fractions show identical amino acid sequence. This perplexing chromatographic behavior appears to be caused by the association of these proteins with different anions, since dissolving notexin in buffers containing different anions produces up to a 30% change in elution volume. Certain anions promote an apparent reduction in the interaction of notexin with gel filtration matrix, hence earlier elution. These anions include citrate, 3-phosphoglycerate and 2-phosphoglycerate, which also inhibit the PLA2 activity of notexin. However, even under conditions which minimize protein-matrix interaction the toxins elute later than expected based on their mol. wt.

Circular-dichroism and fluorescence studies on melittin: effects of C-terminal modifications on tetramer formation and binding to phospholipid vesicles

Studies were performed on a series of melittin analogues with selective alterations to the positively charged amino acid sequence at the C-terminus. Fluorescence studies were undertaken using the sole tryptophan residue in the analogues as an intrinsic fluorescence probe for indications of tetramer formation in free solution, and binding and insertion of the melittins into phospholipid bilayers. Studies were performed under conditions of low-salt buffer with increasing concentrations of phosphate added to promote self-association of the melittin monomers, and also in the presence of phospholipid vesicles. C.d. studies were also performed under conditions of increasing phosphate concentrations and in the presence of lipid vesicles to monitor the alpha-helical content of the melittins. It was found that selective replacement of the C-terminal basic amino acids by glutamine has different effects on self-association, alpha-helix formation and lipid binding of melittin.

Helix promotion in polypeptides by polyols

The helix content of [L-Lys(Me3)]n.ClO4 and [L-Lys(Me3)50,L-Ala50]n.ClO4 in water is markedly increased by the presence of sucrose and glycerol. For [L-Lys(Me3)]n.ClO4 the ellipticity at 222 nm changes from +2 x 10(3) deg cm2 dmole-1 in water to -44 x 10(3) in 50% glycerol. Sucrose does not promote helix formation in melittin at pH 7.2, but glycerol does. At pH 5.5 sucrose and, more so, glycerol, induce helix in melittin. Glycerol induces some helix in methylated melittin, but less than in melittin. The results are discussed in relation to excluded volume effects, delta G of transfer of peptide and hydrophobic groups from water to mixed solvents, electrostatic effects, and preferential hydration.

Permethylation alters the conformational transitions and the complexing ability of melittin: a model for methylated proteins

Melittin exhibits a transition from random-coil monomer to helical tetramer as a function of peptide concentration [J. Bello, H. R. Bello, and E. Granados (1982) Biochemistry, Vol. 21, pp. 461-465]. When permethylated on each of the four amino groups (Gly-1 N alpha and Lys-7, 21, and 23 N epsilon) to yield trimethylammonium groups, melittin exists as a random coil and does not show any concentration-dependent conformational transition (up to 290 microM). Acylation of the amino groups of melittin with glycine or 5-aminopentanoic acid followed by permethylation increases helix formation, but to a lesser extent than for the unmethylated aminoacylmelittin derivatives. The results are discussed in relation to hydrophobicity, charge repulsions, and ion binding. Melittin, and more weakly, permethylated melittin (MLT-Me) form helical hybrids with an anionic random-coil melittin analogue (E-MLT), in which all the lysine and arginine residues of melittin were replaced by glutamate residues. The hybrid between MLT-Me and E-MLT shows a concentration-dependent increase in helicity. E-MLT, when succinylated at the N-terminal glycine (E-MLT-suc), forms a stronger hybrid with MLT-Me, possibly as a result of increased electrostatic interaction between equal but opposite charges in E-MLT-suc (net charge -6) and MLT-Me (net charge +6). The hybrids exhibit both cold- and heat-induced denaturation, similar to the phenomenon exhibited by proteins. The hybrids also exhibit significant residual structures in the temperature range of 80-100 degrees C, which may be similar to the molten globular states that have been suggested for proteins.

Conformational studies of anionic melittin analogues: effect of peptide concentration, pH, ionic strength, and temperature--models for protein folding and halophilic proteins

Melittin (MLT), a 26-residue cationic (net charge +5 at pH 7.2) peptide from bee venom, is well known to be a monomeric, approximately random coil; but when its charges are reduced by titration, by acetylation (net charge +2) or succinylation (net charge -2), or by screening by salt, it goes over to tetrameric alpha-helix. The conversion is promoted by raising the peptide concentration. The tetramer is held together by hydrophobic forces. We have changed the net charge to -6 by acylation with acetylcitric anhydride (a new acylating agent); this anionic derivative forms tetrameric helix at neutral pH, without salt, and at relatively low concentration, conditions under which the cationic MLT does not become helical. Thus, a high net charge is not sufficient to prevent association and helix formation. We have synthesized an anionic melittin analogue of MLT (E-MLT; net charge -4) in which all five lysine and arginine residues are replaced with glutamate, and acetyl and succinyl derivatives of E-MLT (net charges -5 and -6). All three of these are resistant to helix formation. They require much higher NaCl or NaClO4 concentration for helix formation than does MLT. Even CaCl2, MgCl2, and spermine.4HCl are less effective in helicizing E-MLT than MLT. MLT, at pH 7.2, shows increasing helix as the peptide concentration increases (8-120 microM), but E-MLT and its acyl derivatives do not. MLT and acylated MLTs in the helical tetramer show both cold- and heat-induced unfolding, with maximum stability near room temperature. At high temperature, a significant amount of residual structure remains. Heating (to 100 degrees C) monomeric MLT (i.e., MLT at low concentration) or E-MLT results in a monotonic increase in negative ellipticity. In 1.0 M NaCl, E-MLT (at sufficiently high concentration) also shows cold and hot unfolding. The results are discussed in respect to charge-charge and charge-dipole interactions, and hydrophobic effects. E-MLT is also discussed in relation to proteins of halophilic bacteria, which have higher proportions of anionic residues than do corresponding proteins of nonhalophiles.

Similarities in melittin functional group reactivities during self-association and association with lipid bilayers

Competitive labeling of melittin over a range of concentrations in the presence and absence of liposomes provides a series of "snapshots" of the chemical reactivities of melittin's intrinsic nucleophiles. Distinct trends in apparent reactivities were observed for the Gly-1 alpha-amino group and the epsilon-amino groups of Lys-7 and Lys-21 and -23, over a range of concentrations, providing evidence for different forms of associated melittin in solution. The monomer-tetramer transition can be followed, in accord with structural details derived from X-ray crystallography. The reactivity behavior of the alpha-amino group of Gly-1 and the epsilon-amino groups of Lys-21 and Lys-23 suggests these groups undergo similar perturbations in their microenvironments during the monomer-tetramer transition in free solution. Similar changes in reactivity behavior occur upon association of melittin monomers with bilayer-forming lipids. Together, these findings suggest that the local environments of the N- and C-terminal segments have similar physicochemical properties in both the solution tetramer and the lipid-associated complex. The concentration dependence of the chemical properties of melittin is correlated with surface accessibility calculations which are used to provide a framework for interpretation. Aspects of several previously proposed models of membrane lysis can be accounted for by concentration-dependent properties of melittin.

Effect of permethylation on the haemolytic activity of melittin

The cytolytic activity of the bee venom toxin, melittin, is abolished on permethylation of the ammonium groups into quaternary trimethylammonium groups. The loss of activity in permethylated melittin may result partly from the absence of the hydrogen bonding potential and partly from steric effects involving the bulky trimethylammonium groups. Displacing the trimethylammonium groups away from the backbone to relieve steric effects (by acylating melittin with glycine or 5-aminopentanoic acid followed by permethylation) restored moderate activity at 5-fold increase in concentration.

Melittin induced voltage-dependent conductance in DOPC lipid bilayers

Melittin-induced conductance was measured on planar bilayers made from dioleoylphosphatidylcholine. Upon application of a fixed voltage, the current response was monophasic and remained so even after prolonged observation times. The conductance of melittin-doped bilayers increased exponentially with voltage. In addition, an ohmic contribution appeared after some current had passed. The voltage-dependent conductance increased e-fold every 22 mV and was proportional to the fourth power of the aqueous monomeric peptide concentration, for all salt concentrations investigated (0.4-1.8 M NaCl). Discrete conductance steps could be resolved at all these salt concentrations. The amplitudes of these steps were highly variable. In each experiment, conductance was initially only observed for potentials which were positive on the side of peptide addition. As more and more current passed across the bilayer, the current-voltage curves became symmetric. The system needed some time to reach stationary current-voltage characteristics: about 50 min at pH 7 but only about 15 min at pH 8, suggesting involvement of the N-terminus (pK around 7.5) of melittin in the slow formation of a 'prepore'.

Conformational distributions of melittin in water/methanol mixtures from frequency-domain measurements of nonradiative energy transfer

We used fluorescence energy transfer to examine the effects of solvent composition on the distribution of distances between the single tryptophan residue of melittin (residue 19) to the N-terminal alpha-amino group, which was labeled with a dansyl residue. The tryptophan intensity decays, with and without the dansyl acceptor, were measured by the frequency-domain method. The data were analyzed by a least-squares algorithm which accounts for correlation between the parameters. A wide distribution of tryptophan to dansyl distances was found for the random-coil state, with a Gaussian half-width of 25 A. Increasing concentrations of methanol, which were shown to induce and alpha-helical conformation, resulted in a progressive decrease in the width of the distribution, reaching a limiting half-width of 3 A at 80% (v/v) methanol. The distance from the indole moiety of Trp-19 to the dansyl group in 80% (v/v) methanol/water was found to be 25 A, as assessed from the center of the distance distribution. A distance of 24-25 A was recovered from the X-ray crystal structure of the tetramer, which is largely alpha-helical. At low ionic strength (less than 0.01) the CD spectra revealed a small fraction or amount of alpha-helix for melittin in water, which implies a small fraction of residual structure. This residual structure is apparently lost in guanidine hydrochloride as demonstrated by a further broadening in the distribution of distances. These results demonstrate the usefulness of frequency-domain measurements of resonance transfer for resolution of conformational distributions of proteins.

Characterization of selectively 13C-labeled synthetic melittin and melittin analogues in isotropic solvents by circular dichroism, fluorescence, and NMR spectroscopy

The spectroscopic and functional characterization of 13C-labeled synthetic melittin and three analogues is described. Selectively 13C-enriched tryptophan ( [13C delta 1]-L-Trp) and glycine ( [13C alpha]Gly) were incorporated into melittin and three analogues by de novo peptide synthesis. 13C-Labeled tryptophan was incorporated into melittin at position 19 and into single-tryptophan analogues of melittin at positions 17, 11, and 9, respectively. Each of the synthetic peptides contained 13C-labeled glycine at position 12 only. The peptides were characterized functionally in a cytolytic assay, and spectroscopically by CD, fluorescence, and NMR. The behavior of 13C-labeled synthetic melittin was, in all respects, indistinguishable from that of the naturally occurring peptide. All of the analogues were found to be efficient lytic agents and thus were functionally similar to the native peptide, yet no evidence was found for formation of a melittin-like tetramer by any of the analogues in aqueous media, although there was a propensity for apparently nonspecific peptide aggregation, especially for MLT-W9. Since the analogues did exhibit fractional helicities by CD comparable to or even greater than melittin itself in the presence of methanol, we infer that tetramer assembly requires not only the ability to form alpha-helix but also a very precise packing of amino acid side chains of the constituent monomers. The 13C chemical shift of the Gly-12 C alpha was found to be a sensitive marker for helix formation in all of the peptides. For melittin itself, 13C NMR spectra revealed a downfield shift of approximately 1.8 ppm for the Gly-12 13C alpha resonance of the tetramer relative to that observed for the free monomer in D2O. In mixed samples containing melittin monomer and tetramer, two discrete Gly-12 13C alpha peaks were observed simultaneously, suggestive of slow exchange between the two species. We conclude that melittin's ability to form a soluble tetramer is not a prerequisite for cytolytic activity, nor is cytolytic potential precisely correlated with the ability to form an amphiphilic helix.

Incorporation of highly purified melittin into phosphatidylcholine bilayer vesicles

Melittin free of phospholipase A2 was prepared. In the absence of salt this highly pure protein starts to aggregate in solution at a protein concentration of Cp greater than 10(-3) M. In high salt solution (2 M) aggregation starts at Cp greater than 10(-6) M. This was determined from the blue shift of the intrinsic fluorescence of the protein. Reinvestigation of the quenching behaviour clearly shows that self-aggregation cannot be deduced from quenching experiments using nitrate or 2,2,6,6-tetramethylpiperidine-1-oxyl as quencher. The incorporation of melittin into phosphatidylcholine bilayer vesicles was studied by fluorescence quenching and by energy-transfer experiments using 2- and 6-anthroyloxypalmitic acid as acceptor and peptide tryptophan as donor. Incorporation of melittin into small unilamellar vesicles was found to be reduced below the lipid phase transition temperature, Tt, whereas it incorporates and distributes more randomly above Tt. Cooling the temperature below Tt after incubation at T greater than Tt leads to a deeper incorporation of the peptide into the lipid bilayer due to electrostatic interaction between the lipid phosphate groups and the positively charged amino acids. This stabilizing effect is lost above Tt and melittin is extruded to the polar phase. Quenching experiments support this finding. EPR measurements clearly demonstrate that even in the presence of high amounts of melittin up to 10 mol% with respect to the lipid broadening of the phase transition curves was only observed with fatty acid spin labels, where the doxyl group is localized near the bilayer surface. The order degree of the inner part of the bilayer remains almost unchanged even in the presence of high melittin content.

Malonate transport in human red blood cells

Kinetic parameters of [2-14C]malonate uptake by the human erythrocyte membrane have been determined as Km, 24 mM and turnover number, 5 X 10(4) s-1. The translocation of this organic dianion is concentration, pH and temperature dependent. Competitive inhibition of malonate uptake by eosin and inorganic anions, strongly implies that a common route exists for both inorganic anions and organic dianions, namely the anion-exchange Band 3 protein. 14C-Malonate which is nonmetabolized in the erythrocyte, could be a useful probe for monitoring anion-exchange in reconstituted Band 3 systems.

Isolation and structure analysis of bee venom mast cell degranulating peptide

The venom of Apis mellifera was processed by gel permeation chromatography on Sephadex G-50 and by reversed-phase HPLC. The initial gel permeation step was carried out in the presence of phosphate ions (0.5 M). Ion pair reagents were required to resolve the strongly basic peptides, secapin, mast cell degranulating (MCD-) peptide and apamin, by reversed-phase (RP) HPLC. Using this relatively simple procedure it is possible to isolate these peptides essentially free of melittin (less than 1 in 10(7)) and phospholipase (less than 1 in 10(5] in high yield. The CD spectrum and secondary structure analysis are reported for MCD-peptide and on this basis a solution structure is proposed for this toxin.

The role of the phosphate group for the structure of phosphopeptide products of adenosine 3',5'-cyclic monophosphate-dependent protein kinase

By c.d. studies it is shown that liver-pyruvate-kinase-related peptide substrates of cyclic AMP-dependent protein kinase have a high tendency towards non-random structures in non-aqueous media. When phosphorylated, the conformation tendencies decrease. This structural change is explained in terms of the formation of strong intrapeptide phosphate-guanidinium salt links. It is proposed that similar events occur at the catalytic site of protein kinase and that such an interaction could facilitate the removal of the phosphorylated products.