Mechanism of alamethicin insertion into lipid bilayers

Effect of lipid composition on buforin II structure and membrane entry

Buforin II is a 21-amino acid polycationic antimicrobial peptide derived from a peptide originally isolated from the stomach tissue of the Asian toad Bufo bufo gargarizans. It is hypothesized to target a wide range of bacteria by translocating into cells without membrane permeabilization and binding to nucleic acids. Previous research found that the structure and membrane interactions of buforin II are related to lipid composition. In this study, we used molecular dynamics (MD) simulations along with lipid vesicle experiments to gain insight into how buforin II interacts differently with phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE) lipids. Fluorescent spectroscopic measurements agreed with the previous assertion that buforin II does not interact with pure PC vesicles. Nonetheless, the reduced entry of the peptide into anionic PG membranes versus neutral PC membranes during simulations correlates with the experimentally observed reduction in BF2 translocation through pure PG membranes. Simulations showing membrane entry into PC also provide insight into how buforin II may initially penetrate cell membranes. Our MD simulations also allowed us to consider how neutral PE lipids affect the peptide differently than PC. In particular, the peptide had a more helical secondary structure in simulations with PE lipids. A change in structure was also apparent in circular dichroism measurements. PE also reduced membrane entry in simulations, which correlates with decreased translocation in the presence of PE observed in previous studies. Together, these results provide molecular-level insight into how lipid composition can affect buforin II structure and function and will be useful in efforts to design peptides with desired antimicrobial and cell-penetrating properties.

Alamethicin in lipid bilayers: combined use of X-ray scattering and MD simulations

We study fully hydrated bilayers of two di-monounsaturated phospholipids diC18:1PC (DOPC) and diC22:1PC with varying amounts of alamethicin (Alm). We combine the use of X-ray diffuse scattering and molecular dynamics simulations to determine the orientation of alamethicin in model lipids. Comparison of the experimental and simulated form factors shows that Alm helices are inserted transmembrane at high humidity and high concentrations, in agreement with earlier results. The X-ray scattering data and the MD simulations agree that membrane thickness changes very little up to 1/10 Alm/DOPC. In contrast, the X-ray data indicate that the thicker diC22:1PC membrane thins with added Alm, a total decrease in thickness of 4 A at 1/10 Alm/diC22:1PC. The different effect of Alm on the thickness changes of the two bilayers is consistent with Alm having a hydrophobic thickness close to the hydrophobic thickness of 27 A for DOPC; Alm is then mismatched with the 7 A thicker diC22:1PC bilayer. The X-ray data indicate that Alm decreases the bending modulus (K(C)) by a factor of approximately 2 in DOPC and a factor of approximately 10 in diC22:1PC membranes (P/L approximately 1/10). The van der Waals and fluctuational interactions between bilayers are also evaluated through determination of the anisotropic B compressibility modulus.

Conformation and membrane orientation of amphiphilic helical peptides by oriented circular dichroism

Oriented circular dichroism (OCD) was used to characterize and compare in a quantitative manner the secondary structure and concentration dependent realignment of the antimicrobial peptides PGLa and MSI-103, and of the structurally related cell-penetrating peptide MAP in aligned phospholipid bilayers. All these peptides adopt an amphiphilic alpha-helical conformation, and from solid-state NMR analysis they are known to bind to membranes in two distinct orientations depending on their concentration. At low peptide/lipid (P/L) ratio the helices are aligned parallel to membrane surface (S-state), but with increasing concentration they realign to a tilted orientation (T-state), getting immersed into the membrane with an oblique angle supposedly as a result of dimer-formation. In macroscopically aligned liquid crystalline 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine bilayers the two limiting states are represented by distinct OCD spectra, and all spectra at intermediate peptide concentrations can be described by a linear combination of these two line shapes. The corresponding fraction of molecules occupying the T-state was determined by fitting the intermediate spectra with a superposition of the two extreme line shapes. By plotting this fraction versus 1/(P/L), the threshold P/L* ratio for realignment was extracted for each of the three related peptides. Despite their structural similarity distinctly different thresholds were obtained, namely for MSI-103 realignment starts already at a low P/L of approximately 1:236, for a MAP derivative (using a nonaggregating analog containing a D-amino acid) the transition begins at P/L approximately 1:156, whereas PGLa needs the highest concentration to flip into T-state at P/L approximately 1:85. Analysis of the original MAP sequence (containing only L-amino acids) gave OCD spectra compatible with beta-pleated conformation, suggesting that this peptide starts to aggregate with increasing concentration, unlike the other helical peptides. All these changes in peptide conformation and membrane alignment observed here by OCD seem to be functionally relevant, as they can be correlated with the membrane perturbing activities of the three antimicrobial and cell-penetrating sequences.

On the mechanism of pore formation by melittin

The mechanism of pore formation of lytic peptides, such as melittin from bee venom, is thought to involve binding to the membrane surface, followed by insertion at threshold levels of bound peptide. We show that in membranes composed of zwitterionic lipids, i.e. phosphatidylcholine, melittin not only forms pores but also inhibits pore formation. We propose that these two modes of action are the result of two competing reactions: direct insertion into the membrane and binding parallel to the membrane surface. The direct insertion of melittin leads to pore formation, whereas the parallel conformation is inactive and prevents other melittin molecules from inserting, hence preventing pore formation.

Peptide aggregation and pore formation in a lipid bilayer: a combined coarse-grained and all atom molecular dynamics study

We present a simulation study where different resolutions, namely coarse-grained (CG) and all-atom (AA) molecular dynamics simulations, are used sequentially to combine the long timescale reachable by CG simulations with the high resolution of AA simulations, to describe the complete processes of peptide aggregation and pore formation by alamethicin peptides in a hydrated lipid bilayer. In the 1-micros CG simulations the peptides spontaneously aggregate in the lipid bilayer and exhibit occasional transitions between the membrane-spanning and the surface-bound configurations. One of the CG systems at t = 1 micros is reverted to an AA representation and subjected to AA simulation for 50 ns, during which water molecules penetrate the lipid bilayer through interactions with the peptide aggregates, and the membrane starts leaking water. During the AA simulation significant deviations from the alpha-helical structure of the peptides are observed, however, the size and arrangement of the clusters are not affected within the studied time frame. Solid-state NMR experiments designed to match closely the setup used in the molecular dynamics simulations provide strong support for our finding that alamethicin peptides adopt a diverse set of configurations in a lipid bilayer, which is in sharp contrast to the prevailing view of alamethicin oligomers formed by perfectly aligned helical alamethicin peptides in a lipid bilayer.

Reaction phenotyping: current industry efforts to identify enzymes responsible for metabolizing drug candidates

Reaction phenotyping studies to identify specific enzymes involved in the metabolism of drug candidates are increasingly important in drug discovery efforts. Experimental approaches used for CYP reaction phenotyping include incubations with cDNA expressed CYP enzyme systems and incubations containing specific CYP enzyme inhibitors. Since both types of experiments present specific advantages as well as known drawbacks, these studies are generally viewed as complementary approaches. Although glucuronidation pathways are also known to present potential drug-drug interaction issues as well as challenges related to their polymorphic expression, reaction phenotyping approaches for glucuronidation are generally limited to cDNA expressed systems due to lack of availability of specific UGT inhibitors. This article presents a limited review of current approaches to reaction phenotyping studies used within the pharmaceutical industry.

Bilayer interactions of pHLIP, a peptide that can deliver drugs and target tumors

The pH-dependent insertion of pHLIP across membranes is proving to be a useful property for targeting acidic tissues or tumors and delivering drugs attached to its C-terminus. It also serves as a model peptide for studies of protein insertion into membranes, so further elucidation of the insertion mechanism of pHLIP and its features is desirable. We examine how the peptide perturbs a model phosphatidylcholine membrane and how it associates with the lipid bilayer using an array of fluorescence techniques, including fluorescence anisotropy measurements of TMA-DPH anchored in bilayers, quenching of pHLIP fluorescence by brominated lipids and acrylamide, and measurements of energy transfer between aromatic residues of pHLIP and TMA-DPH. When pHLIP is bound to the surface of bilayers near neutral pH, the membrane integrity is preserved whereas the elastic properties of bilayers are changed as reported by an increase of membrane viscosity. When it is inserted, there is little perturbation of the lipids. The results also suggest that pHLIP can bind to the membrane surface in a shallow or a deep mode depending on the phase state of the lipids. Using parallax analysis, the change of the penetration depth of pHLIP was estimated to be 0.4 A from the bilayer center and 2.8 A from the membrane surface after the liquid-to-gel phase transition.

Characterization of the N-terminal segment used by the barley yellow dwarf virus movement protein to promote interaction with the nuclear membrane of host plant cells

The barley yellow dwarf virus movement protein (BYDV-MP) requires its N-terminal sequence to promote the transport of viral RNA into the nuclear compartment of host plant cells. Here, graphical analysis predicts that this sequence would form a membrane interactive amphiphilic alpha-helix. Confirming this prediction, NT1, a peptide homologue of the BYDV-MP N-terminal sequence, was found to be alpha-helical (65%) in the presence of vesicles mimics of the nuclear membrane. The peptide increased the fluidity of these nuclear membrane mimics (rise in wavenumber of circa 0.5-1.0 cm(-1)) and induced surface pressure changes of 2 mN m(-1) in lipid monolayers with corresponding compositions. Taken with isotherm analysis these results suggest that BYDV-MP forms an N-terminal amphiphilic alpha-helix, which partitions into the nuclear membrane primarily through thermodynamically stable associations with the membrane lipid headgroup region. We speculate that these associations may play a role in targeting of the nuclear membrane by BYDM-MP.

Defect formation of lytic peptides in lipid membranes and their influence on the thermodynamic properties of the pore environment

We present an experimental study of the pore formation processes of small amphipathic peptides in model phosphocholine lipid membranes. We used atomic force microscopy to characterize the spatial organization and structure of alamethicin- and melittin-induced defects in lipid bilayer membranes and the influence of the peptide on local membrane properties. Alamethicin induced holes in gel DPPC membranes were directly visualized at different peptide concentrations. We found that the thermodynamic state of lipids in gel membranes can be influenced by the presence of alamethicin such that nanoscopic domains of fluid lipids form close to the peptide pores, and that the elastic constants of the membrane are altered in their vicinity. Melittin-induced holes were visualized in DPPC and DLPC membranes at room temperature in order to study the influence of the membrane state on the peptide induced hole formation. Also differential scanning calorimetry was used to investigate the effect of alamethicin on the lipid membrane phase behaviour.

Pro-apoptotic bax-α1 synthesis and evidence for β-sheet to α-helix conformational change as triggered by negatively charged lipid membranes

Solid phase synthesis of Bax-alpha1, the 25 amino acids domain (14TSSEQIMKTGALLLQGFIQDRAGRM38) of the pro-apoptotic Bax protein has been accomplished using Fmoc chemistry. A new fast and harmless protocol is described for complete TFA removal from the purified peptide powder leading to a final purity greater than 98% as controlled by 19F-NMR, UV and MALDI-TOF mass spectrometry. Secondary structure was determined in various solution and membrane media using UV Circular Dichroism. In water solution, Bax-alpha1 is present as a mixture of beta-sheet and unstructured (random coil) conformations. A marked change from beta-sheet to alpha-helix secondary structures is observed upon interaction with negatively charged phospholipids vesicles whereas neutral lipid membranes have no significant effect on the aqueous peptide conformation. Results are discussed in terms of Bax binding to mitochondrial membranes.

Solid-Phase Synthesis, Characterization, and Antibacterial Activities of Metallocene–Peptide Bioconjugates

This work shows how the introduction of an organometallic group enhances and modifies the specificity of biologically active peptides. Ferrocene was chosen as an organometallic group because it has been shown to alter the pharmacodynamic profile of bioactive compounds. A comparison with the isosteric cobaltocenium group allows one to explore the influence of charge and redox potential on the biological activity of the conjugates. Arginine and tryptophan containing peptides H-WRWRWR-NH(2) and H-RWRWRW-NH(2) and the metallocene peptide bioconjugates [M]-C(O)-RWRWR-NH(2) and [M]-C(O)-WRWRW-NH(2), where [M]=[Co(Cp)(C(5)H(4))](+), [Fe(Cp)(C(5)H(4))] were prepared by solid-phase peptide synthesis (SPPS). They were purified by HPLC, characterized by ESIMS and NMR spectroscopy, and tested for antibacterial properties against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus using the minimum inhibitory concentration (MIC) test. In most cases, no metal-specific activity could be observed. However, the conjugate [Fe(Cp)(C(5)H(4))-C(O)-WRWRW-NH(2)] 6 was found to be particularly effective against the Gram-positive S. aureus. The activity of this metallocene-pentapeptide conjugate (7.1 microM) was even better than the 20 amino acid naturally occurring pilosulin 2, which was used as a positive control. Unlike all other compounds tested, which were most active against the Gram-negative E. coli strain, the ferrocene conjugate 6 was the only compound in this series that was most active against Gram-positive bacteria. Given the health concerns resulting from multidrug resistant S. aureus strains, the incorporation of metallocenes may provide a novel line of attack.

Composition effect on peptide interaction with lipids and bacteria: variants of C3a peptide CNY21

The effect of peptide hydrophobicity and charge on peptide interaction with model lipid bilayers was investigated for the C3a-derived peptide CNY21 by fluorescence spectroscopy, circular dichroism, ellipsometry, z-potential, and photon correlation spectroscopy measurements. For both zwitterionic and anionic liposomes, the membrane-disruptive potency for CNY21 variants increased with increasing net positive charge and mean hydrophobicity and was completely lost on elimination of all peptide positive charges. Analogous effects of elimination of the peptide positive net charge in particular were found regarding bacteria killing for both Pseudomonas aeruginosa and Bacillus subtilis. The peptides, characterized by moderate helix content both in buffer and when attached to the liposomes, displayed high adsorption for the net positively charged peptide variants, whereas adsorption was non-measurable for the uncharged peptide. That electrostatically driven adsorption represents the main driving force for membrane disruption in lipid systems was also demonstrated by a drastic reduction in both liposome leakage and peptide adsorption with increasing ionic strength, and this salt inactivation can be partly avoided by increasing the peptide hydrophobicity. This increased electrolyte resistance translates also to a higher antibacterial effect for the hydrophobically modified variant at high salt concentration. Overall, our findings demonstrate the importance of the peptide adsorption and resulting peptide interfacial density for membrane-disruptive effects of these peptides.

Investigations into the ability of an oblique alpha-helical template to provide the basis for design of an antimicrobial anionic amphiphilic peptide

AP1 (GEQGALAQFGEWL) was shown by theoretical analysis to be an anionic oblique-orientated alpha-helix former. The peptide exhibited a monolayer surface area of 1.42 nm(2), implying possession of alpha-helical structure at an air/water interface, and Fourier transform infrared spectroscopy (FTIR) showed the peptide to be alpha-helical (100%) in the presence of vesicle mimics of Escherichia coli membranes. FTIR lipid-phase transition analysis showed the peptide to induce large decreases in the fluidity of these E. coli membrane mimics, and Langmuir-Blodgett trough analysis found the peptide to induce large surface pressure changes in monolayer mimics of E. coli membranes (4.6 mN.m(-1)). Analysis of compression isotherms based on mixing enthalpy (DeltaH) and the Gibbs free energy of mixing (DeltaG(Mix)) predicted that these monolayers were thermodynamically stable (DeltaH and DeltaG(Mix) each negative) but were destabilized by the presence of the peptide (DeltaH and DeltaG(Mix) each positive). The peptide was found to have a minimum lethal concentration of 3 mm against E. coli and was seen to cause lysis of erythrocytes at 5 mm. In combination, these data clearly show that AP1 functions as an anionic alpha-helical antimicrobial peptide and suggest that both its tilted peptide characteristics and the composition of its target membrane are important determinants of its efficacy of action.

Antimicrobial peptides temporins B and L induce formation of tubular lipid protrusions from supported phospholipid bilayers

The binding of the antimicrobial peptides temporins B and L to supported lipid bilayer (SLB) model membranes composed of phosphatidylcholine and phosphatidylglycerol (4:1, mol/mol) caused the formation of fibrillar protrusions, visible by fluorescent microscopy of both a fluorescent lipid analog and a labeled peptide. Multicolor imaging at low peptide-to-lipid ratios (P/L < approximately 1:5) revealed an initial in-plane segregation of membrane-bound peptide and partial exclusion of lipid from the peptide-enriched areas. Subsequently, at higher P/L numerous flexible lipid fibrils were seen growing from the areas enriched in lipid. The fibrils have diameters <250 nm and lengths of up to approximately 1 mm. Fibril formation reduces the in-plane heterogeneity and results in a relatively even redistribution of bound peptide over the planar bilayer and the fibrils. Physical properties of the lipid fibrils suggest that they have a tubular structure. Our data demonstrate that the peptide-lipid interactions alone can provide a driving force for the spontaneous membrane shape transformations leading to tubule outgrowth and elongation. Further experiments revealed the importance of positive curvature strain in the tubulation process as well as the sufficient positive charge on the peptide (>/=+2). The observed membrane transformations could provide a simplified in vitro model for morphogenesis of intracellular tubular structures and intercellular connections.

Paradoxical lipid dependence of pores formed by the Escherichia coli alpha-hemolysin in planar phospholipid bilayer membranes

alpha-Hemolysin (HlyA) is an extracellular protein toxin (117 kDa) secreted by Escherichia coli that targets the plasma membranes of eukaryotic cells. We studied the interaction of this toxin with membranes using planar phospholipid bilayers. For all lipid mixtures tested, addition of nanomolar concentrations of toxin resulted in an increase of membrane conductance and a decrease in membrane stability. HlyA decreased membrane lifetime up to three orders of magnitude in a voltage-dependent manner. Using a theory for lipidic pore formation, we analyzed these data to quantify how HlyA diminished the line tension of the membrane (i.e., the energy required to form the edge of a new pore). However, in contrast to the expectation that adding the positive curvature agent lysophosphatidylcholine would synergistically lower line tension, its addition significantly stabilized HlyA-treated membranes. HlyA also appeared to thicken bilayers to which it was added. We discuss these results in terms of models for proteolipidic pores.

Structure of magainin and alamethicin in model membranes studied by x-ray reflectivity

We have investigated the structure of lipid bilayers containing varied molar ratios of different lipids and the antimicrobial peptides magainin and alamethicin. For this structural study, we have used x-ray reflectivity on highly aligned solid-supported multilamellar lipid membranes. The reflectivity curves have been analyzed by semi-kinematical reflectivity theory modeling the bilayer density profile rho(z). Model simulations of the reflectivity curves cover a large range of vertical momentum transfer q(z), and yield excellent agreement between data and theory. The structural changes observed as a function of the molar peptide/lipid concentration P/L are discussed in a comparative way.

Peptide-membrane interactions and mechanisms of membrane destruction by amphipathic alpha-helical antimicrobial peptides

Antimicrobial peptides (AMPs) have received considerable interest as a source of new antibiotics with the potential for treatment of multiple-drug resistant infections. An important class of AMPs is composed of linear, cationic peptides that form amphipathic alpha-helices. Among the most potent of these are the cecropins and synthetic peptides that are hybrids of cecropin and the bee venom peptide, mellitin. Both cecropins and cecropin-mellitin hybrids exist in solution as unstructured monomers, folding into predominantly alpha-helical structures upon membrane binding with their long helical axis parallel to the bilayer surface. Studies using model membranes have shown that these peptides intercalate into the lipid bilayer just below the level of the phospholipid glycerol backbone in a location that requires expansion of the outer leaflet of the bilayer, and evidence from a variety of experimental approaches indicates that expansion and thinning of the bilayer are common characteristics during the early stages of antimicrobial peptide-membrane interactions. Subsequent disruption of the membrane permeability barrier may occur by a variety of mechanisms, leading ultimately to loss of cytoplasmic membrane integrity and cell death.

Influence of N-acylation of a peptide derived from human lactoferricin on membrane selectivity

Increasing numbers of bacterial strains being resistant to conventional antibiotics emphasize the urgent need for new antimicrobial agents. One strategy is based on host defence peptides that can be found in every organism including humans. We have studied the antimicrobial peptide LF11, derived from the pepsin cleavage product of human lactoferrin, known for its antimicrobial and lipid A-binding activity, and peptide C12LF11, the N-lauryl-derivative of LF11, which has owing to the attached hydrocarbon chain an additional hydrophobic segment. The influence of this hydrocarbon chain on membrane selectivity was studied using model membranes composed of dipalmitoylphosphatidylglycerol (DPPG), mimicking bacterial plasma membranes, and of dipalmitoylphosphatidylcholine (DPPC), a model system for mammalian membranes. A variety of biophysical techniques was applied. Thereby, we found that LF11 did not affect DPPC bilayers and showed only moderate effects on DPPG membranes in accordance with its non-hemolytic and weak antimicrobial activity. In contrast, the introduction of the N-lauryl group caused significant changes in the phase behaviour and lipid chain packing in both model membrane systems. These findings correlate with the in vitro tests on methicillin resistant S. aureus, E. coli, P. aeruginosa and human red blood cells, showing increased biological activity of C12LF11 towards these test organisms. This provides evidence that both electrostatic and hydrophobic interactions are crucial for biological activity of antimicrobial peptides, whereas a certain balance between the two components has to be kept, in order not to loose the specificity for bacterial membranes.

4-AminobiphenylN-glucuronidation by liver microsomes: optimization of the reaction conditions and characterization of the UDP-glucuronosyltransferase isoforms

4-Aminobiphenyl (4-ABP) is an arylamine that has long been associated with human and animal urinary bladder cancer. N-glucuronidation is an important metabolic pathway that contributes significantly to 4-ABP-bladder carcinogenesis by facilitating transport of the active metabolites from the liver to the bladder. This pathway is carried out by UDP-glucuronosyltransferase (UGTs). These enzymes are located in the inner membrane of the endoplasmic reticulum. Full UGT activity is not achieved until membrane constraints are removed. This study was conducted to optimize the incubation conditions of 4-ABP N-glucuronidation. The kinetic parameters of the isozymes most commonly involved in arylamine glucuronidation, namely UGT1A4 and UGT1A9, were also determined. The UGT reaction was linear in the incubation time (0-90 min) and in the microsomal protein range of 0-0.5 mg. Alamethicin, a pore-forming agent, was found to be the best reagent to activate UGTs. It increased the enzyme activity by nearly 8-fold and this activation was at concentration of 50 microg mg(-1) protein. Interestingly, UGT1A4 glucuronidated 4-ABP with more affinity and efficiency than did UGT1A9. The K(m) and V(max) of UGT1A4 for 4-ABP were 58.8 microm and 234.9 pmol min(-1) mg(-1) protein, respectively, and 227.5 microm and 31.2 pmol min(-1) mg(-1) protein for UGT1A9. Furthermore, hecogenin was found to be a competitive inhibitor for UGT1A4. It increased the K(m) of UGT1A4 for 4-ABP by nearly 10 fold at a concentration of 50 microm. This is the first report that tried to optimize the incubation conditions for 4-ABP N-glucuronidation and characterized the enzyme kinetic parameters of UGT isoforms catalysing 4-ABP N-glucuronidation.

Many-body effect of antimicrobial peptides: on the correlation between lipid's spontaneous curvature and pore formation

Recently we have shown that the free energy for pore formation induced by antimicrobial peptides contains a term representing peptide-peptide interactions mediated by membrane thinning. This many-body effect gives rise to the cooperative concentration dependence of peptide activities. Here we performed oriented circular dichroism and x-ray diffraction experiments to study the lipid dependence of this many-body effect. In particular we studied the correlation between lipid's spontaneous curvature and peptide's threshold concentration for pore formation by adding phosphatidylethanolamine and lysophosphocholine to phosphocholine bilayers. Previously it was argued that this correlation exhibited by magainin and melittin supported the toroidal model for the pores. Here we found similar correlations exhibited by melittin and alamethicin. We found that the main effect of varying the spontaneous curvature of lipid is to change the degree of membrane thinning, which in turn influences the threshold concentration for pore formation. We discuss how to interpret the lipid dependence of membrane thinning.

Structure-function characterization and optimization of a plant-derived antibacterial peptide

Crushed seeds of the Moringa oleifera tree have been used traditionally as natural flocculants to clarify drinking water. We previously showed that one of the seed peptides mediates both the sedimentation of suspended particles such as bacterial cells and a direct bactericidal activity, raising the possibility that the two activities might be related. In this study, the conformational modeling of the peptide was coupled to a functional analysis of synthetic derivatives. This indicated that partly overlapping structural determinants mediate the sedimentation and antibacterial activities. Sedimentation requires a positively charged, glutamine-rich portion of the peptide that aggregates bacterial cells. The bactericidal activity was localized to a sequence prone to form a helix-loop-helix structural motif. Amino acid substitution showed that the bactericidal activity requires hydrophobic proline residues within the protruding loop. Vital dye staining indicated that treatment with peptides containing this motif results in bacterial membrane damage. Assembly of multiple copies of this structural motif into a branched peptide enhanced antibacterial activity, since low concentrations effectively kill bacteria such as Pseudomonas aeruginosa and Streptococcus pyogenes without displaying a toxic effect on human red blood cells. This study thus identifies a synthetic peptide with potent antibacterial activity against specific human pathogens. It also suggests partly distinct molecular mechanisms for each activity. Sedimentation may result from coupled flocculation and coagulation effects, while the bactericidal activity would require bacterial membrane destabilization by a hydrophobic loop.

Alamethicin influence on the membrane bending elasticity

We investigate the bending elasticity of lipid membranes with the increase of the alamethicin concentrations in the membrane via analysis of the thermally induced shape fluctuations of quasi-spherical giant vesicles. Our experimental results prove the strong influence of alamethicin molecules on the bending elasticity of diphytanoyl phosphatidylcholine and dilauroyl phosphatidylcholine membranes even in the range of very low peptide concentrations (less than 10(-3) mol/mol in the membrane). The results presented in this work, testify to the peripheral orientation of alamethicin molecules at low peptide concentrations in the membrane for both types of lipid bilayers. An upper limit of the concentration of the peptide in the membrane is determined below which the system behaves as an ideal two-dimensional solution and the peptide molecules have a planar orientation in the membrane.

Membrane thinning due to antimicrobial peptide binding: an atomic force microscopy study of MSI-78 in lipid bilayers

The interaction of an antimicrobial peptide, MSI-78, with phospholipid bilayers has been investigated using atomic force microscopy, circular dichroism, and nuclear magnetic resonance (NMR). Binding of amphipathic peptide helices with their helical axis parallel to the membrane surface leads to membrane thinning. Atomic force microscopy of supported 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers in the presence of MSI-78 provides images of the membrane thinning process at a high spatial resolution. This data reveals that the membrane thickness is not reduced uniformly over the entire bilayer area. Instead, peptide binding leads to the formation of distinct domains where the bilayer thickness is reduced by 1.1 +/- 0.2 nm. The data is interpreted using a previously published geometric model for the structure of the peptide-lipid domains. In this model, the peptides reside at the hydrophilic-hydrophobic boundary in the lipid headgroup region, which leads to an increased distance between lipid headgroups. This picture is consistent with concentration-dependent 31P and 2H NMR spectra of MSI-78 in mechanically aligned DMPC bilayers. Furthermore, 2H NMR experiments on DMPC-d54 multilamellar vesicles indicate that the acyl chains of DMPC are highly disordered in the presence of the peptide as is to be expected for the proposed structure of the peptide-lipid assembly.

Conformation of peptides in lipid membranes studied by x-ray grazing incidence scattering

Although the antimicrobial, fungal peptide alamethicin has been extensively studied, the conformation of the peptide and the interaction with lipid bilayers as well as the mechanism of channel gating are still not completely clear. As opposed to studies of the crystalline state, the polypeptide structures in the environment of fluid bilayers are difficult to probe. We have investigated the conformation of alamethicin in highly aligned stacks of model lipid membranes by synchrotron-based x-ray scattering. The (wide-angle) scattering distribution has been measured by reciprocal space mappings. A pronounced scattering signal is observed in samples of high molar peptide/lipid ratio which is distinctly different from the scattering distribution of an ideal helix in the transmembrane state. Beyond simple models of ideal helices, the data is analyzed in terms of models based on atomic coordinates from the Brookhaven Protein Data Bank, as well as from published molecular dynamics simulations. The results can be explained by assuming a wide distribution of helix tilt angles with respect to the membrane normal and a partial insertion of the N-terminus into the membrane.

Membrane perturbation induced by interfacially adsorbed peptides

The structural and energetic characteristics of the interaction between interfacially adsorbed (partially inserted) alpha-helical, amphipathic peptides and the lipid bilayer substrate are studied using a molecular level theory of lipid chain packing in membranes. The peptides are modeled as "amphipathic cylinders" characterized by a well-defined polar angle. Assuming two-dimensional nematic order of the adsorbed peptides, the membrane perturbation free energy is evaluated using a cell-like model; the peptide axes are parallel to the membrane plane. The elastic and interfacial contributions to the perturbation free energy of the "peptide-dressed" membrane are evaluated as a function of: the peptide penetration depth into the bilayer's hydrophobic core, the membrane thickness, the polar angle, and the lipid/peptide ratio. The structural properties calculated include the shape and extent of the distorted (stretched and bent) lipid chains surrounding the adsorbed peptide, and their orientational (C-H) bond order parameter profiles. The changes in bond order parameters attendant upon peptide adsorption are in good agreement with magnetic resonance measurements. Also consistent with experiment, our model predicts that peptide adsorption results in membrane thinning. Our calculations reveal pronounced, membrane-mediated, attractive interactions between the adsorbed peptides, suggesting a possible mechanism for lateral aggregation of membrane-bound peptides. As a special case of interest, we have also investigated completely hydrophobic peptides, for which we find a strong energetic preference for the transmembrane (inserted) orientation over the horizontal (adsorbed) orientation.

Perturbation of a lipid membrane by amphipathic peptides and its role in pore formation

We study the structural and energetic consequences of (alpha-helical) amphipathic peptide adsorption onto a lipid membrane and the subsequent formation of a transmembrane peptide pore. Initially, each peptide binds to the membrane surface, with the hydrophobic face of its cylinder-like body inserted into the hydrocarbon core. Pore formation results from subsequent peptide crowding, oligomerization, and eventually reorientation along the membrane normal. We have theoretically analyzed three peptide-membrane association states: interfacially-adsorbed monomeric and dimeric peptides, and the multi-peptide transmembrane pore state. Our molecular-level model for the lipid bilayer is based on a combination of detailed chain packing theory and a phenomenological description of the headgroup region. We show that the membrane perturbation free energy depends critically on peptide orientation: in the transmembrane pore state the lipid perturbation energy, per peptide, is smaller than in the adsorbed state. This suggests that the gain in conformational freedom of the lipid chains is a central driving force for pore formation. We also find a weak, lipid-mediated, gain in membrane perturbation free energy upon dimerization of interfacially-adsorbed peptides. Although the results pertain mainly to weakly-charged peptides, they reveal general properties of the interaction of amphipathic peptides with lipid membranes.

Penetratin-membrane association: W48/R52/W56 shield the peptide from the aqueous phase

Using molecular dynamics simulations, we studied the mode of association of the cell-penetrating peptide penetratin with both a neutral and a charged bilayer. The results show that the initial peptide-lipid association is a fast process driven by electrostatic interactions. The homogeneous distribution of positively charged residues along the axis of the helical peptide, and especially residues K46, R53, and K57, contribute to the association of the peptide with lipids. The bilayer enhances the stability of the penetratin helix. Oriented parallel to the lipid-water interface, the subsequent insertion of the peptide through the bilayer headgroups is significantly slower. The presence of negatively charged lipids considerably enhances peptide binding. Lateral side-chain motion creates an opening for the helix into the hydrophobic core of the membrane. The peptide aromatic residues form a pi-stacking cluster through W48/R52/W56 and F49/R53, protecting the peptide from the water phase. Interaction with the penetratin peptide has only limited effect on the overall membrane structure, as it affects mainly the conformation of the lipids which interact directly with the peptide. Charge matching locally increases the concentration of negatively charged lipids, lateral lipid diffusion locally decreases. Lipid disorder increases, through decreased order parameters of the lipids interacting with the penetratin side chains. Penetratin molecules at the membrane surface do not seem to aggregate.

Trichogin: a paradigm for lipopeptaibols

Lipopeptaibols are members of a novel family of naturally occurring, short peptides with antimicrobial activity, characterized by a lipophilic acyl chain at the N-terminus, a high content of turn/helix inducing alpha-aminoisobutyric acid and a 1,2-amino alcohol at the C-terminus. Using solution methods, the prototypical lipopeptaibol trichogin GA IV and a large series of appropriately designed analogues were synthesized, which allow: (i) determination of the minimal lipid chain and peptide main-chain lengths for the onset of membrane activity, and (ii) exploitation of a number of physico-chemical techniques aimed at assessing the trichogin preferred conformation under a variety of conditions and at investigating its mechanism of interaction with the phospholipid membranes.

Solid-state NMR investigation of the depth of insertion of protegrin-1 in lipid bilayers using paramagnetic Mn2+

The depth of insertion of an antimicrobial peptide, protegrin-1 (PG-1), in lipid bilayers is investigated using solid-state NMR. Paramagnetic Mn(2+) ions bind to the surface of lipid bilayers and induce distance-dependent dipolar relaxation of nuclear spins. By comparing the signal dephasing of the peptide with that of the lipids, whose segmental depths of insertion are known, we determined the depths of several residues of PG-1 in 1,2 dilauryl-sn-glycero-3-phosphotidylcholine (DLPC) bilayers. We found that residues G2 at the N-terminus and F12 at the beta-turn of the peptide reside near the membrane surface, whereas L5 and V16 are embedded in the acyl chain region. The depths increase in the order of G2 < F12 < L5 < V16. These intensity-dephasing results are confirmed by direct measurement of the paramagnetically enhanced (13)C transverse relaxation rates. The relative depths indicate that PG-1 is tilted from the bilayer normal, which is consistent with independent solid-state NMR measurements of PG-1 orientation in the same lipids (Yamaguchi et al., 2001). They also indicate that PG-1 is fully immersed in the lipid bilayer. However, a quantitative mismatch between the bilayer thickness and PG-1 length suggests a local thinning of the DLPC bilayer by 8-10 A. The depth sensitivity of this Mn(2+) dephasing technique is tunable with the Mn(2+) concentration to focus on different regions of the lipid bilayer.

Highly robust lipid membranes on crystalline S-layer supports investigated by electrochemical impedance spectroscopy

In the present work, S-layer supported lipid membranes formed by a modified Langmuir-Blodgett technique were investigated by electrochemical impedance spectroscopy (EIS). Basically two intermediate hydrophilic supports for phospholipid- (DPhyPC) and bipolar tetraetherlipid- (MPL from Thermoplasma acidophilum) membranes have been applied: first, the S-layer protein SbpA isolated from Bacillus sphaericus CCM 2177 recrystallized onto a gold electrode; and second, as a reference support, an S-layer ultrafiltration membrane (SUM), which consists of a microfiltration membrane (MFM) with deposited S-layer carrying cell wall fragments. The electrochemical properties and the stability of DPhyPC and MPL membranes were found to depend on the used support. The specific capacitances were 0.53 and 0.69 microF/cm(2) for DPhyPC bilayers and 0.75 and 0.77 microF/cm(2) for MPL monolayers resting on SbpA and SUM, respectively. Membrane resistances of up to 80 mega Ohm cm(2) were observed for DPhyPC bilayers on SbpA. In addition, membranes supported by SbpA exhibited a remarkable long-term robustness of up to 2 days. The membrane functionality could be demonstrated by reconstitution of membrane-active peptides such as valinomycin and alamethicin. The present results recommend S-layer-supported lipid membranes as promising structures for membrane protein-based biosensor technology.

Solid-state NMR study of antimicrobial peptides from Australian frogs in phospholipid membranes

Antimicrobial peptides, isolated from the dorsal glands of Australian tree frogs, possess a wide spectrum of biological activity and some are specific to certain pathogens. These peptides have the capability of disrupting bacterial membranes and lysing lipid bilayers. This study focused on the following amphibian peptides: (1) aurein 1.2, a 13-residue peptide; (2) citropin 1.1, with 16 residues; and (3) maculatin 1.1, with 21 residues. The antibiotic activity and structure of these peptides have been studied and compared and possible mechanisms by which the peptides lyse bacterial membrane cells have been proposed. The peptides adopt amphipathic alpha-helical structures in the presence of lipid micelles and vesicles. Specifically 15N-labelled peptides were studied using solid-state NMR to determine their structure and orientation in model lipid bilayers. The effect of these peptides on phospholipid membranes was determined by 2H and 31P solid-state NMR techniques in order to understand the mechanisms by which they exert their biological effects that lead to the disruption of the bacterial cell membrane. Aurein 1.2 and citropin 1.1 are too short to span the membrane bilayer while the longer maculatin 1.1, which may be flexible due to the central proline, would be able to span the bilayer as a transmembrane alpha-helix. All three peptides had a peripheral interaction with phosphatidylcholine bilayers and appear to be located in the aqueous region of the membrane bilayer. It is proposed that these antimicrobial peptides have a "detergent"-like mechanism of membrane lysis.

Antimicrobial peptides: current status and therapeutic potential

Antimicrobial peptides (AMPs) are effector molecules of the innate immune system. A variety of AMPs have been isolated from species of all kingdoms and are classified based on their structure and amino acid motifs. AMPs have a broad antimicrobial spectrum and lyse microbial cells by interaction with biomembranes. Besides their direct antimicrobial function, they have multiple roles as mediators of inflammation with impact on epithelial and inflammatory cells influencing diverse processes such as cell proliferation, immune induction, wound healing, cytokine release, chemotaxis and protease-antiprotease balance. AMPs qualify as prototypes of innovative drugs that may be used as antimicrobials, anti-lipopolysaccharide drugs or modifiers of inflammation. Several strategies have been followed to identify lead candidates for drug development, to modify the peptides' structures, and to produce sufficient amounts for pre-clinical and clinical studies. This review summarises the current knowledge about the basic and applied biology of AMPs.

A tethered bilayer sensor containing alamethicin channels and its detection of amiloride based inhibitors

Alamethicin, a small transmembrane peptide, inserts into a tethered bilayer membrane (tBLM) to form ion channels, which we have investigated using electrical impedance spectroscopy. The number of channels formed is dependent on the incubation time, concentration of the alamethicin and the application of DC voltage. The properties of the ion channels when formed in tethered bilayers are similar to those for such channels assembled into black lipid membranes (BLMs). Furthermore, amiloride and certain analogs can inhibit the channel pores, formed in the tBLMs. The potency and concentration of the inhibitors can be determined by measuring the change of impedance. Our work illustrates the possibility of using a synthetic tBLM for the study of small peptide voltage dependent ion channels. A potential application of such a device is as a screening tool in drug discovery processes.

Energetics and self-assembly of amphipathic peptide pores in lipid membranes

We present a theoretical study of the energetics, equilibrium size, and size distribution of membrane pores composed of electrically charged amphipathic peptides. The peptides are modeled as cylinders (mimicking alpha-helices) carrying different amounts of charge, with the charge being uniformly distributed over a hydrophilic face, defined by the angle subtended by polar amino acid residues. The free energy of a pore of a given radius, R, and a given number of peptides, s, is expressed as a sum of the peptides' electrostatic charging energy (calculated using Poisson-Boltzmann theory), and the lipid-perturbation energy associated with the formation of a membrane rim (which we model as being semitoroidal) in the gap between neighboring peptides. A simple phenomenological model is used to calculate the membrane perturbation energy. The balance between the opposing forces (namely, the radial free energy derivatives) associated with the electrostatic free energy that favors large R, and the membrane perturbation term that favors small R, dictates the equilibrium properties of the pore. Systematic calculations are reported for circular pores composed of various numbers of peptides, carrying different amounts of charge (1-6 elementary, positive charges) and characterized by different polar angles. We find that the optimal R's, for all (except, possibly, very weakly) charged peptides conform to the "toroidal" pore model, whereby a membrane rim larger than approximately 1 nm intervenes between neighboring peptides. Only weakly charged peptides are likely to form "barrel-stave" pores where the peptides essentially touch one another. Treating pore formation as a two-dimensional self-assembly phenomenon, a simple statistical thermodynamic model is formulated and used to calculate pore size distributions. We find that the average pore size and size polydispersity increase with peptide charge and with the amphipathic polar angle. We also argue that the transition of peptides from the adsorbed to the inserted (membrane pore) state is cooperative and thus occurs rather abruptly upon a change in ambient conditions.

Solution NMR studies of antiamoebin, a membrane channel-forming polypeptide

Antiamoebin I is a membrane-active peptaibol produced by fungi of the species Emericellopsis which is capable of forming ion channels in membranes. Previous structure determinations by x-ray crystallography have shown the molecule is mostly helical, with a deep bend in the center of the polypeptide, and that the backbone structure is independent of the solvent used for crystallization. In this study, the solution structure of antiamoebin was determined by NMR spectroscopy in methanol, a solvent from which one of the crystal structures was determined. The ensemble of structures produced exhibit a right-handed helical C terminus and a left-handed helical conformation toward the N-terminus, in contrast to the completely right-handed helices found in the crystal structures. The NMR results also suggest that a "hinge" region exists, which gives flexibility to the polypeptide in the central region, and which could have functional implications for the membrane insertion process. A model for the membrane insertion and assembly process is proposed based on the antiamoebin solution and crystal structures, and is contrasted with the assembly and insertion mechanism proposed for other ion channel-forming polypeptides.

Changes in lipid mobility associated with alamethicin incorporation into membranes

The binding state of the antibiotic peptide alamethicin with phospholipid bilayers was investigated in terms of the changes induced in lipid mobility. Fluorescence anisotropy was used for the study. It was found that an increase in peptide concentration induced different changes in lipid mobility above and below a critical peptide concentration. This concentration was also critical for an increase in the cooperative binding of the peptide, as detected by circular dichroism. Above the critical peptide concentration, the mobility of both lipid regions, around the polar head and hydrocarbon chain, became restricted with an increased peptide concentration. Below the critical level, however, an increased peptide concentration induced a "wobbling" of the lipid hydrocarbon chain. These results show that an increase in the cooperative binding of the peptide is accompanied by a change in the dominant configuration of the binding peptide. When the binding peptide increases, the dominant configuration appears to shift from surface association to deep incorporation within the membrane. This shift in configuration means that in the formation of ion-conductive pores, voltage-driven insertion of the peptide is a prominent step below a critical peptide concentration.

Sigmoidal concentration dependence of antimicrobial peptide activities: a case study on alamethicin

The transition of the state of alamethicin from its inactive state to its active state of pore formation was measured as a function of the peptide concentration in three different membrane conditions. In each case the fraction of the alamethicin molecules occupying the active state, phi, showed a sigmoidal concentration dependence that is typical of the activities of antimicrobial peptides. Such a concentration dependence is often interpreted as due to peptide aggregation. However, we will show that a simple effect of aggregation cannot explain the data. We will introduce a model based on the elasticity of membrane, taking into consideration the membrane-thinning effect due to protein inclusion. The elastic energy of membrane provides an additional driving force for aggregation. The model produces a relation that not only predicts the correct concentration dependence but also explains qualitatively how the dependence changes with membrane conditions. The result shows that the membrane-mediated interactions between monomers and aggregates are essential for the strong cooperativity shown in pore formation.

Orientation and dynamics of an antimicrobial peptide in the lipid bilayer by solid-state NMR spectroscopy

The orientation and dynamics of an 18-residue antimicrobial peptide, ovispirin, has been investigated using solid-state NMR spectroscopy. Ovispirin is a cathelicidin-like model peptide (NH(2)-KNLRRIIRKIIHIIKKYG-COOH) with potent, broad-spectrum bactericidal activity. (15)N NMR spectra of oriented ovispirin reconstituted into synthetic phospholipids show that the helical peptide is predominantly oriented in the plane of the lipid bilayer, except for a small portion of the helix, possibly at the C-terminus, which deviates from the surface orientation. This suggests differential insertion of the peptide backbone into the lipid bilayer. (15)N spectra of both oriented and unoriented peptides show a reduced (15)N chemical shift anisotropy at room temperature compared with that of rigid proteins, indicating that the peptide undergoes uniaxial rotational diffusion around the bilayer normal with correlation times shorter than 10(-4) s. This motion is frozen below the gel-to-liquid crystalline transition temperature of the lipids. Ovispirin interacts strongly with the lipid bilayer, as manifested by the significantly reduced (2)H quadrupolar splittings of perdeuterated palmitoyloleoylphosphatidylcholine acyl chains upon peptide binding. Therefore, ovispirin is a curved helix residing in the membrane-water interface that executes rapid uniaxial rotation. These structural and dynamic features are important for understanding the antimicrobial function of this peptide.

Conformation of alamethicin in oriented phospholipid bilayers determined by (15)N solid-state nuclear magnetic resonance

The conformation of the 20-residue antibiotic ionophore alamethicin in macroscopically oriented phospholipid bilayers has been studied using (15)N solid-state nuclear magnetic resonance (NMR) spectroscopy in combination with molecular modeling and molecular dynamics simulations. Differently (15)N-labeled variants of alamethicin and an analog with three of the alpha-amino-isobutyric acid residues replaced by alanines have been investigated to establish experimental structural constraints and determine the orientation of alamethicin in hydrated phospholipid (dimyristoylphosphatidylcholine) bilayers and to investigate the potential for a major kink in the region of the central Pro(14) residue. From the anisotropic (15)N chemical shifts and (1)H-(15)N dipolar couplings determined for alamethicin with (15)N-labeling on the Ala(6), Val(9), and Val(15) residues and incorporated into phospholipid bilayer with a peptide:lipid molar ratio of 1:8, we deduce that alamethicin has a largely linear alpha-helical structure spanning the membrane with the molecular axis tilted by 10-20 degrees relative to the bilayer normal. In particular, we find compatibility with a straight alpha-helix tilted by 17 degrees and a slightly kinked molecular dynamics structure tilted by 11 degrees relative to the bilayer normal. In contrast, the structural constraints derived by solid-state NMR appear not to be compatible with any of several model structures crossing the membrane with vanishing tilt angle or the earlier reported x-ray diffraction structure (Fox and Richards, Nature. 300:325-330, 1982). The solid-state NMR-compatible structures may support the formation of a left-handed and parallel multimeric ion channel.

Dehydration induces lateral expansion of polyunsaturated 18:0-22:6 phosphatidylcholine in a new lamellar phase

To gain a better understanding of the biological role of polyunsaturated phospholipids, infrared (IR) linear dichroism, NMR, and x-ray diffraction studies have been conducted on the lyotropic phase behavior and bilayer dimensions of sn-1 chain perdeuterated 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (SDPC-d35), a mixed-chain saturated (18:0)-polyunsaturated (22:6 omega 3) lipid. SDPC films were hydrated at definite values of temperature (T) and relative humidity (RH). In excess water, the lipid forms exclusively lamellar phases in the temperature range 0--50 degrees C. Upon dehydration the lipid undergoes the main phase transition between the liquid-crystalline (L(alpha)) and gel (L(beta)) phase at T < 15 degrees C. Both the saturated and polyunsaturated chains adopt a stretched conformation in the L(beta) phase, presumably the all-trans (stearoyl) and angle iron or helical (docosahexaenoyl) one. A new fluid lamellar phase (L(alpha)') was found in partially hydrated samples at T > 15 degrees C. SDPC membranes expand laterally and contract vertically in the L(alpha)' phase when water was removed. This tendency is in sharp contrast to typical dehydration-induced changes of membrane dimensions. The slope of the phase transition lines in the RH-T phase diagram reveal that the lyotropic L(alpha)'-L(alpha) and L(beta)-L(alpha) transitions are driven by enthalpy and entropy, respectively The possible molecular origin of the phase transitions is discussed. The properties of SDPC are compared with that of membranes of monounsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d31).

Bilayer thickness and lipid interface area in unilamellar extruded 1,2-diacylphosphatidylcholine liposomes: a small-angle neutron scattering study

Small-angle neutron scattering (SANS) experiments have been performed on large unilamellar liposomes prepared from 1,2-dilauroylphosphatidylcholine (DLPC), 1,2-dimyristoyl-phosphatidylcholine (DMPC) and 1,2-distearoylphosphatidylcholine (DSPC) in heavy water by extrusion through polycarbonate filters with 500 A pores. The neutron scattering intensity I(Q) in the region of scattering vectors Q corresponding to 0.0015 A(-2) < or = Q(2) < or = 0.0115 A(-2) was fitted using a step function model of bilayer neutron scattering length density and supposing that the liposomes are spherical and have a Gaussian distribution of radii. Using the lipid volumetric data, and supposing that the thickness of bilayer polar region equals to d(H) = 9+/-1 A and the water molecular volume intercalated in the bilayer polar region is the same as in the aqueous bulk aqueous phase, the steric bilayer thickness d(L), the lipid surface area A(L) and the number of water molecules per lipid molecule N intercalated in the bilayer polar region were obtained: d(L) = 41.58+/-1.93 A, A(L) = 57.18+/-1.00 A(2) and N = 6.53+/-1.93 in DLPC at 20 degrees C, d(L) = 44.26+/-1.42 A, A(L) = 60.01+/-0.75 A(2) and N = 7.37+/-1.94 in DMPC at 36 degrees C, and d(L) = 49.77+/-1.52 A, A(L) = 64.78+/-0.46 A(2) and N = 8.67+/-1.97 in DSPC at 60 degrees C. After correcting for area thermal expansivity alpha approximately 0.00417 K(-1), the lipid surface area shows a decrease with the lipid acyl chain length at 60 degrees C: A(L) = 67.56+/-1.18 A(2) in DLPC, A(L) = 66.33+/-0.83 A(2) in DMPC and A(L) = 64.78+/-0.46 A(2) in DSPC. It is also shown that a joint evaluation of SANS and small-angle X-ray scattering on unilamellar liposomes can be used to obtain the value of d(H) and the distance of the lipid phosphate group from the bilayer hydrocarbon region d(H1).

High-performance liquid chromatographic assay for acetaminophen glucuronide in human liver microsomes

A rapid and specific high-performance liquid chromatographic assay was developed for the determination of acetaminophen glucuronide formed by human liver microsomes. In addition, incubation conditions were systematically evaluated. Conditions that yielded the optimal rate of acetaminophen glucuronide formation over various concentrations of acetaminophen (0.15-30 mM) consisted of the following: 0.1 M potassium phosphate buffer, 1 mM magnesium chloride, 30 microg/mg alamethicin, 4 mM uridine 5'-diphosphoglucuronic acid at a pH of 7.1. Alamethicin produced higher and more consistent APAPG formation rates compared to Brij-58. Adding saccharolactone to the incubation medium reduced the velocity of the reaction. Acetaminophen glucuronide, acetaminophen, and the internal standard (paraxanthine), were analyzed on a C18 column with UV detection at 250 nm. The mean correlation coefficient (r2) of the standard curves for acetaminophen glucuronide was >0.99 over the range of 0.1-25 nmol. The intra- and inter-day coefficients of variation were <4%. This method is suitable for in vitro studies using acetaminophen glucuronide formation as an index reaction for UGT activity.

Effects of peptide dimerization on pore formation: Antiparallel disulfide-dimerized magainin 2 analogue

To elucidate the effects of peptide dimerization on pore formation by magainin 2 (MG2), a covalently linked antiparallel dimer of the MG2 analogue [(F5Y, L6C, F16W, I20C-MG2)(2): II] was synthesized based on the dimer structure revealed by our NMR study. The interactions of the dimer with lipid bilayers were investigated by CD and fluorescence in comparison with a monomer analogue (F5Y, F16W-MG2: I). Similar to I, II was found to form a peptide-lipid supramolecular complex pore accompanied with lipid flip-flop and peptide translocation. The pore formed by II was characterized by a slightly larger pore diameter and a threefold longer lifetime than that of I, although the pore formation rate of the dimer was lower than that of the monomer. The coexistence of the dimer and the monomer exhibited slight but significant synergism in membrane permeabilization, which was maximal at a monomer/dimer ratio of 3. Therefore, we concluded that a pentameric pore composed of one pore-stabilizing dimer and three monomers maximized the overall leakage activity in keeping with our kinetic prediction.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.