Kinetics and Mechanism of the Recognition of Endotoxin by Polymyxin B

A molecular overview of the polymyxin-LPS interaction in the context of its mode of action and resistance development

With the rising incidences of antimicrobial resistance (AMR) and the diminishing options of novel antimicrobial agents, it is paramount to decipher the molecular mechanisms of action and the emergence of resistance to the existing drugs. Polymyxin, a cationic antimicrobial lipopeptide, is used to treat infections by Gram-negative bacterial pathogens as a last option. Though polymyxins were identified almost seventy years back, their use has been restricted owing to toxicity issues in humans. However, their clinical use has been increasing in recent times resulting in the rise of polymyxin resistance. Moreover, the detection of "mobile colistin resistance (mcr)" genes in the environment and their spread across the globe have complicated the scenario. The mechanism of polymyxin action and the development of resistance is not thoroughly understood. Specifically, the polymyxin-bacterial lipopolysaccharide (LPS) interaction is a challenging area of investigation. The use of advanced biophysical techniques and improvement in molecular dynamics simulation approaches have furthered our understanding of this interaction, which will help develop polymyxin analogs with better bactericidal effects and lesser toxicity in the future. In this review, we have delved deeper into the mechanisms of polymyxin-LPS interactions, highlighting several models proposed, and the mechanisms of polymyxin resistance development in some of the most critical Gram-negative pathogens.

Polymyxin B stabilized DNA micelles for sustained antibacterial and antibiofilm activity against P. aeruginosa

Nucleic acid-based materials showcase an increasing potential for antimicrobial drug delivery. Although numerous reports on drug-loaded DNA nanoparticles outline their pivotal antibacterial activities, their potential as drug delivery systems against bacterial biofilms awaits further studies. Among different oligonucleotide structures, micellar nanocarriers derived from amphiphilic DNA strands are of particular interest due to their spontaneous self-assembly and high biocompatibility. However, their clinical use is hampered by structural instability upon cation depletion. In this work, we used a cationic amphiphilic antibiotic (polymyxin B) to stabilize DNA micelles destined to penetrate P. aeruginosa biofilms and exhibit antibacterial/antibiofilm properties. Our study highlights how the strong affinity of this antibiotic enhances the stability of the micelles and confirms that antibacterial activity of the novel micelles remains intact. Additionally, we show that PMB micelles can penetrate P. aeruginosa biofilms and impact their metabolic activity. Finally, PMB micelles were highly safe and biocompatible, highlighting their possible application against P. aeruginosa biofilm-colonized skin wounds.

Lipid Microenvironment Modulates the Pore-Forming Ability of Polymyxin B

The ability of polymyxin B, an antibiotic used to treat infections caused by multidrug-resistant Gram-negative bacteria as a last-line therapeutic option, to form ion pores in model membranes composed of various phospholipids and lipopolysaccharides was studied. Our data demonstrate that polymyxin B predominantly interacts with negatively charged lipids. Susceptibility decreases as follows: Kdo2-Lipid A >> DOPG ≈ DOPS >> DPhPG ≈ TOCL ≈ Lipid A. The dimer and hexamer of polymyxin B are involved in the pore formation in DOPG(DOPS)- and Kdo2-Lipid A-enriched bilayers, respectively. The pore-forming ability of polymyxin B significantly depends on the shape of membrane lipids, which indicates that the antibiotic produces toroidal lipopeptide-lipid pores. Small amphiphilic molecules diminishing the membrane dipole potential and inducing positive curvature stress were shown to be agonists of pore formation by polymyxin B and might be used to develop innovative lipopeptide-based formulations.

Functionalized core-shell nanogel scavenger for immune modulation therapy in sepsis

Sepsis is a complex, life-threatening hyperinflammatory syndrome associated with organ failure and high mortality due to lack of effective treatment options. Here we report a core-shell hydrogel nanoparticle with the core functionalized with telodendrimer (TD) nanotrap (NT) to control hyperinflammation in sepsis. The combination of multi-valent charged and hydrophobic moieties in TD enables effective binding with biomolecules in NT. The higher crosslinking in the shell structure of nanogel excludes the abundant large serum proteins and allows for size-selectivity in scavenging the medium-sized septic molecules (10-30 kDa), e.g., lipopolysaccharides (LPS, a potent endotoxin in sepsis), thus reducing cytokine production. At the same time, the core-shell TD NT nanogel captures the over-flowing proinflammatory cytokines effectively both in vitro and in vivo from biological fluids to further control hyperinflammation. Intraperitoneal injection of core-shell TD NT nanogel effectively attenuates NF-κB activation and cytokine production in LPS-induced septic mouse models. These results indicate the potential applications of the injectable TD NT core-shell nanogel to attenuate local or systemic inflammation.

Recent advances in nanomedicine for sepsis treatment

Sepsis and septic shock are life-threating conditions, which form a continuum of the body's response to overwhelming infection. The current treatment consists of fluid and metabolic resuscitation, hemodynamic and end-organ support, and timely initiation of antibiotics. However, these measures may be ineffective and the sepsis-related mortality toll remains substantial; therefore, an urgent need exists for new therapies. Recently, several nanoparticle (NP) systems have shown excellent protective effects against sepsis in preclinical models, suggesting a potential utility in the management of sepsis and septic shock. These NPs serve as antibacterial agents, provide platforms to immobilize endotoxin adsorbents, interact with inflammatory cells to restore homeostasis and detect biomarkers of sepsis for timely diagnosis. This review discusses the recent developments in NP-based approaches for the treatment of sepsis.

A polymyxin B-silver nanoparticle colloidal system and the application of lipopolysaccharide analysis

Polymyxin B (PMB) is a small cyclic polycationic lipopeptide, which can be used as an antibiotic drug against Gram-negative bacteria. In this work, the interaction between citrate capped silver nanoparticles (AgNPs) and PMB is studied. Experimental results show that the association constant is extremely high and the binding event leads to a significant variation in the localized surface plasmon resonance (LSPR) of AgNPs. Moreover, the PMB-AgNP colloidal system can be further exploited as a sensitive analytical platform. Taking lipopolysaccharide (LPS) as an example, we demonstrate a facile colorimetric LPS detection method. LPS is a major constituent of the Gram-negative bacterial cell wall which is used as an optimal biomarker for some diseases like urinary tract infections and sepsis. In this study, target LPS is able to tightly bind to PMB, which effectively inhibits the PMB induced aggregation of AgNPs. By monitoring the UV-vis absorption spectra, the LPS concentration can be quantitatively determined with high sensitivity. The sensing strategy is quite simple, which allows effortless diagnosis of many diseases at different stages.

Antibacterial activity and safety of commercial veterinary cationic steroid antibiotics and neutral superoxidized water

Antibiotic resistance of bacteria common to the ocular surface is an evolving problem. Thus, novel treatment options with new modes of action are required. We investigated the antibacterial activity and safety of three commercially available topical veterinary ophthalmic products (cationic steroid antibiotics, products A and B, and a neutral superoxidized water, product C) to determine their potential use as antimicrobial alternatives. The minimum inhibitory concentrations (MIC) of the three products were determined against 17 antibiotic resistant bacterial clinical isolates from the ocular surface. Using a standard cytotoxicity assay, the products at varying concentrations were evaluated with a corneal fibroblast cell line and a macrophage-like cell line to determine their potential toxic effect in vitro. The commercial ophthalmic solutions, ofloxacin 0.3%, tobramycin 0.3% and gentamicin 0.3% were used as positive controls for the MIC and tobramycin 0.3% was used as positive control for the cytotoxicity assays. For the MIC, Product C showed no inhibition of growth for any organisms, while Products A and B showed inhibition of growth similar to slightly less than the positive controls. For the cytotoxicity assays, Product C exhibited minimal toxicity while Products A and B exhibited toxicity similar to the controls. In conclusion, Product C had no antibacterial activity in these assays, while Products A and B had antibacterial profiles similar to slightly less than common topical ophthalmic antibiotics and cytotoxicity profiles similar to common topical ophthalmic antibiotics. To our knowledge, this is the first report on the antibacterial activity and safety of the cationic steroid antibiotics and superoxidized water.

Domestic exposure to endotoxin and respiratory morbidity in former smokers with COPD

Indoor air pollution has been linked to adverse chronic obstructive pulmonary disease (COPD) health, but specific causative agents have not yet been identified. We evaluated the role of indoor endotoxin exposure upon respiratory health in former smokers with COPD. Eighty-four adults with moderate to severe COPD were followed longitudinally and indoor air and dust samples collected at baseline, 3 and 6 months. Respiratory outcomes were repeatedly assessed at each time point. The associations between endotoxin exposure in air and settled dust and health outcomes were explored using generalizing estimating equations in multivariate models accounting for confounders. Dust endotoxin concentrations in the main living area were highest in spring and lowest in fall, while airborne endotoxins remained steady across seasons. Airborne and dust endotoxin concentrations were weakly correlated with one another (rs  = +0.24, P = 0.005). Endotoxin concentrations were not significantly associated with respiratory symptoms, rescue medication use, quality of life, or severe exacerbations. In vitro whole-blood assays of the pro-inflammatory capacity of PM10 filters with and without endotoxin depletion demonstrated that the endotoxin component of indoor air pollution was not the primary trigger for interleukin-1β release. Our findings support that endotoxin is not the major driver in the adverse effects of indoor PM upon COPD morbidity.

A Molecularly Complete Planar Bacterial Outer Membrane Platform

The bacterial outer membrane (OM) is a barrier containing membrane proteins and liposaccharides that fulfill crucial functions for Gram-negative bacteria. With the advent of drug-resistant bacteria, it is necessary to understand the functional role of this membrane and its constituents to enable novel drug designs. Here we report a simple method to form an OM-like supported bilayer (OM-SB), which incorporates native lipids and membrane proteins of gram-negative bacteria from outer membrane vesicles (OMVs). We characterize the formation of OM-SBs using quartz crystal microbalance with dissipation (QCM-D) and fluorescence microscopy. We show that the orientation of proteins in the OM-SB matches the native bacterial membrane, preserving the characteristic asymmetry of these membranes. As a demonstration of the utility of the OM-SB platform, we quantitatively measure antibiotic interactions between OM-SBs and polymyxin B, a cationic peptide used to treat Gram-negative infections. This data enriches understanding of the antibacterial mechanism of polymyxin B, including disruption kinetics and changes in membrane mechanical properties. Combining OM-SBs with microfluidics will enable higher throughput screening of antibiotics. With a broader view, we envision that a molecularly complete membrane-scaffold could be useful for cell-free applications employing engineered membrane proteins in bacterial membranes for myriad technological purposes.

Review: Towards antibacterial strategies: studies on the mechanisms of interaction between antibacterial peptides and model membranes

Lipopolysaccharides (LPSs) play a dual role as inflammation-inducing and as membrane-forming molecules. The former role attracts significantly more attention from scientists, possibly because it is more closely related to sepsis and septic shock. This review aims to focus the reader's attention to the other role, the function of LPS as the major constituent of the outer layer of the outer membrane of Gram-negative bacteria, in particular those of enterobacterial strains. In this function, LPS is a necessary component of the cell envelope and guarantees survival of the bacterial organism. At the same time, it represents the first target for attacking molecules which may either be synthesized by the host's innate or adaptive immune system or administered to the human body. The interaction of these molecules with the outer membrane may not only directly cause the death of the bacterial organism, but may also lead to the release of LPS into the circulation. Here, we review membrane model systems and their application for the study of molecular mechanisms of interaction of peptides such as those of the human complement system, the bactericidal/permeability-increasing protein (BPI), cationic antibacterial peptide 18 kDa (CAP18) as an example of cathelicidins, defensins, and polymyxin B (PMB). Emphasis is on electrical measurements with a reconstitution system of the lipid matrix of the outer membrane which was established in the authors' laboratory as a planar asymmetric bilayer with one leaflet being composed solely of LPS and the other of the natural phospholipid mixture. The main conclusion, which can be drawn from these investigations, is that LPS and in general its negative charges are the dominant determinants for specific peptide—membrane interactions. However, the detailed mechanisms of interaction, which finally lead to bacterial killing, may involve further steps and differ for different antibacterial peptides.

Endotoxin removal: history of a mission

One of the key molecules involved in the pathogenesis of severe sepsis and septic shock is lipopolysaccharide (LPS) or endotoxin, which is a component of the cellular wall of Gram-negative bacteria. Clinical studies have shown that the level of circulating LPS is correlated with illness severity (APACHE II), the onset and amount of organ dysfunction (SOFA) and intensive care unit mortality. Many therapeutic strategies have attempted to neutralize the pathogenic activity of endotoxin in order to interrupt the progression of a septic state towards a worsened clinical framework, i.e. severe sepsis of septic shock. Over the past decades the role of extracorporeal hemoperfusion by means of polymyxin B-based cartridges (PMX-DHP) to bind and neutralize LPS from whole blood has increased in clinical relevance. This is due to an increasing number of studies confirming that a directed therapy of endotoxic shock could significantly influence the course of the septic cascade. This review will outline the meaning of the targeted approach to endotoxin, both highlighting the specific immunologic effect of endotoxin removal by polymyxin B and the evidence of clinical improvements following this kind of therapy in terms of recovery of organ function.

Endotoxin removal devices for the treatment of sepsis and septic shock

A substantial body of experimental and clinical evidence suggests that neutralising or removing lipopolysaccharide endotoxin would be an effective adjunctive approach to the management of Gram-negative sepsis. Polymyxins are a group of cyclic cationic polypeptide antibiotics. Although they have useful antimicrobial activity against Gram-negative bacteria, their clinical use has been limited because of toxicity. However, in addition to their antimicrobial property, polymyxins can bind to and neutralise endotoxin. Thus, investigators have explored the possibility of using polymyxin bound to a solid-phase carrier for specific haem-adsorption in patients with sepsis, thereby retaining the lipopolysaccharide-binding properties but minimising systemic toxic effects. This system has been widely used in Japan for many years, but convincing clinical evidence of efficacy is lacking. A recent Italian study has some promising data. Although polymyxin has been the principal agent used to explore this approach, other molecules have the ability to bind endotoxin, and some of these have very recently been proposed as the basis for other endotoxin-removal devices. The available evidence is reviewed to assess the potential use of such devices in clinical practice.

A specific interaction of small molecule entry inhibitors with the envelope glycoprotein complex of the Junín hemorrhagic fever arenavirus

Arenaviruses are responsible for acute hemorrhagic fevers worldwide and are recognized to pose significant threats to public health and biodefense. Small molecule compounds have recently been discovered that inhibit arenavirus entry and protect against lethal infection in animal models. These chemically distinct inhibitors act on the tripartite envelope glycoprotein (GPC) through its unusual stable signal peptide subunit to stabilize the complex against pH-induced activation of membrane fusion in the endosome. Here, we report the production and characterization of the intact transmembrane GPC complex of Junín arenavirus and its interaction with these inhibitors. The solubilized GPC is antigenically indistinguishable from the native protein and forms a homogeneous trimer in solution. When reconstituted into a lipid bilayer, the purified complex interacts specifically with its cell-surface receptor transferrin receptor-1. We show that small molecule entry inhibitors specific to New World or Old World arenaviruses bind to the membrane-associated GPC complex in accordance with their respective species selectivities and with dissociation constants comparable with concentrations that inhibit GPC-mediated membrane fusion. Furthermore, competitive binding studies reveal that these chemically distinct inhibitors share a common binding pocket on GPC. In conjunction with previous genetic studies, these findings identify the pH-sensing interface of GPC as a highly vulnerable target for antiviral intervention. This work expands our mechanistic understanding of arenavirus entry and provides a foundation to guide the development of small molecule compounds for the treatment of arenavirus hemorrhagic fevers.

Structure-function studies of Bubalus bubalis lingual antimicrobial peptide analogs

Antimicrobial peptides expressed on different epithelial lining are major components of the innate immune system. Based on the deduced amino acid sequence of Bubalus bubalis lingual antimicrobial peptide (LAP) cDNA (Accession No. DQ458768), five overlapping peptides LAP(23-55), LAP(42-64), LAP(21-64), LAP(1-26) and LAP(1-64) were synthesized using solid phase fluorenylmethoxycarbonyl (Fmoc) chemistry. Circular Dichroism spectroscopy of synthesized peptides revealed predominantly beta-structure for LAP(23-55,) LAP(42-64) and LAP(21-64) with less alpha-helix in different solutions. Quantitation of secondary structure indicated the highest beta-structure for all these three peptides in membrane mimetic SDS solution. The helicogenic solvent TFE could not induce helix in LAP(23-55) however TFE induced helical propensity was observed in LAP(42-64) and LAP(21-64). The quantitation of secondary structure indicated the highest ordered structure for LAP(23-55) followed by LAP(42-64) and LAP(21-64). The antibacterial activity of LAP(23-55) was found to be more potent against Staphylococcus aureus, Listeria monocytogens, Escherichia coli and Salmonella typhimurium followed by LAP(42-64) and LAP(21-64). Minimum inhibitory concentration (MIC) also showed similar trend with lowest value for LAP(23-55) followed by LAP(42-64) and LAP(21-64). Haemolysis and cytotoxicity was observed above 3 fold for LAP(21-64,) above six fold for LAP(23-55) and LAP(42-64) of their MIC. The LAP(1-26) and LAP(1-64) could not produce any characteristic CD spectra and did not show any antimicrobial activity, indicating that N- terminal of the peptide negates the antimicrobial activity.

Alexidine and chlorhexidine bind to lipopolysaccharide and lipoteichoic acid and prevent cell activation by antibiotics

OBJECTIVES

Many antibiotics used to treat infections cause release of immunostimulatory cell wall components from bacteria. Therefore, a combination of antimicrobial and endotoxin-neutralizing activity is desired to prevent inflammation induced by destroyed bacteria. Chlorhexidine and alexidine are amphipathic bisbiguanides and could neutralize bacterial membrane components as stimulators of Toll-like receptors (TLRs).

METHODS

Binding of chlorhexidine and alexidine to lipopolysaccharide (LPS) and lipoteichoic acid (LTA) was determined by fluorescence displacement assay and isothermal calorimetric titration. Neutralization of the biological effect of LPS and LTA on TLR-activated cellular activation was determined by NF-kappaB reporter luciferase activation on cells transfected with specific TLRs and NO production of murine macrophages in the presence of isolated agonists and antibiotic-treated bacteria.

RESULTS

Alexidine and chlorhexidine bind not only to LPS but also to LTA from Gram-positive bacteria. Alexidine has a higher affinity than chlorhexidine for both compounds. Calorimetric titration shows an initial endothermic contribution indicating participation of hydrophobic interactions in LPS binding, while binding to LTA displayed initial exothermic contribution. Both compounds prevent cell activation of TLR4 and TLR2 by LPS and LTA, respectively. The addition of both compounds suppressed NO production by macrophages in the presence of bacteria treated with different types of antibiotics.

CONCLUSIONS

Chlorhexidine and alexidine suppress bacterial membrane-induced cell activation at concentrations two orders of magnitude lower than that used in topical applications. Combining biocides with different types of antibiotics prevented macrophage activation in the presence of bacteria and demonstrated the potential of chlorhexidine and alexidine to suppress inflammatory responses caused by activation of TLRs.

Endotoxin detection in a competitive electrochemical assay: Synthesis of a suitable endotoxin conjugate

A biotin-lipopolysaccharide (biotin-LPS) conjugate was synthesized from LPS smooth from Salmonella minnesota, yielding a conjugate with a biotin/LPS ratio equal to 1:1 and endotoxic activity of 0.08 EU ng(-1). The conjugate was used in an amperometric competitive assay to determine endotoxins with endotoxin-neutralizing protein (ENP) as the recognition element. The assay is performed on a modified electrode, involving the covalent binding of carboxymethyl dextran (CMDex) to a cystamine-modified gold electrode and then the covalent binding of the recognition protein, ENP, to CMDex. The assay is carried out by incubating the modified electrode in an LPS sample to which biotin-LPS was added. Both species compete for the recognition sites on the modified surface. After the incubation stage and a careful rinsing, the electrode is immersed in a solution containing neutravidin-horseradish peroxidase conjugate (N-HRP), which binds to the sites containing biotin-LPS on the electrode. The system is rinsed and a current signal is generated by the addition of hydrogen peroxide and a redox mediator. The assay is able to detect LPS from Salmonella minnesota at concentrations as low as 0.1 ng ml(-1), equivalent to 0.07 EU ml(-1).

Polymyxin B: an ode to an old antidote for endotoxic shock

Endotoxic shock, a syndrome characterized by deranged hemodynamics, coagulation abnormalities, and multiple system organ failure is caused by the release into the circulation of lipopolysaccharide (LPS), the structurally diverse component of Gram-negative bacterial outer membranes, and is responsible for 60% mortality in humans. Polymyxin B (PMB), a cyclic, cationic peptide antibiotic, neutralizes endotoxin but induces severe side effects in the process. The potent endotoxin neutralizing ability of PMB, however, offers possibilities for designing non-toxic therapeutic agents for combating endotoxicosis. Amongst the numerous approaches for combating endotoxic shock, peptide mediated neutralization of LPS seems to be the most attractive one. The precise mode of binding of PMB to LPS and the structural features involved therein have been elucidated only recently using a variety of biophysical approaches. These suggest that efficient neutralization of endotoxin by PMB is not achieved by mere binding to LPS but requires its sequestration from the membrane. Incorporation of this feature into the design of endotoxin neutralizing peptides should lead to the development of effective antidotes for endotoxic shock.

The antimicrobial antiproteinase elafin binds to lipopolysaccharide and modulates macrophage responses

Lipopolysaccharides (LPS) of the outer membrane of Gram-negative bacteria represent a primary target for innate immune responses. We demonstrate here that the antimicrobial/anti-neutrophil elastase full-length elafin (FL-EL) is able to bind both smooth and rough forms of LPS. The N-terminus was shown to bind both forms of LPS more avidly. We demonstrate that the lipid A core-binding proteins polymyxin B (PB) and LPS-binding protein (LBP) compete with elafin for binding, and that LBP is able to displace prebound elafin from LPS. When PB, FL-EL, N-EL, and C-EL were pre-incubated with LPS before addition to immobilized LBP, PB was the most potent inhibitor of LPS transfer to LBP. These data prompted us to examine the biological consequences of elafin binding to LPS, using tumor necrosis factor (TNF)-alpha release by murine macrophages. In serum-containing conditions, N-EL had no effect, whereas both C-EL and FL-EL inhibited TNF-alpha production. In serum-free conditions, however, all moieties had a stimulatory activity on TNF-alpha release, with C-EL being the most potent at the highest concentration. The differential biological activity of elafin in different conditions suggests a role for this molecule in either LPS detoxification or activation of innate immune responses, depending on the external cellular environment.

Kinetic and structural analysis of the increased affinity of enoyl-ACP (acyl-carrier protein) reductase for triclosan in the presence of NAD+

The binding of enoyl-ACP (acyl-carrier protein) reductase from Plasmodium falciparum (PfENR) with its substrates and inhibitors has been analysed by SPR (surface plasmon resonance). The binding of the substrate analogue crotonoyl-CoA and coenzyme NADH to PfENR was monitored in real time by observing changes in response units. The binding constants determined for crotonoyl-CoA and NADH were 1.6x10(4) M(-1) and 1.9x10(4) M(-1) respectively. Triclosan, which has recently been demonstrated as a potent antimalarial agent, bound to the enzyme with a binding constant of 1.08x10(5) M(-1). However, there was a 300-fold increase in the binding constant in the presence of NAD+. The increase in the binding constant was due to a 17 times increase in the association rate constant (k(1)) from 741 M(-1) x s(-1) to 1.3x10(4) M(-1) x s(-1) and a 16 times decrease in the dissociation rate constant (k(-1)) from 6.84x10(-3) s(-1) to 4.2x10(-4) s(-1). These values are in agreement with those determined by steady-state kinetic analysis of the inhibition reaction [Kapoor, Reddy, Krishnasastry, N. Surolia and A. Surolia (2004) Biochem. J. 381, 719-724]. In SPR experiments, the binding of NAD+ to PfENR was not detected. However, a binding constant of 6.5x10(4) M(-1) was obtained in the presence of triclosan. Further support for these observations was provided by the crystal structures of the binary and ternary complexes of PfENR. Thus the dramatic enhancement in the binding affinity of both triclosan and NAD+ in the ternary complex can be explained by increased van der Waals contacts in the ternary complex, facilitated by the movement of residues 318-324 of the substrate-binding loop and the nicotinamide ring of NAD+. Interestingly, the results of the present study also provide a rationale for the increased affinity of NAD+ for the enzyme in the ternary complex.

Synthesis of lipid A derivatives and their interactions with polymyxin B and polymyxin B nonapeptide

Lipid A is the causative agent of Gram-negative sepsis, a leading cause of mortality among hospitalized patients. Compounds that bind lipid A can limit its detrimental effects. Polymyxin B, a cationic peptide antibiotic, is one of the simplest molecules capable of selectively binding lipid A and may serve as a model for further development of lipid A binding agents. However, association of polymyxin B with lipid A is not fully understood, primarily due to the low solubility of lipid A in water and inhomogeneity of lipid A preparations. To better understand lipid A-polymyxin B interaction, pure lipid A derivatives were prepared with incrementally varied lipid chain lengths. These compounds proved to be more soluble in water than lipid A, with higher aggregation concentrations. Isothermal titration calorimetric studies of these lipid A derivatives with polymyxin B and polymyxin B nonapeptide indicate that binding stoichiometries (peptide to lipid A derivative) are less than 1 and that affinities of these binding partners correlate with the aggregation states of the lipid A derivatives. These studies also suggest that cooperative ionic interactions dominate association of polymyxin B and polymyxin B nonapeptide with lipid A.

Plasticity in structure and interactions is critical for the action of indolicidin, an antibacterial peptide of innate immune origin

The comparative analysis of two cationic antibacterial peptides of the cathelicidin family-indolicidin and tritrypticin-enabled addressing the structural features critical for the mechanism of indolicidin activity. Functional behavior of retro-indolicidin was found to be identical to that of native indolicidin. It is apparent that the gross conformational propensities associated with retro-peptides resemble those of the native sequences, suggesting that native and retro-peptides can have similar structures. Both the native and the retro-indolicidin show identical affinities while binding to endotoxin, the initial event associated with the antibacterial activity of cationic peptide antibiotics. The indolicidin-endotoxin binding was modeled by docking the indolicidin molecule in the endotoxin structure. The conformational flexibility associated with the indolicidin residues, as well as that of the fatty acid chains of endotoxin combined with the relatively strong structural interactions, such as ionic and hydrophobic, provide the basis for the endotoxin-peptide recognition. Thus, the key feature of the recognition between the cationic antibacterial peptides and endotoxin is the plasticity of molecular interactions, which may have been designed for the purpose of maintaining activity against a broad range of organisms, a hallmark of primitive host defense.

Thermodynamic and kinetic analysis of porphyrin binding to Trichosanthes cucumerina seed lectin

The interaction of several metallo-porphyrins with the galactose-specific lectin from Trichosanthes cucumeirna (TCSL) has been investigated. Difference absorption spectroscopy revealed that significant changes occur in the Soret band region of the porphyrins upon binding to TCSL and these changes have been monitored to obtain association constants (Ka) and stoichiometry of binding (n). The dimeric lectin binds two porphyrin molecules and the presence of the specific saccharide lactose did not affect porphyrin binding significantly, indicating that the sugar and the porphyrin bind at different sites. The Ka values obtained for the binding of different porphyrins with TCSL at 25 degrees C were in the range of 2 x 10(3)-5 x 10(5) m(-1). Association constants for meso-tetra(4-sulphonatophenyl)porphyrinato copper(II) (CuTPPS), a porphyrin bearing four negative charges and meso-tetra(4-methylpyridinium)porphyrinato copper(II) (CuTMPyP), a porphyrin with four positive charges, were determined at several temperatures; from the temperature dependence of the association constants, the thermodynamic parameters change in enthalpy (DeltaH degrees ) and change in entropy (DeltaS degrees ) associated with the binding process were estimated. The thermodynamic data indicate that porphyrin binding to TCSL is driven largely by a favourable entropic contribution; the enthalpic contribution is very small, suggesting that the binding process is governed primarily by hydrophobic forces. Stopped-flow spectroscopic measurements show that binding of CuTMPyP to TCSL takes place by a single-step process and at 20 degrees C, the association and dissociation rate constants were 1.89 x 10(4) m(-1).s(-1) and 0.29 s(-1), respectively.

Kinetic and thermodynamic analysis of the interactions of 23-residue peptides with endotoxin

Many naturally occurring peptides exhibit lipopolysaccharide binding properties. In this work we describe the endotoxin binding properties of a series of 23-residue peptides based on the sequence corresponding to the antisense strand of the magainin gene. Biochemical and biophysical characterization of these peptides reveals that they have the tendency to perturb both the inner and outer membranes of test pathogens. Structurally these peptides are amphiphilic and adopt helical conformations in membranes. Three of the seven peptides tested have high affinities for endotoxin that approach the values shown by polymyxin B, a cyclic cationic acylated decapeptide, which is used clinically in treating extreme cases of sepsis. The kinetic parameters obtained using stopped-flow methods and BIAcore analysis, when considered in conjunction with the isothermal titration calorimetry-derived thermodynamic parameters, allow us to highlight the key structural features essential for lipopolysaccharide (LPS) recognition by these peptides. The studies stress the role of ionic forces in the initial recognition of LPS. The fortification of the strength of these ionic charges increases affinity for LPS, whereas the hydrophobic residues involved in interactions are more amenable to disruptions in contiguity. Peptides that improve these features further are expected to perform better as endotoxin-neutralizing agents.

Stages of polymyxin B interaction with the Escherichia coli cell envelope

The effects of polymyxin B (PMB) on the Escherichia coli outer (OM) and cytoplasmic membrane (CM) permeabilities were studied by monitoring the fluxes of tetraphenylphosphonium, phenyldicarbaundecaborane, and K(+) and H(+) ions. At concentrations between 2 and 20 microgram/ml, PMB increased the OM permeability to lipophilic compounds and induced a leakage of K(+) from the cytosol and an accumulation of lipophilic anions in the cellular membranes but did not cause the depolarization of the CM. At higher concentrations, PMB depolarized the CM, forming ion-permeable pores in the cell envelope. The permeability characteristics of PMB-induced pores mimic those of bacteriophage- and/or bacteriocin-induced channels. However, the bactericidal effect of PMB took place at concentrations below 20 microgram/ml, indicating that this effect is not caused by pore formation. Under conditions of increased ionic strength, PMB made the OM permeable to lipophilic compounds and decreased the K(+) gradient but was not able to depolarize the cells. The OM-permeabilizing effect of PMB can be diminished by increasing the concentration of Mg(2+). The major new findings of this work are as follows: (i) the OM-permeabilizing action of PMB was dissected from its depolarizing effect on the CM, (ii) the PMB-induced ion-permeable pores in bacterial envelope were registered, and (iii) the pore formation and depolarization of the CM are not obligatory for the bactericidal action of PMB and dissipation of the K(+) gradient on the CM.

B cell responses to a peptide epitope. X. Epitope selection in a primary response is thermodynamically regulated

We examine the etiological basis of hierarchical immunodominance of B cell epitopes on a multideterminant Ag. A model T-dependent immunogen, containing a single immunodominant B cell epitope, was used. The primary IgM response to this peptide included Abs directed against diverse determinants presented by the peptide. Interestingly, affinity of individual monomeric IgM Abs segregated around epitope recognized and was independent of their clonal origins. Furthermore, affinity of Abs directed against the immunodominant epitope were markedly higher than that of the alternate specificities. These studies suggested that the affinity of an epitope-specific primary response, and variations therein, may be determined by the chemical composition of epitope. This inference was supported by thermodynamic analyses of monomer IgM binding to Ag, which revealed that this interaction occurs at the expense of unfavorable entropy changes. Permissible binding required compensation by net enthalpic changes. Finally, the correlation between chemical composition of an epitope, the resultant affinity of the early primary humoral response, and its eventual influence on relative immunogenicity could be experimentally verified. This was achieved by examining the effect of various amino-terminal substitutions on immunogenicity of a, hitherto cryptic, amino-terminal determinant. Such experiments permitted delineation of a hierarchy of individual amino acid residues based on their influence; which correlated well with calculated Gibbs-free energy changes that individual residue side chains were expected to contribute in a binding interaction. Thus, maturation of a T-dependent humoral response is initiated by a step that is under thermodynamic control.

Polymyxin B binds to anandamide and inhibits its cytotoxic effect

Anandamide (ANA), an endogenous cannabinoid, can be generated by activated macrophages during endotoxin shock and is thought to be a paracrine contributor to hypotension. We discovered that ANA in saline/ethanol solution and in serum was efficiently adsorbed in a polymyxin B (PMB)-immobilized beads column and eluted with ethanol. We confirmed the direct binding of PMB to ANA by using surface plasmon resonance. The adsorption of ANA by PMB may abolish the diverse effects of ANA such as hypotension, immunosuppression, and cytotoxicity, and may suggest a new therapeutic strategy for endotoxin shock.

Lipid-induced conformation and lipid-binding properties of cytolytic and antimicrobial peptides: determination and biological specificity

While antimicrobial and cytolytic peptides exert their effects on cells largely by interacting with the lipid bilayers of their membranes, the influence of the cell membrane lipid composition on the specificity of these peptides towards a given organism is not yet understood. The lack of experimental model systems that mimic the complexity of natural cell membranes has hampered efforts to establish a direct correlation between the induced conformation of these peptides upon binding to cell membranes and their biological specificities. Nevertheless, studies using model membranes reconstituted from lipids and a few membrane-associated proteins, combined with spectroscopic techniques (i.e. circular dichroism, fluorescence spectroscopy, Fourier transform infra red spectroscopy, etc.), have provided information on specific structure-function relationships of peptide-membrane interactions at the molecular level. Reversed phase-high performance chromatography (RP-HPLC) and surface plasmon resonance (SPR) are emerging techniques for the study of the dynamics of the interactions between cytolytic and antimicrobial peptides and lipid surfaces. Thus, the immobilization of lipid moieties onto RP-HPLC sorbent now allows the investigation of peptide conformational transition upon interaction with membrane surfaces, while SPR allows the observation of the time course of peptide binding to membrane surfaces. Such studies have clearly demonstrated the complexity of peptide-membrane interactions in terms of the mutual changes in peptide binding, conformation, orientation, and lipid organization, and have, to a certain extent, allowed correlations to be drawn between peptide conformational properties and lytic activity.

Surface plasmon resonance studies resolve the enigmatic endotoxin neutralizing activity of polymyxin B

Polymyxin B (PMB), a cyclic cationic peptide antibiotic, despite its severe side effects continues to occupy a premiere position for treating endotoxicosis. Its mode of neutralization of endotoxin has remained elusive for the last three decades. Several synthetic peptide mimics of PMB, capable of binding endotoxin, have been made. However, the binding ability alone appears to be a deceptive indicator of endotoxin neutralizing activity as molecules with similar binding propensities could either sequester or opsonize the toxin. Hence identification of additional physical parameters which describe adequately the outcome of PMB-endotoxin interaction become imperative. Surface plasmon resonance (SPR) studies reported here show that several mimics of PMB despite exhibiting lipopolysaccharide binding affinities comparable with it but, unlike it, do not sequester the endotoxin. These studies thus provide a striking illustration of the difference in the behavior of PMB, vis a vis its mimics toward the endotoxin lamellae, and define further, in chemical terms, mechanism of the action of PMB and allow us to posit that the design of molecules as effective antidotes for sepsis should incorporate the ability to sequester endotoxin specifically.

Kinetics of the interaction of endotoxin with polymyxin B and its analogs: a surface plasmon resonance analysis

Lipopolysaccharide, the invariant structural component of Gram-negative bacteria, when present in minute amounts in the circulation in humans elicits 'endotoxic shock' syndrome, which is fatal in 60% of the cases. Polymyxin B (PMB), a cyclic cationic peptide, neutralizes the endotoxin, but also induces many harmful side effects. Many peptide-based drugs mimicking the activity of PMB have been synthesized in an attempt to reduce toxicity while still retaining the anti-endotoxic activity. The study attempts to use the recent technique of surface plasmon resonance (SPR), in determining the kinetics of association and dissociation involved in the interaction of endotoxin with a few selected peptides that have structural features resembling PMB. The results, in conjunction with the thermodynamic data derived using isothermal titration calorimetry (ITC), stress the vital role played by amphiphilicity of the peptides and hydrophobic forces in this biologically important interaction.