Purification of gramicidin A

Membrane-mediated action of the endocannabinoid anandamide on membrane proteins: implications for understanding the receptor-independent mechanism

Endocannabinoids are amphiphilic molecules that play crucial neurophysiological functions acting as lipid messengers. Antagonists and knockdown of the classical CB1 and CB2 cannabinoid receptors do not completely abolish many endocannabinoid activities, supporting the idea of a mechanism independent of receptors whose mode of action remains unclear. Here we combine gramicidin A (gA) single channel recordings and membrane capacitance measurements to investigate the lipid bilayer-modifying activity of endocannabinoids. Single channel recordings show that the incorporation of endocannabinoids into lipid bilayers reduces the free energy necessary for gramicidin channels to transit from the monomeric to the dimeric conformation. Membrane capacitance demonstrates that the endocannabinoid anandamide has limited effects on the overall structure of the lipid bilayers. Our results associated with the theory of membrane elastic deformation reveal that the action of endocannabinoids on membrane proteins can involve local adjustments of the lipid/protein hydrophobic interface. The current findings shed new light on the receptor-independent mode of action of endocannabinoids on membrane proteins, with important implications towards their neurobiological function.

Temporizin and Temporizin-1 Peptides as Novel Candidates for Eliminating Trypanosoma cruzi

Tropical diseases caused by parasitic infections continue to cause socioeconomic distress worldwide. Among these, Chagas disease has become a great concern because of globalization. Caused by Trypanosoma cruzi, there is an increasing need to discover new, more effective methods to manage infections that minimize disease onset. Antimicrobial peptides represent a possible solution to this challenge. As effector molecules of the innate immune response against pathogens, they are the first line of defense found in all multi-cellular organisms. In amphibians, temporins are a large family of antimicrobial peptides found in skin secretions. Their functional roles and modes of action present unique properties that indicate possible candidates for therapeutic applications. Here, we investigated the trypanocide activity of temporizin and temporizin-1. Temporizin is an artificial, hybrid peptide containing the N-terminal region of temporin A, the pore-forming region of gramicidin and a C-terminus consisting of alternating leucine and lysine. Temporizin-1 is a modification of temporizin with a reduction in the region responsible for insertion into membranes. Their activities were evaluated in a cell permeabilization assay by flow cytometry, an LDH release assay, electron microscopy, an MTT assay and patch clamp experiments. Both temporizin and temporizin-1 demonstrated toxicity against T. cruzi with temporizin displaying slightly more potency. At concentrations up to 100 μg/ ml, both peptides exhibited low toxicity in J774 cells, a macrophage lineage cell line, and no toxicity was observed in mouse primary peritoneal macrophages. In contrast, the peptides showed some toxicity in rat adenoma GH3 cells and Jurkat human lymphoma cells with temporizin-1 displaying lower toxicity. In summary, a shortened form of the hybrid temporizin peptide, temporizin-1, was efficient at killing T. cruzi and it has low toxicity in wild-type mammalian cells. These data suggest that temporizin-1 might be a candidate for Chagas disease therapy.

A model for the interfacial kinetics of phospholipase D activity on long-chain lipids

The membrane-active enzyme phospholipase D (PLD) catalyzes the hydrolysis of the phosphodiester bond in phospholipids and plays a critical role in cell signaling. This catalytic reaction proceeds on lipid-water interfaces and is an example of heterogeneous catalysis in biology. Recently we showed that planar lipid bilayers, a previously unexplored model membrane for these kinetic studies, can be used for monitoring interfacial catalytic reactions under well-defined experimental conditions with chemical and electrical access to both sides of the lipid membrane. Employing an assay that relies on the conductance of the pore-forming peptide gramicidin A to monitor PLD activity, the work presented here reveals the kinetics of hydrolysis of long-chain phosphatidylcholine lipids in situ. We have developed an extension of a basic kinetic model for interfacial catalysis that includes product activation and substrate depletion. This model describes the kinetic behavior very well and reveals two kinetic parameters, the specificity constant and the interfacial quality constant. This approach results in a simple and general model to account for product accumulation in interfacial enzyme kinetics.

Gramicidin pores report the activity of membrane-active enzymes

Phospholipases constitute a ubiquitous class of membrane-active enzymes that play a key role in cellular signaling, proliferation, and membrane trafficking. Aberrant phospholipase activity is implicated in a range of diseases including cancer, inflammation, and myocardial disease. Characterization of these enzymes is therefore important, both for improving the understanding of phospholipase catalysis and for accelerating pharmaceutical and biotechnological applications. This paper describes a novel approach to monitor, in situ and in real-time, the activity of phospholipase D (PLD) and phospholipase C (PLC) on planar lipid bilayers. This method is based on lipase-induced changes in the electrical charge of lipid bilayers and on the concomitant change in ion concentration near lipid membranes. The approach reports these changes in local ion concentration by a measurable change in the single channel ion conductance through pores of the ion channel-forming peptide gramicidin A. This enzyme assay takes advantage of the amplification characteristics of gramicidin pores to sense the activity of picomolar to nanomolar concentrations of membrane-active enzymes without requiring labeled substrates or products. The resulting method proceeds on lipid bilayers without the need for detergents, quantifies enzyme activity on native lipid substrates within minutes, and provides unique access to both leaflets of well-defined lipid bilayers; this method also makes it possible to generate planar lipid bilayers with transverse lipid asymmetry.

A semi-synthetic ion channel platform for detection of phosphatase and protease activity

Sensitive methods to probe the activity of enzymes are important for clinical assays and for elucidating the role of these proteins in complex biochemical networks. This paper describes a semi-synthetic ion channel platform for detecting the activity of two different classes of enzymes with high sensitivity. In the first case, this method uses single ion channel conductance measurements to follow the enzyme-catalyzed hydrolysis of a phosphate group attached to the C-terminus of gramicidin A (gA, an ion channel-forming peptide) in the presence of alkaline phosphatase (AP). Enzymatic hydrolysis of this phosphate group removes negative charges from the entrance of the gA pore, resulting in a product with measurably reduced single ion channel conductance compared to the original gA-phosphate substrate. This technique employs a standard, commercial bilayer setup and takes advantage of the catalytic turnover of enzymes and the amplification characteristics of ion flux through individual gA pores to detect picomolar concentrations of active AP in solution. Furthermore, this technique makes it possible to study the kinetics of an enzyme and provides an estimate for the observed rate constant (k(cat)) and the Michaelis constant (K(M)) by following the conversion of the gA-phosphate substrate to product over time in the presence of different concentrations of AP. In the second case, modification of gA with a substrate for proteolytic cleavage by anthrax lethal factor (LF) afforded a sensitive method for detection of LF activity, illustrating the utility of ion channel-based sensing for detection of a potential biowarfare agent. This ion channel-based platform represents a powerful, novel approach to monitor the activity of femtomoles to picomoles of two different classes of enzymes in solution. Furthermore, this platform has the potential for realizing miniaturized, cost-effective bioanalytical assays that complement currently established assays.

Designing nanosensors based on charged derivatives of gramicidin A

Detection of chemical processes on a single molecule scale is the ultimate goal of sensitive analytical assays. We recently reported the possibility to detect chemical modifications on individual molecules by monitoring a change in the single ion channel conductance of derivatives of gramicidin A (gA) upon reaction with analytes in solution. These peptide-based nanosensors detect reaction-induced changes in the charge of gA derivatives that were engineered to carry specific functional groups near their C-terminus.1 Here, we discuss five key design parameters to optimize the performance of such chemomodulated ion channel sensors. In order to realize an effective sensor that measures changes in charge of groups attached to the C-terminus of a gA pore, the following conditions should be fulfilled: (1) the change in charge should occur as close to the entrance of the pore as possible; (2) the charge before and after reaction should be well-defined within the operational pH range; (3) the ionic strength of the recording buffer should be as low as possible while maintaining a detectable flow of ions through the pore; (4) the applied transmembrane voltage should be as high as possible while maintaining a stable membrane; (5) the lipids in the supporting membrane should either be zwitterionic or charged differently than the derivative of gA. We show that under the condition of high applied transmembrane potential (>100 mV) and low ionic strength of the recording buffer (< or =0.10 M), a change in charge at the entrance of the pore is the dominant requirement to distinguish between two differently charged derivatives of gA; the conductance of the heterodimeric gA pore reported here does not depend on a difference in charge at the exit of the pore. We provide a simple explanation for this asymmetric characteristic based on charge-induced local changes in the concentration of cations near the lipid bilayer membrane. Charge-based ion channel sensors offer tremendous potential for ultrasensitive functional detection since a single chemical modification of each individual sensing element can lead to readily detectable changes in channel conductance.

Direct analysis of gramicidin double helices using packed column supercritical fluid chromatography

Direct analysis of the monomeric and four double helical dimeric conformations of gramicidin has been achieved using packed column supercritical fluid chromatography (pSFC). Using a PRP-1 polymeric column and typical conditions of 40 degrees C column temperature, 25 MPa column pressure, and 35% n-pentanol modifier addition, all of the gramicidin conformers were readily separated. To evaluate the method, the dynamic characteristics of the monomer and dimer species were monitored as a function of solvent type, incubation time, solvent temperature, and initial concentration. The findings agree with those previously obtained by other methods but also yield new information about the relative amounts of two closely related dimers (species 1 and 2) as well as the simultaneous changes in the full dimer/monomer distribution. Results indicate that the developed pSFC method can be an informative complimentary tool for readily monitoring changes in the full profile of gramicidin species present in different environments.

Separation of linear gramicidins using carbon dioxide-containing mobile phases

Packed-column supercritical-fluid chromatography (pSFC) is presented as a novel method for separating and analyzing gramicidin samples. By use of methanol-modified carbon dioxide as a mobile phase the pentadecapeptides gramicidin A (gA), gramicidin B (gB), and gramicidin C (gC) are readily separated and eluted from a PRP-1 poly(styrene-divinylbenzene) column. Although optimum separation conditions are typically achieved near a column temperature of 40 degrees C, a column pressure of 11 MPa, and 30% methanol modifier, pressure and modifier gradients around these values are also found to improve the overall separation time. Measurements indicate that the mobile phase solubility of gramicidin under these conditions is 5.0+/-0.4 microg mL(-1). Collection of individual peaks during chromatography achieved analytical-scale isolation of 2 microg refined gC from 20 microg injected gramicidin D. Further, supercritical-fluid extraction of 200 microg gramicidin D from a Chromosorb 102 support packed into the vessel produced 57 microg gA in 90% purity. The results establish that carbon dioxide-based mobile phases can be successfully used for the separation of individual gramicidin species.

Preparative protein purification on underivatized silica

This article describes the use of underivatized silica gel as a preparative stationary phase for process purification of proteins. Although silica has been frequently used as a stationary phase backbone matrix, direct adsorption of proteins on underivatized silica has not been widely exploited for industrial applications. In this study an effort was made to fundamentally understand the interaction mechanisms between a protein and silica surface by using several proteins with a wide range of isoelectric points (pIs) and surface hydrophobicity. Interactions in silica were found to be largely dominated by a combination of ionic and hydrophobic forces. Accordingly, a predictive model was derived for describing linear retention of proteins on silica. Finally, a case study is described investigating the role of silica in an industrial purification process. It was found that the integration of the two modes of interaction confers silica with a unique selectivity that can be very effectively utilized in downstream bioprocessing.

Detector for Liquid Chromatography Based on Acoustic Emissions from an Oscillating Flame

The acoustic flame detector (AFD) is examined as a novel detector for liquid chromatography (LC). It is based upon the acoustic emission frequency of an oscillating hydrogen/oxygen premixed flame and produces a universal response toward organic molecules. A stable frequency near 1000 Hz, which further depends on mobile-phase composition, is achieved for flow rates in the microliter per minute range. The mass flow sensitivity of the AFD demonstrates a linear response over 3 orders of magnitude and a detection limit (S/sigma = 3) of approximately 15 ng of C/s for a series of alcohols. For cyclopentanol, this amounts to an injected mass of approximately 77 ng based on a 0.5-microL injection of a 196 ppm solution in methanol (flow rate 20 microL/min methanol; peak width 30 s). Similar sensitivity is observed using a water mobile phase. Low-frequency (1/f ) noise contributions are dominant with or without mobile phase present. The AFD demonstrates a uniform molar sensitivity toward carbon compounds independent of their optical properties or volatility. Results suggest the device might serve as a simple, inexpensive universal LC detector.

Purification of insulin-like growth factor-I and related proteins using underivatized silica

Adsorption chromatography using underivatized porous glass can be an effective capture step for the purification of recombinant proteins. Classical desorption techniques using chaotropic agents or harsh chemical solvents often result in elution of inactive material and may not be economical at the process scale. More recently, elution schemes have used tetramethylammonium chloride (TMAC) to obtain biologically active material. A TMAC elution was shown to be effective in the initial purification steps for the recovery of recombinant human insulin-like growth factor-I (rhIGF-I) from an Escherichia coli fermentation broth. However, TMAC also elutes other, more hydrophobic, proteins that are difficult to remove in subsequent purification steps. This paper describes the capture of IGF-I from a crude fermentation broth and a more specific elution using a combination of ethanol and NaCl rather than TMAC. This elution also can be used with other proteins including an IGF-I binding protein (BP3) expressed in mammalian cell culture.

The structure of an integral membrane peptide: a deuterium NMR study of gramicidin

Solid state deuterium NMR was employed on oriented multilamellar dispersions consisting of 1,2-dilauryl-sn-glycero-3-phosphatidylcholine and deuterium (2H) exchange-labeled gramicidin D, at a lipid to protein molar ratio (L/P) of 15:1, in order to study the dynamic structure of the channel conformation of gramicidin in a liquid crystalline phase. The corresponding spectra were used to discriminate between several structural models for the channel structure of gramicidin (based on the left- and right-handed beta 6.3 LD helix) and other models based on a structure obtained from high resolution NMR. The oriented spectrum is complicated by the fact that many of the doublets, corresponding to the 20 exchangeable sites, partially overlap. Furthermore, the asymmetry parameter, eta, of the electric field gradient tensor of the amide deuterons is large (approximately 0.2) and many of the amide groups are involved in hydrogen bonding, which is known to affect the quadrupole coupling constant. In order to account for these complications in simulating the spectra in the fast motional regime, an ab initio program called Gaussian 90 was employed, which permitted us to calculate, by quantum mechanical means, the complete electric field gradient tensor for each residue in gramicidin (using two structural models). Our results indicated that the left-handed helical models were inconsistent with our observed spectra, whereas a model based on the high-resolution structure derived by Arseniev and coworkers, but relaxed by a simple energy minimization procedure, was consistent with our observed spectra. The molecular order parameter was then estimated from the motional narrowing assuming the relaxed (right-handed) Arseniev structure. Our resultant order parameter of SZZ = 0.91 translates into an rms angle of 14 degrees, formed by the helix axis and the local bilayer normal. The strong resemblance between our spectra (and also those reported for gramicidin in 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) multilayers) and the spectra of the same peptide incorporated in a lyotropic nematic phase, suggests that the lyotropic nematic phase simulates the local environment of the lipid bilayer.

Interaction of multidrug-resistant Chinese hamster ovary cells with the peptide ionophore gramicidin D

A major form of multidrug resistance results from the overexpression of P-glycoprotein, a 170 kDa membrane protein. Multidrug resistant (MDR) Chinese hamster ovary (CHO) cells and mdrl transfectants displayed cross-resistance to the channel-forming peptide ionophore gramicidin D, which was reversed by various chemosensitizers, thus directly implicating P-glycoprotein as the mediator of resistance. However, gramicidin D was not able to inhibit [3H]azidopine photolabelling of P-glycoprotein. MDR cells were not resistant to other pore-forming ionophores, but showed a modest level of cross-resistance to the mobile ionophore valinomycin. There was no difference in 125I-gramicidin D uptake by resistant and sensitive cells. Resistant cells showed lower 86Rb+ uptake, relative to the drug-sensitive parent. Addition of GmD increased both the rate and the level of 86Rb+ uptake in sensitive cells, but had no effect on MDR cells. MDR cells also showed much lower rates of gramicidin D-dependent 86Rb+ efflux than sensitive cells, and this was greatly increased by verapamil. These results suggest that P-glycoprotein interferes with the formation of ion-conducting gramicidin D channels. In contrast, valinomycin had the same effect on gramicidin D-dependent cation efflux in MDR and sensitive cells. Gramicidin D is thus unique among the ionophores is being a substrate for P-glycoprotein, which appears to greatly reduce the formation of active dimeric channels in the plasma membrane of MDR cells.

Model ion channels: gramicidin and alamethicin

We have discussed in some detail a variety of experimental studies which were designed to elucidate the conformational and dynamic properties of gramicidin and alamethicin. Although the behavior of these peptides is by no means fully characterized, these studies have already permitted aspects of ion channel activity to be understood in molecular terms. Studies with gramicidin in a variety of organic solutions have revealed conformational heterogeneity of this peptide; at least five major isomers exist, several of which have been characterized in detail using NMR spectroscopy and X-ray crystallography. When added to lipid membranes gramicidin undergoes a further conformational conversion. Although the conformation of gramicidin in membranes is not as well characterized as the solution conformation(s) and an X-ray structure is not yet available, detailed data, particularly from solid-state NMR studies, continue to become available and a right-handed beta 6.3 helical conformation of the peptide backbone is now generally accepted. Two of these beta 6.3 helices joined at their N-termini are believed to form the conducting channel. The conformational behavior of the side-chains of gramicidin in the membrane-bound form is not well established and several NMR, CD, fluorescence and theoretical studies are now focussed on this. Although the side-chains do not directly contact the permeating ions, they can have distinct effects on conductance and selectivity by altering the electrostatic environment sensed by the ion. The dynamics of both side-chain and backbone conformations of gramicidin appear critical to a detailed understanding of the ion transport process in this channel. As the description of the membrane-bound conformation of gramicidin becomes more detailed, simulations of ion transport using computational methods are likely to improve and will further our understanding of the processes of ion transport. As well as internal motion of the backbone and side-chains, gramicidin undergoes rotational and translational motion in the plane of the membrane. These motions do not appear to be essential for the process of ion transport but can affect channel lifetime since lifetime is determined by the rate of association and dissociation of gramicidin monomers. Gramicidin-membrane interactions are also likely to be involved in the frequency of occurrence of channel subconductance states, the frequency of channel flickering and fundamentally in the stability of the membrane-bound gramicidin conformation. Alamethicin forms channels in membranes which are strongly voltage-dependent. The molecular origin of voltage-dependent conductances has been a fundamental problem in biophysics for many years.(ABSTRACT TRUNCATED AT 400 WORDS)

The structure of gramicidin A in dimethylsulfoxide/acetone

It has been demonstrated by two-dimensional NMR cross-relaxation spectroscopy that gramicidin A exists in dimethylsulfoxide/acetone solution in random coil form. This contradicts earlier conclusions by Hawkes et al. [Hawkes, G. E., Lian, L. Y., Randall, E. W., Sales, K. D. & Curzon, E. H. (1987) Eur. J. Biochem. 166, 437-445] that were based on the interpretation of vicinal proton coupling constants.