Interaction of noncompetitive inhibitors with the alpha3beta2 nicotinic acetylcholine receptor investigated by affinity chromatography and molecular docking

Moment Analysis Method for Measurement of Reaction Equilibrium and Rate Constants by Using High-Performance Liquid Chromatography

The moment analysis method was developed for the determination of association equilibrium constant (KA) and association (ka) and dissociation (kd) rate constants of intermolecular interactions between solute and ligand molecules. They are accurately determined by using moment equations from elution peak profiles because they are measured by using high-performance liquid chromatography (HPLC) under preferable conditions that neither immobilization nor chemical modification (i.e., fluorescence labeling) of solute and ligand molecules is required. To demonstrate the effectiveness of the method, it was applied to the inclusion complex formation system between dibenzo-18-crown-6 (DB18C6) and alkaline earth metal cations, i.e., Mg2+, Ca2+, and Sr2+, as a concrete example. Because the diameter of the three metal cations is smaller than that of the inner cavity of DB18C6, the values of KA, ka, and kd were analytically determined by assuming the stoichiometry of 1:1 between DB18C6 and the metal cation. They reflected the influence of the difference in the size between the inner cavity of DB18C6 and the metal cations on the inclusion complex formation. It seems that the moment analysis method based on HPLC separation is effective for the multifaceted analysis of chemical reactions because some characteristics of the method are different from those of other conventional methods. It must contribute to the dissemination of an opportunity for the study of chemical reactions to many researchers because of the versatility of HPLC.

Moment Theory of Chromatography for the Analysis of Reaction Kinetics of Intermolecular Interactions

Moment theory was applied to the kinetic study of intermolecular interactions. The association equilibrium constant (K_A) and association (k_a) and dissociation (k_d) rate constants of chemical reactions were analytically determined on the basis of the moment theory from elution peak profiles measured by high-performance liquid chromatography (HPLC). The HPLC data were measured under the conditions that neither immobilization nor fluorescence labeling of solute and ligand molecules is required. These are the advantages of the moment analysis method for determining accurate values of K_A, k_a, and k_d. Moment equations were developed on the basis of the Einstein equation for diffusion, the random walk model, and the general rate model of chromatography. The moment analysis method was applied to the inclusion complex formation system between dibenzo-18-crown-6 or dibenzo-15-crown-5 and alkali metal cations. It was demonstrated that the values of K_A, k_a, and k_d can be determined on the assumption that the stoichiometry between crown ethers and cations is 1:1 or 2:1. The influence of the difference in the size between the inner cavity of crown ethers and cations on the association and dissociation of the inclusion complex was considered. The moment analysis method using HPLC is effective for analyzing intermolecular interactions from various perspectives because it is based on the separation technique and has different characteristics from other methods such as spectroscopy. The results of this study contribute to the dissemination of an opportunity for studying intermolecular interactions from equilibrium and kinetic points of view to many researchers because HPLC is widespread.

Kinetic Analysis by Affinity Chromatography

Important information on chemical processes in living systems can be obtained by the rates at which these biological interactions occur. This review will discuss several techniques based on traditional and high-performance affinity chromatography that may be used to examine the kinetics of biological reactions. These methods include band-broadening measurements, techniques for peak fitting, split-peak analysis, peak decay studies, and ultrafast affinity extraction. The general principles and theory of each method, as applied to the determination of rate constants, will be discussed. The applications of each approach, along with its advantages and limitations, will also be considered.

A Review on the Receptor-ligand Molecular Interactions in the Nicotinic Receptor Signaling Systems

Nicotine is regarded as the main active addictive ingredient in tobacco products driving continued tobacco abuse behavior (smoking) to the addiction behavior, whereas nicotinic acetylcholine receptors (nAChR) is the crucial effective apparatus or molecular effector of nicotine and acetylcholine and other similar ligands. Many nAChR subunits have been revealed to bind to either neurotransmitters or exogenous ligands, such as nicotine and acetylcholine, being involved in the nicotinic receptor signal transduction. Therefore, the nicotinic receptor signalling molecules and the receptor-ligand molecular interactions between nAChRs and their ligands are universally regarded as crucial mediators of cellular functions and drug targets in medical treatment and clinical diagnosis. Given numerous endeavours have been made in defining the roles of nAChRs in response to nicotine and other addictive drugs, this review focuses on studies and reports in recent years on the receptor-ligand interactions between nAChR receptors and ligands, including lipid-nAChR and protein-nAChR molecular interactions, relevant signal transduction pathways and their molecular mechanisms in the nicotinic receptor signalling systems. All the references were carefully retrieved from the PubMed database by searching key words "nicotine", "acetylcholine", "nicotinic acetylcholine receptor(s)", "nAChR*", "protein and nAChR", "lipid and nAChR", "smok*" and "tobacco". All the relevant referred papers and reports retrieved were fully reviewed for manual inspection. This effort intend to get a quick insight and understanding of the nicotinic receptor signalling and their molecular interactions mechanisms. Understanding the cellular receptor-ligand interactions and molecular mechanisms between nAChRs and ligands will lead to a better translational and therapeutic operations and outcomes for the prevention and treatment of nicotine addiction and other chronic drug addictions in the brain's reward circuitry.

Drug Discovery on Natural Products: From Ion Channels to nAChRs, from Nature to Libraries, from Analytics to Assays

Natural extracts are complex mixtures that may be rich in useful bioactive compounds and therefore are attractive sources for new leads in drug discovery. This review describes drug discovery from natural products and in explaining this process puts the focus on ion-channel drug discovery. In particular, the identification of bioactives from natural products targeting nicotinic acetylcholine receptors (nAChRs) and serotonin type 3 receptors (5-HT3Rs) is discussed. The review is divided into three parts: "Targets," "Sources," and "Approaches." The "Targets" part will discuss the importance of ion-channel drug targets in general, and the α7-nAChR and 5-HT3Rs in particular. The "Sources" part will discuss the relevance for drug discovery of finding bioactive compounds from various natural sources such as venoms and plant extracts. The "Approaches" part will give an overview of classical and new analytical approaches that are used for the identification of new bioactive compounds with the focus on targeting ion channels. In addition, a selected overview is given of traditional venom-based drug discovery approaches and of diverse hyphenated analytical systems used for screening complex bioactive mixtures including venoms.

Moment Analysis Theory for Size Exclusion Capillary Electrochromatography with Chemical Reaction of Intermolecular Interaction

New moment equations were developed for size exclusion capillary electrochromatography (SECEC), in which intermolecular chemical reactions simultaneously took place. They explain how the first absolute and second central moments of elution peaks are correlated with some fundamental equilibrium and kinetic parameters of mass transfer and chemical reaction in SECEC column. In order to demonstrate the effectiveness of the moment equations, they were used to predict chromatographic behavior under hypothetical SECEC conditions. It was quantitatively studied how the association and dissociation rate constants of intermolecular interaction affected the position and spreading of elution peaks. It was indicated that both the intermolecular reaction kinetics and axial dispersion of solute molecules in a capillary column had a predominant contribution to the band broadening.

Kinetic Study of the Intermolecular Interaction between 2-Phenoxypropionic Acid and β-Bromo-cyclodextrin Affixed on the Stationary Phase by Liquid Chromatography

The intermolecular interaction between 2-phenoxypropionic acid and β-bromo-cyclodextrin affixed on the stationary phase surface in a chiral HPLC system was studied by the moment analysis method. At first, pulse response and peak parking experiments were conducted to measure some parameters concerning the column geometry, adsorption equilibrium, and mass-transfer kinetics. Then, the first absolute moment (μ1) and second central moment (μ2') of the elution peaks were analyzed by the moment equations, which were developed by assuming that the reaction kinetics between the solute molecules and the functional ligands can be represented by the Langmuir-type rate equation. Finally, the flow-rate dependence of HETP calculated from μ1 and μ2' was analyzed by using the values of the parameters to determine the association and dissociation rate constants of the intermolecular interaction. It was demonstrated that the combination of the chromatographic experiments and moment analysis is one of the effective strategies for the kinetic study of intermolecular interactions.

Defining the putative inhibitory site for a selective negative allosteric modulator of human α4β2 neuronal nicotinic receptors

Neuronal nicotinic receptors (nAChRs) have been implicated in several diseases and disorders such as autism spectrum disorders, Alzheimer's disease, Parkinson's disease, epilepsy, and nicotine addiction. To understand the role of nAChRs in these conditions, it would be beneficial to have selective molecules that target specific nAChRs in vitro and in vivo. Our laboratory has previously identified a novel allosteric site on human α4β2 nAChRs using a series of computational and in vitro approaches. At this site, we have identified negative allosteric modulators that selectively inhibit human α4β2 nAChRs, a subtype implicated in nicotine addiction. This study characterizes the allosteric site via site-directed mutagenesis. Three amino acids (Phe118, Glu60, and Thr58) on the β2 subunit were shown to participate in the inhibitory properties of the selective antagonist KAB-18 and provided insights into its antagonism of human α4β2 nAChRs. SAR studies with KAB-18 analogues and various mutant α4β2 nAChRs also provided information concerning how different physiochemical features influence the inhibition of nAChRs through this allosteric site. Together, these studies identify the amino acids that contribute to the selective antagonism of human α4β2 nAChRs at this allosteric site. Finally, these studies define the physiochemical features of ligands that influence interaction with specific amino acids in this allosteric site.

Pharmaceutical and biomedical applications of affinity chromatography: recent trends and developments

Affinity chromatography is a separation technique that has become increasingly important in work with biological samples and pharmaceutical agents. This method is based on the use of a biologically related agent as a stationary phase to selectively retain analytes or to study biological interactions. This review discusses the basic principles behind affinity chromatography and examines recent developments that have occurred in the use of this method for biomedical and pharmaceutical analysis. Techniques based on traditional affinity supports are discussed, but an emphasis is placed on methods in which affinity columns are used as part of HPLC systems or in combination with other analytical methods. General formats for affinity chromatography that are considered include step elution schemes, weak affinity chromatography, affinity extraction and affinity depletion. Specific separation techniques that are examined include lectin affinity chromatography, boronate affinity chromatography, immunoaffinity chromatography, and immobilized metal ion affinity chromatography. Approaches for the study of biological interactions by affinity chromatography are also presented, such as the measurement of equilibrium constants, rate constants, or competition and displacement effects. In addition, related developments in the use of immobilized enzyme reactors, molecularly imprinted polymers, dye ligands and aptamers are briefly considered.

Interaction of ibogaine with human alpha3beta4-nicotinic acetylcholine receptors in different conformational states

The interaction of ibogaine and phencyclidine (PCP) with human (h) alpha3beta4-nicotinic acetylcholine receptors (AChRs) in different conformational states was determined by functional and structural approaches including, radioligand binding assays, Ca2+ influx detections, and thermodynamic and kinetics measurements. The results established that (a) ibogaine inhibits (+/-)-epibatidine-induced Ca2+ influx in h(alpha)3beta4 AChRs with approximately 9-fold higher potency than that for PCP, (b) [3H]ibogaine binds to a single site in the h(alpha)3beta4 AChR ion channel with relatively high affinity (Kd = 0.46 +/- 0.06 microM), and ibogaine inhibits [3H]ibogaine binding to the desensitized h(alpha)3beta4 AChR with slightly higher affinity compared to the resting AChR. This is explained by a slower dissociation rate from the desensitized ion channel compared to the resting ion channel, and (c) PCP inhibits [3H]ibogaine binding to the h(alpha)3beta4 AChR, suggesting overlapping sites. The experimental results correlate with the docking simulations suggesting that ibogaine and PCP interact with a binding domain located between the serine (position 6') and valine/phenylalanine (position 13') rings. This interaction is mediated mainly by van der Waals contacts, which is in agreement with the observed enthalpic contribution determined by non-linear chromatography. However, the calculated entropic contribution also indicates local conformational changes. Collectively our data suggest that ibogaine and PCP bind to overlapping sites located between the serine and valine/phenylalanine rings, to finally block the AChR ion channel, and in the case of ibogaine, to probably maintain the AChR in the desensitized state for longer time.

Kinetic studies of biological interactions by affinity chromatography

The rates at which biological interactions occur can provide important information on the mechanism and behavior of such processes in living systems. This paper will discuss how affinity chromatography can be used as a tool to examine the kinetics of biological interactions. This approach, referred to here as biointeraction chromatography, uses a column with an immobilized binding agent to examine the association or dissociation of this agent with other compounds. The use of HPLC-based affinity columns in kinetic studies has received particular attention in recent years. Advantages of using HPLC with affinity chromatography for this purpose include the ability to reuse the same ligand within a column for a large number of experiments, and the good precision and accuracy of this approach. A number of techniques are available for kinetic studies through the use of affinity columns and biointeraction chromatography. These approaches include plate height measurements, peak profiling, peak fitting, split-peak measurements, and peak decay analysis. The general principles for each of these methods are discussed in this paper and some recent applications of these techniques are presented. The advantages and potential limitations of each approach are also considered.

Interaction of bupropion with muscle-type nicotinic acetylcholine receptors in different conformational states

To characterize the binding sites and the mechanisms of inhibition of bupropion on muscle-type nicotinic acetylcholine receptors (AChRs), structural and functional approaches were used. The results established that bupropion (a) inhibits epibatidine-induced Ca(2+) influx in embryonic muscle AChRs, (b) inhibits adult muscle AChR macroscopic currents in the resting/activatable state with approximately 100-fold higher potency compared to that in the open state, (c) increases the desensitization rate of adult muscle AChRs from the open state and impairs channel opening from the resting state, (d) inhibits binding of [(3)H]TCP and [(3)H]imipramine to the desensitized/carbamylcholine-bound Torpedo AChR with higher affinity compared to the resting/alpha-bungarotoxin-bound AChR, (e) binds to the Torpedo AChR in either state mainly by an entropy-driven process, and (f) interacts with a binding domain located between the serine (position 6') and valine (position 13') rings, by a network of van der Waals, hydrogen bond, and polar interactions. Collectively, our data indicate that bupropion first binds to the resting AChR, decreasing the probability of ion channel opening. The remnant fraction of open ion channels is subsequently decreased by accelerating the desensitization process. Bupropion interacts with a luminal binding domain shared with PCP that is located between the serine and valine rings, and this interaction is mediated mainly by an entropy-driven process.

Measurement of drug-protein dissociation rates by high-performance affinity chromatography and peak profiling

The rate at which a drug or other small solute interacts with a protein is important in understanding the biological and pharmacokinetic behavior of these agents. One approach that has been developed for examining these rates involves the use of high-performance affinity chromatography (HPAC) and estimates of band-broadening through peak profiling. Previous work with this method has been based on a comparison of the statistical moments for a retained analyte versus nonretained species at a single, high flow rate to obtain information on stationary phase mass transfer. In this study an alternative approach was created that allows a broad range of flow rates to be used for examining solute-protein dissociation rates. Chromatographic theory was employed to derive equations that could be used with this approach on a single column, as well as with multiple columns to evaluate and correct for the impact of stagnant mobile phase mass transfer. The interaction of L-tryptophan with human serum albumin was used as a model system to test this method. A dissociation rate constant of 2.7 (+/-0.2) s(-1) was obtained by this approach at pH 7.4 and 37 degrees C, which was in good agreement with previous values determined by other methods. The techniques described in this report can be applied to other biomolecular systems and should be valuable for the determination of drug-protein dissociation rates.

The preparation and development of cellular membrane affinity chromatography columns

Cellular membrane affinity chromatography is a technique that is based on the immobilization of a target trans-membrane protein onto a stationary phase. The target protein is isolated by homogenization and solubilization of a source (e.g., cell line) followed by immobilization on either the immobilized artificial membrane-phosphatidyl choline (IAM-PC) stationary phase or the surface of an open tubular capillary during a dialysis step. The procedure typically takes 3-4 d for the IAM-PC stationary phase, whereas the open-tubular method takes an extra week for the preparation of the capillary. The resulting columns can then be used to characterize binding sites on the target protein through frontal chromatographic and/or nonlinear chromatographic studies using a wide variety of ligands including small molecules and polypeptides. The columns have been used in drug discovery as well as in the screening of tobacco smoke condensates.

Determination and modelling of stereoselective interactions of ligands with drug transporters: a key dimension in the understanding of drug disposition

1. Stereochemistry is an important dimension in pharmacology and plays a role in every aspect of the pharmacological fate of chiral xenobiotics. This includes small molecule-drug transporter binding. 2. This paper reviews the reported stereoselectivities of substrate and inhibitor interactions with P-glycoprotein and the organic cation transporter obtained using standard functional and binding studies, as well as data obtained from online cellular membrane affinity chromatography studies. 3. The use of stereochemical data in quantitative structure-activity relationship (QSAR) and pharmacophore modelling is also addressed as is the effect of ignoring the fact that small molecule-drug transporter interactions take place in three-dimensional and asymmetric space.

Exploring enantiospecific ligand-protein interactions using cellular membrane affinity chromatography: chiral recognition as a dynamic process

The chiral recognition mechanisms responsible for the enantioselective binding on the alpha3beta4 nicotinic acetylcholine receptor (alpha3 beta4 nAChR) and human organic cation transporter 1 (hOCT1) have been reviewed. The results indicate that chiral recognition on the alpha3beta4 nAChR is a process involving initial tethering of dextromethorphan and levomethorphan at hydrophobic pockets within the central lumen followed by hydrogen bonding interactions favoring dextromethorphan. The second step is the defining enantioselective step. Studies with the hOCT1 indentified four binding sites within the transporter that participated in chiral recognition. Each of the enantiomers of the compounds used in the study interacted with three of these sites, while (R)-verapamil interacted with all four. Chiral recognition arose from the conformational adjustments required to produce optimum interactions. With respect to the prevailing interaction-based models, the data suggest that chiral recognition is a dynamic process and that the static point-based models should be amended to reflect this.