Interaction of Drosophila acetylcholinesterases with D-tubocurarine: an explanation of the activation by an inhibitor

Inhibitory Mechanism of Baicalein on Acetylcholinesterase: Inhibitory Interaction, Conformational Change, and Computational Simulation

Alzheimer’s disease (AD) is the most prevalent chronic neurodegenerative disease in elderly individuals, causing dementia. Acetylcholinesterase (AChE) is regarded as one of the most popular drug targets for AD. Herbal secondary metabolites are frequently cited as a major source of AChE inhibitors. In the current study, baicalein, a typical bioactive flavonoid, was found to inhibit AChE competitively, with an associated IC₅₀ value of 6.42 ± 0.07 µM, through a monophasic kinetic process. The AChE fluorescence quenching by baicalein was a static process. The binding constant between baicalein and AChE was an order of magnitude of 10⁴ L mol⁻¹, and hydrogen bonding and hydrophobic interaction were the major forces for forming the baicalein−AChE complex. Circular dichroism analysis revealed that baicalein caused the AChE structure to shrink and increased its surface hydrophobicity by increasing the α-helix and β-turn contents and decreasing the β-sheet and random coil structure content. Molecular docking revealed that baicalein predominated at the active site of AChE, likely tightening the gorge entrance and preventing the substrate from entering and binding with the enzyme, resulting in AChE inhibition. The preceding findings were confirmed by molecular dynamics simulation. The current study provides an insight into the molecular-level mechanism of baicalein interaction with AChE, which may offer new ideas for the research and development of anti-AD functional foods and drugs.

Slow-binding inhibitors of acetylcholinesterase of medical interest

Certain ligands slowly bind to acetylcholinesterase. As a result, there is a slow establishment of enzyme-inhibitor equilibrium characterized by a slow onset of inhibition prior reaching steady state. Three mechanisms account for slow-binding inhibition: a) slow binding rate constant k_on, b) slow ligand induced-fit following a fast binding step, c) slow conformational selection of an enzyme form. The slow equilibrium may be followed by a chemical step. This later that can be irreversible has been observed with certain alkylating agents and substrate transition state analogs. Slow-binding inhibitors present long residence times on target. This results in prolonged pharmacological or toxicological action. Through several well-known molecules (e.g. huperzine) and new examples (tocopherol, trifluoroacetophenone and a 6-methyluracil alkylammonium derivative), we show that slow-binding inhibitors of acetylcholinesterase are promising drugs for treatment of neurological diseases such as Alzheimer disease and myasthenia gravis. Moreover, they may be of interest for neuroprotection (prophylaxis) against organophosphorus poisoning. This article is part of the special issue entitled 'Acetylcholinesterase Inhibitors: From Bench to Bedside to Battlefield'.

Third intracellular loop of glucagon like-peptide-1 receptor is coupled with endogenous mono-ADP-ribosyltransferase - novel type of receptor regulation?

Our previous studies revealed the main role of the third intracellular loop (IC(3)) of glucagon-like peptide-1 receptor (GLP-1 receptor), in G-protein activation, where the presence or absence of agonist and the receptor phosphorylation seemed to be the only regulatory mechanisms. In order to further study the signaling mechanisms of GLP-1 receptor, we investigated the effect of the third intracellular loop-derived peptide on endogenous mono-ADP-ribosyltransferase mediated mono-ADP-ribosylation of G-proteins β subunit in CHO cells. Results showed an inhibitory effect of IC(3) peptide on mono-ADP-ribosylation of β subunit, obviously via the mechanism of competitive inhibition. Excluding the activity of this inhibitory mechanism via pertussis toxin-sensitive G proteins, the direct functional coupling of IC(3) of GLP-1 receptor and endogenous mono-ADP-ribosyltransferase was confirmed. We suggest that this arginine specific enzymatic posttranslational modification of third intracellular loop of GLP-1 receptor might represent a possible novel mechanism of receptor activity regulation and the pharmacological potential in treatment of diabetes mellitus type 2.

Insights into substrate and product traffic in the Drosophila melanogaster acetylcholinesterase active site gorge by enlarging a back channel

To test a product exit differing from the substrate entrance in the active site of acetylcholinesterase (EC 3.1.1.7), we enlarged a channel located at the bottom of the active site gorge in the Drosophila enzyme. Mutation of Trp83 to Ala or Glu widens the channel from 5 A to 9 A. The kinetics of substrate hydrolysis and the effect of ligands that close the main entrance suggest that the mutations facilitate both product exit and substrate entrance. Thus, in the wild-type, the channel is so narrow that the 'back door' is used by at most 5% of the traffic, with the majority of traffic passing through the main entrance. In mutants Trp83Ala and Trp83Glu, ligands that close the main entrance do not inhibit substrate hydrolysis because the traffic can pass via an alternative route, presumably the enlarged back channel.

Concentration-Dependent Kinetics of Acetylcholinesterase Inhibition by the Organophosphate Paraoxon

For decades the interaction of the anticholinesterase organophosphorus compounds with acetylcholinesterase has been characterized as a straightforward phosphylation of the active site serine (Ser-203) which can be described kinetically by the inhibitory rate constant k(i). However, more recently certain kinetic complexities in the inhibition of acetylcholinesterase by organophosphates such as paraoxon (O,O-diethyl O-(p-nitrophenyl) phosphate) and chlorpyrifos oxon (O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphate) have raised questions regarding the adequacy of the kinetic scheme on which k(i) is based. The present article documents conditions in which the inhibitory capacity of paraoxon towards human recombinant acetylcholinesterase appears to change as a function of oxon concentration (as evidenced by a changing k(i)), with the inhibitory capacity of individual oxon molecules increasing at lower oxon concentrations. Optimization of a computer model based on an Ordered Uni Bi kinetic mechanism for phosphylation of acetylcholinesterse determined k(1) to be 0.5 nM(-1)h(-1), and k(-1) to be 169.5 h(-1). These values were used in a comparison of the Ordered Uni Bi model versus a k(i) model in order to assess the capacity of k(i) to describe accurately the inhibition of acetylcholinesterase by paraoxon. Interestingly, the k(i) model was accurate only at equilibrium (or near equilibrium), and when the inhibitor concentration was well below its K(d) (pseudo first order conditions). Comparisons of the Ordered Uni Bi and k(i) models demonstrate the changing k(i) as a function of inhibitor concentrations is not an artifact resulting from inappropriate inhibitor concentrations.

Rational polynomial equation helps to select among homeomorphic kinetic models for cholinesterase reaction mechanism

The hydrolysis of substrates by cholinesterases does not follow the Michaelis-Menten reaction mechanism. In addition to the inhibition by excess substrate, these enzymes often show an unexpectedly high activity at low substrate concentrations. It seems that these deviations are the consequence of an unusual architecture of the active site, buried deep inside the core of the molecule. Kinetic data and structural evidence allow for a detailed prediction of the events during a very fast substrate turnover. Recently, we presented a procedure which provides an unbiased framework for mathematical modelling of the complex cholinesterase reaction [J. Stojan, M. Golicnik, D. Fournier, Rational polynomial equation as an unbiased approach for the kinetic studies of Drosophila melanogaster acetylcholinesterase reaction mechanism, Biochim. Biophys. Acta 1703 (2004) 53-61]. It is based on regression analysis of a rational polynomial using classical initial rate data. Here, we extend the use of the rational polynomial rate equation for finding and comparing some selected homeomorphic reaction schemes useful for the mechanistic interpretation of cholinesterase kinetic data.

Inhibition of human plasma cholinesterase by malachite green and related triarylmethane dyes: Mechanistic implications

The inhibitory effects of the cationic triarylmethane (TAM+) dyes, pararosaniline (PR+), malachite green (MG+), and methyl green (MeG+) on human plasma cholinesterase (BChE) were studied at 25 degrees C in 100 mM Mops, pH 8.0, with butyrylthiocholine as substrate. PR+ and MG+ caused linear mixed inhibition of enzyme activity. The respective inhibitory parameters were K(i) = 1.9 +/- 0.23 microM, alpha = 13 +/- 48, beta = 0 and K(i) = 0.28 +/- 0.037 microM, alpha = 23 +/- 7.4, beta = 0. MeG+ acted as a competitive inhibitor with K(i) = 0.12 +/- 0.017 microM (alpha, infinity, beta, not applicable). The K(i) values were within the same range reported for a number of ChE inhibitors including propidium ion, donepezil, and the phenothiazines, suggesting that TAM+s are active site ligands. On the other hand, the alpha values failed to correlate with values previously reported for a number of ChE inhibitors. It appears that mixed inhibition is the combined result of more than one type of binding and S-I interference. The impact of ligands at the choline-specific and peripheral anionic sites (or, possibly, accessory structural domains) on BChE activity needs to be studied in further detail.

Rational polynomial equation as an unbiased approach for the kinetic studies of Drosophila melanogaster acetylcholinesterase reaction mechanism

The hydrolysis of substrates by cholinesterases does not follow the Michaelis-Menten reaction mechanism. The well-known inhibition by excess substrate is often accompanied by an unexpectedly high activity at low substrate concentrations. It appears that these peculiarities are the consequence of an unusual architecture of the active site, which conducts the substrate molecule over many stages before it is cleaved and released. Structural and kinetic data also suggest that two substrate molecules can attach at the same time to the free, as well as to the acetylated, enzyme. We present a procedure which provides an unbiased framework for mathematical modelling of such complex reaction mechanisms. It is based on regression analysis of a rational polynomial using classical initial rate data. The determination of polynomial degree reveals the number of independent parameters that can be evaluated from the available information. Once determined, these parameters can substantially facilitate the construction and evaluation of a kinetic model reflecting the expected molecular events in an enzymic reaction. We also present practical suggestions for testing the postulated kinetic model, using an original thermodynamic approach and an isolated effect in a specifically mutated enzyme.

Localization of mRNAs encoding acetylcholinesterase and butyrylcholinesterase in the rat spinal cord by nonradioactive in situ hybridization

In spite of intensive investigations, the roles of acetylcholinesterase (AChE; EC 3.1.1.7) and butyrylcholinesterase (BuChE; EC 3.1.1.8) in the central nervous system (CNS) remain unclear. A role recently proposed for BuChE as an explanation for survival of AChE knockout mice is compensation for AChE activity if it becomes insufficient. Neuronal contribution of both enzymes to the cholinesterase pool in the neuromuscular junction has also been suggested. These proposals imply that BuChE expression follows that of AChE and that, in addition to AChE, BuChE is also expressed in alpha-motor neurons. However, these assumptions have not yet been properly tested. Histochemical approaches to these problems have been hampered by a number of problems that prevent unambiguous interpretation of results. In situ hybridization (ISH) of mRNAs encoding AChE and BuChE, which is the state-of-the-art approach, has not yet been done. Here we describe rapid nonradioactive ISH for the localization of mRNAs encoding AChE and BuChE. Various probes and experimental conditions had been tested to obtain reliable localization. In combination with RT-PCR, ISH revealed that, in rat spinal cord, cells expressing AChE mRNA also express BuChE mRNA but in smaller quantities. alpha-Motor neurons had the highest levels of both mRNAs. Virtual absence of transcripts encoding AChE and BuChE in glia might reflect a discrepancy between mRNA and enzyme levels previously reported for cholinesterases.

Multi-step analysis as a tool for kinetic parameter estimation and mechanism discrimination in the reaction between tight-binding fasciculin 2 and electric eel acetylcholinesterase

The mechanism of action of a potent peptidic inhibitor fasciculin 2 (Fas2) on electric eel acetylcholinesterase (eleelAChE) has been examined in a three-level analysis. Classical steps included equilibration experiments for the evaluation of high affinity binding constant and the existence of residual hydrolytic activity in a solution of completely Fas2 saturated enzyme. The two rate constants for the association (k(on)) and the dissociation (k(off)) of Fas2 with free enzyme were determined by the time course of residual enzyme activity measurements. In the third step, with a nonclassical progress curve analysis, we found that the Fas2-enzyme complex exhibited hydrolytic activity in a butyrylcholinesterase-like kinetics. The switch appears to be a consequence of steric obstruction, but also the consequence of subtle rapid conformational changes around catalytic site, upon slow single-step binding of large Fas2 molecule at the peripheral site. An unusual unilateral effect of bound Fas2 is reflected by acylation-independent association and dissociation rates and might indeed be due to inability of small acylation agent to influence the binding of a large opponent.

On a nonelementary progress curve equation and its application in enzyme kinetics

The analytical equation describing progress curves of an enzyme catalyzed reaction acting upon the Michaelis-Menten mechanism has been known for the case in which only the free enzyme incurs a loss of its activity, either spontaneously or as a result of an irreversible inhibitor action. The solution of differential equations which defines the rates of enzyme inactivation and substrate utilization is expressed by a nonelementary function in equation of an implicit type that precludes direct calculation of the extent of reaction at any time. Previously, the implicit equations have been rearranged to the alternative formulas and solved by the Newton-Raphson method, but this procedure may fail when used upon the presented equation. For this reason the other root-finding numerical method was applied, and the enzyme kinetic parameters of such numerically solved implicit equation for the reaction mechanism of irreversibly inhibited acetylcholinesterase were fitted to the experimental data by a nonlinear regression computer program.

Acceleration of Drosophila melanogaster acetylcholinesterase methanesulfonylation: peripheral ligand D-tubocurarine enhances the affinity for small methanesulfonylfluoride

D-Tubocurarine, a reversible peripheral inhibitor of cholinesterases accelerates methanesulfonylation of Drosophila melanogaster wild type and W359L mutant. The kinetic evaluation of the process was performed in a step-by-step analysis. The second order overall sulfonylation rate constants, determined from classical residual activity measurements, were used in the subsequent analysis of progress curves. The latter were obtained by measuring the hydrolysis of acetylthiocholine in a complex reaction system of enzyme, substrate, irreversible and reversible inhibitor. The underlying kinetic mechanisms, from such a complex data, could only be untangled by targeted inspection and successive incorporation of reaction steps for which experimental evidence existed. The study showed that the peripheral ligand D-tubocurarine, by binding at the entrance into the active site of the two investigated enzymes (Golicnik et al., Biochemistry 40 (2001) 1214), enhances the affinity for small methanesulfonylfluoride, rather to speeding up the formation of a stable covalent enzyme-inhibitor complex. The specific arrangements at the rim of the active site of each individual enzyme dictate the actual events which can be detected by kinetic means.

Concentration-dependent reversible activation-inhibition of human butyrylcholinesterase by tetraethylammonium ion

Tetraalkylammonium (TAA) salts are well known reversible inhibitors of cholinesterases. However, at concentrations around 10 mm, they have been found to activate the hydrolysis of positively charged substrates, catalyzed by wild-type human butyrylcholinesterase (EC 3.1.1.8) [Erdoes, E.G., Foldes, F.F., Zsigmond, E.K., Baart, N. & Zwartz, J.A. (1958) Science 128, 92]. The present study was undertaken to determine whether the peripheral anionic site (PAS) of human BuChE (Y332, D70) and/or the catalytic substrate binding site (CS) (W82, A328) are involved in this phenomenon. For this purpose, the kinetics of butyrylthiocholine (BTC) hydrolysis by wild-type human BuChE, by selected mutants and by horse BuChE was carried out at 25 degreeC and pH 7.0 in the presence of tetraethylammonium (TEA). It appears that human enzymes with more intact structure of the PAS show more prominent activation phenomenon. The following explanation has been put forward: TEA competes with the substrate at the peripheral site thus inhibiting the substrate hydrolysis at the CS. As the inhibition by TEA is less effective than the substrate inhibition itself, it mimics activation. At the concentrations around 40 mm, well within the range of TEA competition at both substrate binding sites, it lowers the activity of all tested enzymes.

Kinetic model of ethopropazine interaction with horse serum butyrylcholinesterase and its docking into the active site

The action of a potent tricyclic cholinesterase inhibitor ethopropazine on the hydrolysis of acetylthiocholine and butyrylthiocholine by purified horse serum butyrylcholinesterase (EC 3.1.1.8) was investigated at 25 and 37 degrees C. The enzyme activities were measured on a stopped-flow apparatus and the analysis of experimental data was done by applying a six-parameter model for substrate hydrolysis. The model, which was introduced to explain the kinetics of Drosophila melanogaster acetylcholinesterase [Stojan et al. (1998) FEBS Lett. 440, 85-88], is defined with two dissociation constants and four rate constants and can describe both cooperative phenomena, apparent activation at low substrate concentrations and substrate inhibition by excess of substrate. For the analysis of the data in the presence of ethopropazine at two temperatures, we have enlarged the reaction scheme to allow primarily its competition with the substrate at the peripheral site, but the competition at the acylation site was not excluded. The proposed reaction scheme revealed, upon analysis, competitive effects of ethopropazine at both sites; at 25 degrees C, three enzyme-inhibitor dissociation constants could be evaluated; at 37 degrees C, only two constants could be evaluated. Although the model considers both cooperative phenomena, it appears that decreased enzyme sensitivity at higher temperature, predominantly for the ligands at the peripheral binding site, makes the determination of some expected enzyme substrate and/or inhibitor complexes technically impossible. The same reason might also account for one of the paradoxes in cholinesterases: activities at 25 degrees C at low substrate concentrations are higher than at 37 degrees C. Positioning of ethopropazine in the active-site gorge by molecular dynamics simulations shows that A328, W82, D70, and Y332 amino acid residues stabilize binding of the inhibitor.

Progress curves analysis as an alternative for exploration of activation-inhibition phenomena in cholinesterases

The kinetic behaviour of insect acetylcholinesterases deviates from the Michaelis-Menten pattern. These deviations are known as activation or inhibition at various substrate concentrations and can be more or less observable depending on mutations around the active site of the enzyme. Most kinetic studies on these enzymes still rely on initial rate measurements. It is demonstrated here that according to this method one of the deviations can be overlooked. We attempt to point out that in such cases a detailed step-by-step progress curves analysis is successful. The study is focused on two different methods of analysing progress curves: (i) the first one is based on an integrated initial rate equation which can sufficiently fit truncated progress curves under corresponding conditions; and (ii) the other one precludes the algebraic formulae, but uses numerical integration for searching a non analytical solution of ordinary differential equations describing a kinetic model. All methods are tested on three different acetylcholinesterase mutants from Drosophila melanogaster. The results indicate that kinetic parameters for the E107K mutant with highly expressive activation and inhibition can be well evaluated applying any analysis method. It is quite different for E107W and E107Y mutants where latent activation is present, but discovered only using one or the other progress curves analysis methods.