Kinetic analysis of butyrylcholinesterase inhibition with N,N-dimethyl-2-phenylaziridinium ion

The kinetics of the butyrylcholinesterase interaction with N,N-dimethyl-2-phenylaziridinium ion, which probably alkylates the anionic site of the enzyme, was investigated at pH 7.5 and 25°C. It was found that the main product of the spontaneous hydrolysis of the aziridinium compound in water, N,N-dimethyl-2-hydroxy-2-phenylethylamine, binds effectively in the enzyme active center and protects it against alkylation. This binding equilibrium as well as the kinetics of the spontaneous decomposition of the aziridinium inhibitor were taken into consideration in calculating the rate and equilibrium constants for the enzyme alkylation reaction. The kinetics of the formation of the aziridinium compound and of the spontaneous hydrolysis reaction were investigated separately at pH 5.8–7.8, and the rate constants obtained from these experiments agree with the corresponding data calculated from the enzyme inhibition kinetics.     

Kinetic analysis of product inhibition in human manganese superoxide dismutase

Manganese superoxide dismutase (MnSOD) cycles between the Mn(II) and Mn(III) states during the catalyzed disproportionation of O(2)(*-), a catalysis that is limited at micromolar levels of superoxide by a peroxide-inhibited complex with the metal. We have investigated the role in catalysis and inhibition of the conserved residue Trp161 which forms a hydrophobic side of the active site cavity of MnSOD. Crystal structures of mutants of human MnSOD in which Trp161 was replaced with Ala or Phe showed significant conformational changes on adjacent residues near the active site, particularly Gln143 and Tyr34 which in wild-type MnSOD participate in a hydrogen bond network believed to support proton transfer during catalysis. Using pulse radiolysis and observing the UV absorbance of superoxide, we have determined rate constants for the catalytic dismutation of superoxide. In addition, the rates of formation and dissociation of the product-inhibited complex of these mutants were determined by direct observation of the characteristic visible absorption of the oxidized and inhibited states. Catalysis by W161A and W161F MnSOD was associated with a decrease of at least 100-fold in the catalytic rate of reduction of superoxide, which then promotes a competing pathway leading to product inhibition. The structural changes caused by the mutations at position 161 led to small changes, at most a 6-fold decrease, in the rate constant for formation of the inhibited complex. Solvent hydrogen isotope effects support a mechanism in which formation of this complex, presumably the peroxide dianion bound to the manganese, involves no rate-contributing proton transfer; however, the dissociation of the complex requires proton transfer to generate HO(2)(-) or H2O2.     

Kinetic analysis of enzyme reactions with slow-binding inhibition.

In this paper we present a general kinetic study of slow-binding inhibition processes, i.e. enzyme reactions that do not respond instantly to the presence of a competitive inhibitor. The analysis that we present is based on the equation that describes the formation of products with time in each case on the experimental progress curve. It is carried out under the condition of limiting enzyme concentration and allows the discrimination between the different cases of slow-binding inhibition. The mechanism in which the formation of complex enzyme-inhibitor is a single or two slow steps or follow a rapid equilibrium, has been considered. The corresponding explicit equations of each case have been obtained and checked by numerical integration. A kinetic data analysis to evaluate the corresponding kinetic parameters is suggested. We illustrate the method, numerically by computer simulation, of the reaction and present some numerical examples that demonstrate the applicability of our procedure.     

Selective reversible inhibition of human butyrylcholinesterase by aryl amide derivatives of phenothiazine

Evidence suggests that specific inhibition of butyrylcholinesterase may be an appropriate focus for the development of more effective drugs to treat dementias such as Alzheimer's disease. Butyrylcholinesterase is a co-regulator of cholinergic neurotransmission and its activity is increased in Alzheimer's disease, and is associated with all neuropathological lesions in this disease. Some selective butyrylcholinesterase inhibitors have already been reported to increase acetylcholine levels and to reduce the formation of abnormal amyloid found in Alzheimer's disease. Synthesized N-(10)-aryl and N-(10)-alkylaryl amides of phenothiazine are specific inhibitors of butyrylcholinesterase. In some cases, inhibition constants in the nanomolar range are achieved. Enzyme specificity and inhibitor potency of these molecules can be related to molecular volumes, steric and electronic factors. Computed logP values indicate high potential for these compounds to cross the blood-brain barrier. Use of such butyrylcholinesterase inhibitors could provide direct evidence for the importance of this enzyme in the normal nervous system and in Alzheimer's disease.     

Synthesis of novel purine nucleosides towards a selective inhibition of human butyrylcholinesterase

The search for new and potent cholinesterase inhibitors is an ongoing quest mobilizing many organic chemistry groups around the world as these molecules have been shown to treat the late symptoms of Alzheimer’s disease as well as to act as neuroprotecting agents. In this work, we disclose the synthesis of novel 2-acetamidopurine nucleosides and, for the first time, regioselective N⁷-glycosylation with 2-acetamido-6-chloropurine, promoted by trimethylsilyl triflate, was accomplished by tuning the reaction conditions (acetonitrile as solvent, 65 °C, 5 h) starting from 1-acetoxy bicyclic glycosyl donors, or by direct coupling of a methyl glucopyranoside with the nucleobase to obtain only N⁷ nucleosides in reasonable yield (55–60%). The nucleosides as well as their sugar precursors were screened for acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibition. While none of the compounds tested inhibited AChE, remarkably, some of the N⁷ nucleosides and sugar bicyclic derivatives showed potent inhibition towards BChE. Nanomolar inhibition was obtained for one compound competing well with rivastigmine, a drug currently in use for the treatment of Alzheimer’s disease. Experimental results showed that the presence of benzyl groups on the carbohydrate scaffold and the N⁷-linked purine nucleobase were necessary for strong BChE inactivation. A preliminary evaluation of the acute cytotoxicity of the elongated bicyclic sugar precursors and nucleosides was performed indicating low values, in the same order of magnitude as those of rivastigmine.     

Kinetic analysis of the inhibition of sulfate transport in human red blood cells by isothiocyanates

Summary A kinetic analysis of anion self-exchange in human red blood cells, in the presence of an irreversible inhibitor, is presented and applied to the study of the inactivation of sulfate transport by three isothiocyanates: 3-isothiocyano-1,5-naphthalenedisulfonic acid, disodium salt (INDS), 1-isothiocyano-4-naphthalene sulfonic acid, sodium salt, monohydrate (INS), and 1-isothiocyano-4-benzenesulfonic acid, sodium salt, monohydrate (IBS). The time dependence of the inhibition of sulfate transport by the isothiocyanates used could be described by a single exponential and could be shown to contain a reversible and an irreversible component. In each case a portion of sulfate efflux was found to be resistant to inactivation. The residual portion of the sulfate efflux varied with inhibition: 4% for INS, 16% for INDS, and 34% for IBS. INS showed the largest reversible inhibitory effect (12% of the flux remaining at 0.2mm inhibitor concentration), while INDS showed the weakest effect (92% of the flux remaining at 0.3mm inhibitor concentration). IBS had the highest rate of inactivation while INDS had the lowest. The kinetic analysis further suggests that all three isothiocyanates bind reversibly to an inhibitory site on the membrane before they bind covalently, and therefore irreversibly, to the same site on the membrane. The equilibrium constant for the dissociation of the reversibly-bound complex,K _i, and the rate of irreversible inactivation after all membrane sites are reversibly bound,k _max, have been computed for all three inhibitors: INDS (K _i=420m,k _max=5.04 hr^–1), INS (K _i=148 m,k _max=6.48 hr^–1), and IBS (K _i=208 m,k _max=8.11 hr^–1).     

Microcalorimetric study of the inhibition of butyrylcholinesterase by carbamates

The inhibition of horse serum butyrylcholinesterase (EC 3.1.1.8) by three carbamates (eserine, neostigmine, and rivastigmine) was studied by flow microcalorimetry at 37 degrees C in Tris buffer (pH 7.5). The kinetics of carbamylation was studied in the absence or presence of the substrate, butyrylcholine, using an extension of the model described by Stojan and coworkers (FEBS Lett. 440 (1998) 85-88). The model was fitted to the data by a nonlinear regression procedure using simulated annealing followed by Marquardt's method. The affinity of the carbamates for the free enzyme increased in the order neostigmine相似文献   

Microcalorimetric study of the inhibition of butyrylcholinesterase by paraoxon

The inhibition of horse serum butyrylcholinesterase (EC 3.1.1.8) by the organophosphorus compound paraoxon (diethyl 4-nitrophenyl phosphate) was studied by flow microcalorimetry at 37 °C in Tris buffer (pH 7.5) using a modification of the kinetic model described by Stojan and coworkers [J. Stojan, V. Marcel, S. Estrada-Mondaca, A. Klaebe, P. Masson, D. Fournier, A putative kinetic model for substrate metabolisation by Drosophila acetylcholinesterase, FEBS Lett. 440 (1998) 85-88]. The reversible steps of the inhibition were studied in the mixing cell of the calorimeter, whereas the irreversible step was studied in the flow-through cell. A new pseudo-first-order approximation was developed to allow the kinetic analysis of inhibition progress curves in the presence of substrate when a significant amount of substrate is transformed. This approximation also allowed one to compute an analytical expression of the calorimetric curves using a gamma distribution to describe the impulse response of the calorimeter. Fitting models to data by nonlinear regression, with simulated annealing as a stochastic optimization method, allowed the determination of all kinetic parameters. It was found that paraoxon binds to both the enzyme and acyl-enzyme, but with weak affinities (K_i = 0.123 mM and K′_i = 5.5 mM). A slight activation was observed at the lowest paraoxon concentrations and was attributed to the binding of the substrate to the enzyme-inhibitor complex. The bimolecular inhibition rate constant k_i = 2.8 × 10⁴ M⁻¹ s⁻¹ was in agreement with previous studies. It is hoped that the methods developed in this work will contribute to extending the application range of microcalorimetry in the field of irreversible inhibitors.     

Calcium-activated butyrylcholinesterase in human skin protects acetylcholinesterase against suicide inhibition by neurotoxic organophosphates

The human epidermis holds an autocrine acetylcholine production and degradation including functioning membrane integrated and cytosolic butyrylcholinesterase (BuchE). Here we show that BuchE activities increase 9-fold in the presence of calcium (0.5x10(-3)M) via a specific EF-hand calcium binding site, whereas acetylcholinesterase (AchE) is not affected. (45)Calcium labelling and computer simulation confirmed the presence of one EF-hand binding site per subunit which is disrupted by H(2)O(2)-mediated oxidation. Moreover, we confirmed the faster hydrolysis by calcium-activated BuchE using the neurotoxic organophosphate O-ethyl-O-(4-nitrophenyl)-phenylphosphonothioate (EPN). Considering the large size of the human skin with 1.8m(2) surface area with its calcium gradient in the 10(-3)M range, our results implicate calcium-activated BuchE as a major protective mechanism against suicide inhibition of AchE by organophosphates in this non-neuronal tissue.     

DNA mutations associated with the human butyrylcholinesterase J-variant.

The J-variant of human serum butyrylcholinesterase (BChE) causes both an approximately two-thirds reduction of circulating enzyme molecules and a corresponding decrease in the level of BChE activity present in serum. Since the level of serum BChE activity and the duration of succinylcholine apnea are inversely correlated, this marked decrease in activity makes individuals with the J-variant more susceptible than usual subjects to prolonged apnea from succinylcholine. We reinvestigated the same family in which Garry et al. identified the J-variant phenotype. The atypical, fluoride, and K-variant mutations were also identified in members of the 47-person pedigree. DNA amplification by PCR, followed by direct sequencing of the amplified DNA, led to the finding that the J-variant phenotype of human serum BChE was associated with two DNA point mutations in the coding region. One of these was the mutation previously identified with the K-variant phenotype (GCA----ACA; Ala539----Thr). The other was an adenine-to-thymine transversion at nucleotide 1490, which changed amino acid 497 from glutamic acid to valine (GAA----GTA; Glu497----Val). This latter point mutation was named the J-variant mutation (formal name BCHE*497V). The J-variant mutation has not been identified without the K-variant mutation. The J-variant mutation created an RsaI-enzyme RFLP. Two additional point mutations, located in the noncoding regions of the gene, were also found to be linked with the J-variant and K-variant point mutations on the same allele. These noncoding polymorphic mutations had previously been found linked to the atypical and K-variant point mutations. A summary table shows dibucaine, fluoride, and Hoffmann-La Roche compound Ro 2-0683 inhibition numbers for 119 samples whose DNA has been sequenced. Eighteen BChE genotypes are represented.     

Kinetic evidence for thermally induced conformational change of butyrylcholinesterase

The effect of temperature on the kinetics of human plasma butyrylcholinesterase-catalyzed reactions was studied. The Arrhenius plot of o-nitrophenylbutyrate hydrolysis presents a break at 21 degrees C. However, nucleophilic competition data indicate that there is no change in the rate-limiting step of the overall reaction. In addition, the temperature dependence of the bimolecular rate constant of enzyme carbamylation shows a break at 18 degrees C. These results argue for the existence of thermally induced conformational active states of the enzyme tetramer. It is suggested that the effects of this transition on kinetics arise at the acylation step.     

Kinetic properties and small-molecule inhibition of human myosin-6

Myosin-6 is an actin-based motor protein that moves its cargo towards the minus-end of actin filaments. Mutations in the gene encoding the myosin-6 heavy chain and changes in the cellular abundance of the protein have been linked to hypertrophic cardiomyopathy, neurodegenerative diseases, and cancer. Here, we present a detailed kinetic characterization of the human myosin-6 motor domain, describe the effect of 2,4,6-triiodophenol on the interaction of myosin-6 with F-actin and nucleotides, and show how addition of the drug reduces the number of myosin-6-dependent vesicle fusion events at the plasma membrane during constitutive secretion.     

Kinetic characterization of inhibition of human thrombin with DNA aptamers by turbidimetric assay

A sensitive turbidimetric method for detecting fibrin association was used to study the kinetics of fibrinogen hydrolysis with thrombin. The data were complemented by high-performance liquid chromatography (HPLC) measurements of the peptide products, fibrinopeptides released during hydrolysis. Atomic force microscopy (AFM) data showed that the fibril diameter is the main geometric parameter influencing the turbidity. The turbidimetric assay was validated using thrombin with the standard activity. To study thrombin inhibitors, a kinetic model that allows estimating the inhibition constants and the type of inhibition was proposed. The kinetic model was used to study the inhibitory activity of the two DNA aptamers 15-TBA (thrombin-binding aptamer) and 31-TBA, which bind to thrombin exosites. For the first time, 31-TBA was shown to possess the competitive inhibition type, whereas the shortened aptamer 15-TBA has the noncompetitive inhibition type.     

Flow microcalorimetric study of butyrylcholinesterase kinetics and inhibition

The enzymatic hydrolysis of butyrylcholine, catalyzed by horse serum butyrylcholinesterase (EC 3.1.1.8), was studied at 37 degrees C in Tris buffer (pH 7.5) by flow microcalorimetry. A convolution procedure, using the Gamma distribution to represent the impulse response of the calorimeter, was developed to analyze the microcalorimetric curves. After correction for buffer protonation, the hydrolysis reaction was found to be slightly endothermic, with Delta H=+9.8 kJ mol(-1). Enzyme kinetics was studied with both the differential and integrated forms of the Michaelis equation with equivalent results: Michaelis constant K(m)=3.3mM, catalytic constant k(cat)=1.7 x 10(3)s(-1), bimolecular rate constant k(s)=5.1 x 10(5)M(-1)s(-1). The reaction product, choline, was found to be a competitive inhibitor with a dissociation constant K(i)=9.1mM. Betaine had a slightly higher affinity for the enzyme, but the inhibition was only partial. This study confirms the usefulness of microcalorimetry for the kinetic study of enzymes and their inhibitors.     

Aminopeptidase M from human liver. II. Kinetic analysis of inhibition of the enzyme by bile acids

The mechanism of inhibition of aminopeptidase M by bile acids was analyzed by application of the specific velocity plot that was introduced by Baici [Eur. J. Biochem. 119, 9-14 (1981)]. Kinetic studies with three bile acids (cholic acid, deoxycholic acid, and chenodeoxycholic acid) and three substrates (Leu-Met, Leu-Gly, and Leu-pNA) showed that the inhibition constants Ki for the bile acids were appreciably different from each other, but that the Ki for each was not affected by the substrates used, being 0.89-1.03 mM for cholic acid, 0.42-0.66 mM for deoxycholic acid, and 0.24-0.31 mM for chenodeoxycholic acid. The values of the kinetic coefficient alpha [(apparent Ks in the presence of inhibitor)/Ks] for cholic acid with Leu-Met and Leu-Gly were 9.0 and 2.5, respectively. These values were very similar to those for chenodeoxycholic acid (7.0 and 2.7) but smaller than those for deoxycholic acid (21 and 11). The values of the other kinetic coefficient beta [(apparent kp in the presence of inhibitor)/kp] were 0 except in the case of the combinations of Leu-Gly with cholic acid (0.33) and Leu-Gly with chenodeoxycholic acid (0.13). On the basis of these kinetic parameters, the inhibitions by bile acids were classified into 4 types: competitive-noncompetitive linear mixed type (1 less than alpha less than infinity, beta = 0), noncompetitive-uncompetitive linear mixed type (0 less than alpha less than 1, beta = 0), pure noncompetitive type (alpha = 1, beta = 0), and hyperbolic mixed type (1 less than alpha less than infinity, 0 less than beta less than 1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of human butyrylcholinesterase variants with bambuterol and terbutaline

Bambuterol, a dimethylcarbamate, carbamoylates butyrylcholinesterase (BChE; EC 3.1.1.8). The carbamoylated enzyme is not very stable and the final product of the two-step hydrolysis is a bronchodilator drug, terbutaline (1-(3,5-dihydroxyphenyl)-2-t-butylamino-ethanol sulphate). Both bambuterol and terbutaline inhibit BChE, but their affinities differ in human serum BChE variants (U, A, F, K and S) due to their positive charge. Bambuterol inhibition rate constants for the homozygous usual (UU), Kalow (KK), fluoride-resistant (FF) or atypical (AA) variant ranged from 4.4 to 0.085min (-1)microM(-1). Terbutaline showed competitive reversible inhibition for all BChE variants. The dissociation constants for UU, FF and AA homozygotes were 0.18, 0.31 and 3.3 mM, respectively. The inhibition rate or dissociation constants for heterozygotes were distributed between the respective constants for the corresponding homozygotes. A 50-fold difference in inhibition between the UU and AA enzyme might affect terbutaline release in humans. The affinity of all studied BChE variants for terbutaline was low, which suggests that terbutaline originating from bambuterol hydrolysis should not affect the hydrolysis of bambuterol by BChE.     

Kinetic properties and inhibition of human T lymphoblast deoxycytidine kinase

The kinetic properties of 50,000-fold purified cultured human T lymphoblast (MOLT-4) deoxycytidine kinase were examined. The reaction velocity had an absolute requirement for magnesium. Maximal activity was observed at pH 6.5-7.0 with Mg:ATP for 1:1. High concentrations of free Mg2+ or free ATP were inhibitory. Double reciprocal plots of initial velocity studies yielded intersecting lines for both deoxycytidine and MgATP2-. dCMP was a competitive inhibitor with respect to deoxycytidine and ATP. ADP was a competitive inhibitor with respect to ATP and a mixed inhibitor with respect to deoxycytidine. dCTP, an important end product, is a very potent inhibitor and was a competitive inhibitor with respect to deoxycytidine and a non-competitive inhibitor with respect to ATP. TTP reversed dCTP inhibition. The data suggest that (a) MgATP2- is the true substrate of deoxycytidine kinase; (b) the kinetic mechanism of deoxycytidine kinase is consistent with rapid equilibrium random Bi Bi; (c) deoxycytidine kinase may be regulated by its product ADP and its end product dCTP as well as the availability of deoxycytidine. While many different nucleotides potently inhibit deoxycytidine kinase, their low intracellular concentrations make their regulatory role less important.     

Kinetic and structural analysis of the inhibition of adenosine deaminase by acetaminophen

Kinetic and thermodynamic studies have been made on the effect of acetaminophen on the activity and structure of adenosine deaminase in 50 mM sodium phosphate buffer pH 7.5, at two temperatures of 27 and 37 degrees C using UV spectrophotometry, circular dichroism (CD) and fluorescence spectroscopy. Acetaminophen acts as a competitive inhibitor at 27 degrees C (Ki = 126 microM) and an uncompetitive inhibitor at 37 degrees C (Ki = 214 microM). Circular dichroism studies do not show any considerable effect on the secondary structure of adenosine deaminase by increasing the temperature from 27 to 37 degrees C. However, the secondary structure of the protein becomes more compact at 37 degrees C in the presence of acetaminophen. Fluorescence spectroscopy studies show considerable change in the tertiary structure of the protein by increasing the temperature from 27 to 37 degrees C. Also, the fluorescence spectrum of the protein incubated with different concentrations of acetaminophen show different inhibition behaviors by the effector at the two temperatures.     

Electrophoretic study of aged butyrylcholinesterase after inhibition by soman

The following states of purified tetrameric form (C₄) of human plasma butyrylcholinesterase were studied by electrophoretic techniques: native, inhibited by soman and by methane sulfonyl fluoride and soman-aged.In order to detect a significant conformational change of the aged cholinesterase as compared to the non-inhibited (native) species, enzymes were treated with a set of bifunctional reagents (diimidates) of different chain lengths. After denaturation, the cross-link products were subjected to sodium dodecyl-sulfate polyacrylamide gel electrophoresis. The peak areas of the cross-linked species and the parameters of cross-linkability were calculated from densitometric data, versus the maximal effective reagent length.The effect of occupancy of the esteratic site by substituted phosphonyl group and by methyl-sulfonyl residue on the binding activity of the anionic site was studied by affinity electrophoresis at varying temperatures with immobilized-procaïnamide as ligand. Apparent dissociation-constants of the enzyme-ligand complexes were estimated from measurement of mobilities versus ligand concentration. Corresponding thermodynamic quantities were calculated from Van't Hoff plots and basic thermodynamic equations.The reactivity of aged-cholinesterase with diimidates was similar to that of the native enzyme. Affinity for immobilized-procaïnamide was slightly lowered in aged and inhibited enzymes as compared to the native and sulfonylated enzymes. As for the ligand-induced isomerization of anionic site (A → B), revealed by affinity electrophoresis, the ligand concentration at the midpoint of transition ($\text{A}\text{= 0.5}$) was slightly greater for the aged enzyme than for the native one.From these results, the following conclusions can be drawn: the dealkylation of somancholinesterase conjugate (aging) does not seem to induce structural changes detectable in the cross-linkability of lysyle residues at the subunit interfaces andthe surface of the tetrameric enzyme. On the other hand, the affinity of the anionic site and ligand-induced isomerization process are altered in soman-inhibited and aged enzymes. These data suggest the occurence of a weak conformational change of the active center and/or the formation of non-covalent bonds between the methylphosphonyl residue and side chain groups as a result of the dealkylation reaction.