Sensitivity of bovine retinal acetylcholinesterase (E.C. 3.1.1.7) toward tacrine: kinetic characterization

This work addresses the kinetic analysis of the interaction of tacrine with bovine retina acetylcholinesterase (A ChE, E.C. 3.1.1.7). It was found that the tacrine effect was reversible in nature. Tacrine inhibited bovine retinal AChE activity in a concentration-dependent manner; IC50 was fo to be 8.07 nM. The Michaelis-Menten constant (Ka) for the hydrolysis of acetylthiocholine iodide (ASCh) by AChE was 0.061 mM in the control system, and this value was increased by 54-67% in the tacrine-treated systems. The Vmax was 0.701 mumole/min per milligram protein for the control system, but it was decreased by 26-69% in the tacrine-treated systems. The Lineweaver-Burk plot, Dixon plot, and their secondary replots indicated that the nature of the inhibition was of the partial mixed type, that is, a mixture of competitive and noncompetitive inhibition. The values of Ki and Kt were estimated to be as 4.475 and 8.517 nM, respectively.

Thermodynamic analysis of human retinal acetylcholinesterase inhibition using an anti-Alzheimer's drug, tacrine, through the development of a dual substrate and temperature model

The present study determines the energy parameters, such as the Gibb's free energy change (deltaG), enthalpy change (deltaH), heat of activation (deltaH*), entropy change (deltaS), temperature coefficient (Q10) and activation energy (Ea), of human retinal acetylcholinesterase (AChE, EC 3.1.1.7) inhibition by tacrine. The stereo-frequency collisions factor (PZ, the number of sterically and energetically favorable collisions occurring between tacrine and AChE) was also studied in this investigation. Tacrine significantly increased the value of deltaG, deltaH, deltaH*, Q10, Ea and PZ factor, and decreased the value of deltaS for AChE. Since there is no known report on the inhibition of human retinal AChE by tacrine, these results were compared with the reported values for the energy parameters of camel retinal and chicken brain AChE inhibition by an anti-cancer drug, cyclophosphamide. The uniqueness of this approach lies in the development of the 'dual substrate and dual temperature' model, which may open up a new, more efficient avenue for the study of various enzyme catalyzed reactions.

Kinetic analysis of the toxicological effect of tacrine (Cognex) on human retinal acetylcholinesterase activity

For the first time, kinetic parameters of the effect of tacrine, an anti-cholinesterase inhibitor of therapeutic potential in Alzheimer's disease has been studied on human retinal acetyl-cholinesterase (AChE). Tacrine inhibited the AChE activity in a concentration dependent manner, the IC(50) being about 45 nM. The Michaelis-Menten constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.120 mM and this value was increased by 4-52.8% in the presence of tacrine. V(max) was observed to be 2.23 micromol/h per mg protein for the control system, while it was decreased by 14.73-56.25% in the tacrine treated systems. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the mixed type, i. e. a combination of competitive and noncompetitive inhibition. The values of K(i) and K(I) were estimated to be as 37.76 and 64.36 nM, respectively.

Human erythrocyte acetylcholinesterase inhibition by cis-diamminediaquaplatinum (II): a novel kinetic approach

The present work addresses the analyses of some novel kinetic parameters (k(t), K(v), t50, K(ir), t(c), m(c), IC50, IC99 and Ki) of human erythrocyte membrane-bound acetylcholinesterase (AChE, EC 3.1.1.7) inhibition by cis-diamminediaquaplatinum II (PDC). PDC is under a clinical trial for use as an antineoplastic drug. The authors recently reported that PDC and cisplatin have the ability to inhibit AChE activity in vitro. Therefore this study was designed to determine the estimation of time constant (k(t)), velocity constant (K(v)), 50% inhibition time (t50), inhibition rate constant (K(ir)), transition concentration (t(c)), meeting concentration (m(c)), 50% inhibition (IC50), 99% inhibition (IC99) and inhibition constant (Ki) by novel methods. The details are described in the text.

Kinetics of human erythrocyte acetylcholinesterase inhibition by a novel derivative of physostigmine: phenserine

The effect of phenserine, a novel cholinesterase inhibitor, was assessed for the first time on kinetic parameters of human erythrocyte acetylcholinesterase (AChE). Phenserine (0.025-0.40 microM) inhibited the activity of human erythrocyte AChE in a concentration-dependent fashion, the IC50 was 0.0453 microM. The Michaelis-Menten constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.124 mM and the Vmax was 0.980 mumol/min/mg protein. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the noncompetitive type. The value of Ki was estimated as 0.048 microM by the primary and secondary replots of the Dixon as well as secondary replots of the Lineweaver-Burk plot. A novel relationship between Ki and substrate concentration was also identified which permits more precise prediction of the specific type of noncompetitive inhibition of various enzymes by a wide variety of drugs, chemicals and, in some circumstances, by their own substrates.

Kinetics of the inhibition of acetylcholinesterase in camel retina by cisplatin

The inhibitory effect of cisplatin (CDDP) on camel retina acetylcholinesterase (AChE) was characterized. The CDDP effect was independent of the time of incubation with AChE before the addition of substrate, indicative of reversible inhibition. Moreover, dilution data prove that CDDP is a reversible inhibitor of camel retina AChE. Cisplatin inhibited AChE activity of camel retina in a concentration- and time-dependent manner, the IC50 values being 5.32 and 0.196 mM at 5 min and 24 h incubation times, respectively. The IC50 has dual components, i.e. directly proportional and inversely proportional to 0-1.5 h and 1.5-24 h incubation periods, respectively. The Michaelis-Menten constant (Km) for the hydrolysis of acetylthiocholine iodide (ASCh) was found to be 0.0796 mM and Vmax was 0.668 micromol/min/mg protein. Kmapp and Vmaxapp both decreased as the CDDP concentration increased. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the pure uncompetitive type. The value of Ki was estimated as 0.811 mM by the primary and secondary replots of the Lineweaver-Burk and Dixon plots. Kiapp decreased while Vmaxiapp increased after increasing the ASCh concentration.

On the inhibition of camel retina acetylcholinesterase activity by cycloheximide in vitro

The kinetic parameters of inhibition of camel retinal acetylcholinesterase (AChE) activity by cycloheximide (CH) were investigated. For the control system, the Michaelis-Menten constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.076 mmol/L and the Vmax was 0.547 mumol/min per mg protein. In contrast, these parameters were decreased in the CH-treated systems. Dixon and Lineweaver-Burk plots, and their secondary replots, indicated that the inhibition was of the linear mixed type, which seems to be a combination of partial competitive and pure noncompetitive inhibition. The values of K'i(slope) and KI(intercept) were estimated to be 3.50 and 5.68 mmol/L, respectively. Ki was greater than K'i, indicating that CH has a greater binding affinity for the peripheral site than the active site.

Evaluation of the nature of camel retinal acetylcholinesterase: inhibition by hexamethonium

Acetylcholinesterase (AChE, EC 3.1.1.7) has been demonstrated in retinas of several species, however, the nature of the interaction of AChE with specific inhibitors are very limited in the literature and the mode of inhibition of camel retinal AChE by hexamethonium has been studied. Hexamethonium reversibly inhibited AChE in a concentration dependent manner, the IC50 value being c. 2.52 mM. The Km for the hydrolysis of acetylthiocholine iodide was found to be 0.087 mM and the Vmax was 0.63 mumol/min/mg protein. Dixon, as well as Lineweaver-Burk, plots and their secondary replots indicated that the nature of the inhibition is of the hyperbolic (partial) mixed type, which is considered to be a partial competitive and non-competitive mixture. The values of Ki(slope) and KI(intercept) from a Lineweaver-Burk plot were estimated as 0.30 mM and 0.17 mM, respectively, while Ki from a Dixon plot was estimated as 0.725 mM. The Ki was greater than KI indicating that hexamethonium has a greater affinity of binding for the active site than the peripheral site of the camel retina AChE.

Investigation of the effect of lannate on kinetic parameters of acetylcholinesterase: slightly concave mixed-type of inhibition system

The effect of lannate on kinetic parameters of camel retina acetylcholinesterase is investigated in the present study. The Ks for the hydrolysis of acetylthiocholine iodide was found to be 0.086 mM in the control system; a value that increased in the lannate treated systems. The Vmax was 0.853 mumole/min/mg protein for the control system while it decreased in the lannate treated systems. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition is of the slightly concave mixed type, which is considered to be a 'semi-pure competitive' and 'semi-partial as well as semi-pure non-competitive' mixture. The values of Ki(slope) and KI(intercept) were estimated as 0.143 microM and 0.179 microM respectively. The KI was greater than Ki indicating that lannate has a greater affinity of binding for the peripheral site than the active site of the camel retina AChE.

Estimation and correlation of IC50 for the inhibition of human erythrocyte acetylcholinesterase by cis-diamminediaquaplatinum (II)

The present work addresses the estimation and mode of aquiring fifty per cent inhibition of human erythrocyte membrane-bound acetylcholinesterase (AChE, EC 3.1.1.7) by cis-diamminediaqua-platinum II (DDP), which is presently in clinical trials for use as an antineoplastic drug. It has been recently reported that cisplatin itself has the ability to inhibit the AChE activity in vitro [Al-Jafari, et al, 1995; Kamal and Al-Jafari, 1996]. Therefore, this study was focused on the estimation of the IC50 of AChE inhibition by DDP, and its correlation with reaction times. It was found that 0.0-20.0% and 53.8-94.5% AChE inhibition takes place at 3.0 to 60 minutes after 0.025 and 0.40 mM DDP administration, respectively. The IC50 was proportional to the reaction period, and gave values of 0.057 to 0.918 mMat reaction times ranging from 3.0 to 60.0 minutes, respectively. The DDP has 1025 and 67 times higher inhibitory potency than cisplatin for human erythrocyte AChE at 3.0 and 60.0 minutes reaction time respectively. In the light of these findings, particular attention should be paid to DDP in tumor therapy and its inhibitory effect on AChE must be considered during the decision whether to use it as an antineoplastic drug.

Kinetics of human erythrocyte acetylcholinesterase inhibition by cis-diamminediaquaplatinum (II)

This work addresses the kinetic studies for the interaction of cis-diamminediaquaplatinum (II) ¿cis-[Pt(NH3)2(H2O)2]2+¿, (DDP) with human erythrocyte acetylcholinesterase (AChE). The Ks for the hydrolysis of acetylthiocholine iodide (ASCh) by AChE was 0.077 mM in the control system, the value decreased by 14-38% in the DDP treated systems. The Vmax was 1.34 mumole/min/mg protein for the control system while it was decreased by 26-57% in the DDP treated systems. The Lineweaver-Burk plot, Dixon plot and their secondary replots indicated that the nature of the inhibition was of the linear mixed type, i.e. uncompetitive and noncompetitive. The values of Ki and Ki were estimated as 0.203 and 0.096 mM, respectively. The estimated values of Ki and KI in the present investigation were compared with reported values for human erythrocyte and camel retina AChE inhibition by some antineoplastic drugs: cisplatin, cyclophosphamide and methotrexate.

Dual temperature model for the estimation of energetics parameters for acetylcholinesterase inhibition by cyclophosphamide

The present work addresses the 'dual temperature' model for the estimation of Gibb's free energy change (delta G), enthalpy change (delta H), heat of activation (delta H.) entropy change (delta S), temperature coefficient (Q 10) and activation energy (Ea) of chicken brain acetylcholinesterase (AChE) inhibited by cyclophosphamide monohydrate (CP). In this investigation, the PZ factor (number of sterically and energy wise favorable collisions occurring between CP and AChE) has also been studied. The inhibitor has considerably increased all energetics parameters except delta S and Q10. These results are in general agreement with the data from the other reported studies. The significance of the use of CP in cancer therapy and various aspects of thermodynamic parameters have also been discussed.

Effect of malathion on kinetic parameters of acetylcholinesterase (EC 3.1.1.7) in vitro

Kinetic analysis of the interaction of malathion with camel erythrocyte acetylcholinesterase was investigated in the present study. The Michaelis-Menten constant (K(m)) for the hydrolysis of acetylthiocholine iodide (ASCh) was found to be 53.15 microM and the Vmax was 0.287 mumol/min/mg protein. The Kmapp and Vmaxapp were both decreased by increased malathion concentration. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the pure uncompetitive type with Ki value estimated as 102.1 ppm. The Kiapp decreased while Vmaxiapp increased by an increased concentration in ASCh.

Effect of sevin on kinetic parameters of camel retina acetylcholinesterase

The present investigation addresses the effect of sevin (insecticide) on kinetic parameters of camel retina membrane-bound acetylcholinesterase (AChE, EC 3.1.1.7). The Michaelis-Menten constant (Ks) for the hydrolysis of acetylthiocholine iodide (ASCh) by AChE was 0.072 mM in the control system, a value decreased by 38-55% in the sevin treated systems. The Vmax was 0.967 mumole/min/mg protein for the control system while it was decreased by 50-84% in the sevin treated systems. The Lineweaver-Burk plot, Dixon plot, and their secondary replots indicated that the nature of the inhibition was of the linear mixed type, i.e. uncompetitive and noncompetitive. The values of Ki(slope) and KI(intercept) were estimated as 6.194 and 2.811 microM, respectively. The turnover number (Kcat) and specificity constant (Ksp) were 74.65 min-1 and 10.37 x 10(5) (M.min)-1 in the control system respectively, while the values for both parameters were significantly decreased in the sevin treated systems.

Thermodynamic investigation of camel retina acetylcholinesterase inhibition by cyclophosphamide

The present work addresses the estimation of energy parameters such as Gibb's free energy change (delta G), enthalpy change (delta H); entropy change (delta S) and activation energy (E a) of acetylcholinesterase (AChE, EC 3.1.1.7) from camel retina in the absence and presence of the antineoplastic drug cyclophosphamide (CP). A spectrophotometric method was used for the determination of AChE activity, which was the basis for determination of these parameters. The PZ factor (number of sterically and energetically favorable collisions occurring between CP and AChE) have also been studied in this investigation. The energy parameters obtained in the present investigation were compared with the values reported elsewhere.

Characterization of human erythrocyte membrane-bound acetylcholinesterase inhibition by cisplatin at reversible phase

Acetylcholinesterase (AChE) is a perfect hydrolyzing enzyme, and it has been recently reported [Al-Jafari et al 1995; Kamal and Al-Jafari, 1996] that cisplatin, which is cytotoxic, has the ability to inhibit the AChE activity in vitro. I therefore studied the characterization of this inhibition with respect to establishment of kinetic parameters at the reversible stage only. Conventional kinetic methods have been used in the present study. Some secondary replots, equilibrium schemes and some equations for this type of inhibition are novel for precise kinetic analysis in this inhibition system. The Michaelis-Menten constant (Km) for the hydrolysis of acetylthiocholine iodide (ASCh) was found to be 0.0994 mM and the Vmax was 1.179 mumol/minute/mg protein. The Kmapp and Vmaxapp were both decreased by increasing cisplatin concentrations. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the pure uncompetitive type. The value of Ki was estimated as 3.593 mM by the primary and secondary replots of the Lineweaver-Burk plot and Dixon plot. The Kiapp decreased while Vmaxiapp increased with increases in the ASCh concentration. This opens the possibilities for explaining the mechanism of AChE interaction with cisplatin as well as approaches towards understanding the causes of the neuro-type side effects of this drug.

Kinetics of the inhibition of acetylcholinesterase from desert cobra (Walterinnesia aegyptia) venom by local anesthetics: procaine and tetracaine

The kinetic parameters of W. aegyptia venom acetylcholinesterase (AChE) inhibition by procaine and tetracaine hydrochloride were investigated in the present study. Procaine and tetracaine reversibly inhibited the AChE activity in a concentration-dependent manner, the IC50 being about 0.28 and 0.04 mM, respectively. The Michaelis-Menten constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.051 mM with Vmax 10.2 mumole/min/mg protein. Both K(m) and Vmax were affected by procaine while only Vmax decreased with tetracaine. A Lineweaver-Burk plot and its secondary replot indicated that the nature of the inhibition is of the linear mixed type for procaine which is considered to be a mixture of competitive and noncompetitive types while the inhibition was noncompetitive for tetracaine. The values of Ki(slope) and K(intercept were estimated as 0.133 mM and 0.451 mM for procaine and 7.2 x 10(-3) mM for tetracaine, respectively, by the secondary replots of the Lineweaver-Burk plot.

The inhibitory effect of cyclophosphamide on camel retina acetylcholinesterase activity

Kinetic parameters for the effect of cyclophosphamide (CP) on the camel retina acetylcholinesterase (AChE) activity were investigated for the first time in the present study. It was found that 18 micrograms of retina protein and an incubation time of 4.0 min were suitable conditions for linear of AChE activity. The CP effect was independent of time of incubation with AChE before the addition of substrate, which shows it reversible action. Moreover, dilution data prove that CP is a reversible inhibitor of camel retina AChE. Cyclophosphamide (0.2-2.4 mM) inhibited activity of camel retina in a concentration-dependent fashion, the IC50 being about 1.17 mM. The Michaelis constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.106 mM and the Vmax was 0.765 mumol/min/mg protein. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the pure noncompetitive type. The value of Ki was estimated as 0.763 mM by the primary Dixon and secondary replots of the Lineweaver-Burk plot.

Kinetics for camel (Camelus dromedarius) retina acetylcholinesterase inhibition by methotrexate in vitro

This work addresses the kinetic analysis of the interaction of methotrexate (MTX) with camel retina acetylcholinesterase (A ChE, EC 3.1.1.7). It was found that the MTX effect was reversible in nature. The IC50 was determined, by two methods, to be 1.362 mM. The Michaelis-Menten constant (Ks) for the hydrolysis of acetylthiocholine iodide (ASCh) by AChE was 0.123 mM in the control system, and the MTX-treated systems showed a 10-35% decrease in this value. The Vmax was 0.789 mumol/min/mg protein for the control system, while it was decreased by 23-76% in the MTX-treated systems. The Lineweaver-Burk plot, Dixon plot and their secondary replots indicated that the inhibition was a linear mixed type; i.e., uncompetitive and noncompetitive. The values of Ki and KI were estimated as 0.782 and 0.404 mM, respectively. The use of camel retina as a model for the study of human retina may open new avenues for studying various aspects of AChE.

Investigation of the effect of tetrahydroaminoacridine on turnover kinetics of camel (Camelus dromedarius) retina acetylcholinesterase

The effect of tetrahydroaminoacridine (THA) on turnover kinetics of camel retina acetylcholinesterase has been investigated. The turnover number (Kcat) and specificity constant (Ksp) were 62.1 min-1 and 9.92 x 10(5) (M. min)-1 in the control system while the values for both parameters were lowered by 25-68% and 48-87% in the THA (0.05-0.4 microM) treated systems respectively. Therefore, THA's effect on turnover number must be considered at the time of therapy to the Alzheimer's disease patients.

In vitro inhibition of human erythrocyte acetylcholinesterase (EC3.1.1.7) by an antineoplastic drug methotrexate

This work addresses the kinetic analysis of the interaction of methotrexate (MTX) with human erythrocyte membrane-bound acetylcholinesterase (AChE, EC 3.1.1.7). It was found that the MTX effect was independent of time of incubation with AChE before the addition of substrate which proves its reversible action. The IC50 was determined, by three methods, to be 0.73 mM. The Michaelis-Menten constant (Ks) for the hydrolysis of acetylthiocholine iodide (ASCh) by AChE was 0.13 mM in the control system, a value decreased by 30-61% in the MTX treated systems. The Vmax was 1.27 mumole/min/mg protein for the control system while it was decreased by 44-77% in the MTX treated systems. The Lineweaver-Burk plot Dixon Plot, and their secondary replots indicated that the nature of the inhibition was of the linear mixed type, i.e. uncompetitive and noncompetitive. The values of Ki(slope) and Ki(intercept) were estimated as 1.67 and 0.34 mM, respectively.

The preparation and kinetic analysis of multiple forms of human erythrocyte acetylcholinesterase

A method for preparing various forms of acetylcholinesterase (AChE) from human erythrocyte has been established and they have been characterized in terms of kinetic parameters such as K(m), rate constant (k), turnover number (kcat), specificity constant (ksp), Vmax, half-life (t1/2), IC50 and Ki for tetrabutylammonium hydroxide, procaine and physostigmine in the present study. The solubility experiments show that, there is one major form of AChE i.e. membrane bound AChE (MBAChE) and one minor form i.e. water soluble form (WSAChE). The MBAChE shows several subforms, and on the basis of percentage activity only three MBAChE forms have been selected for complete characterization by various kinetic parameters and found that these three forms of MBAChE demonstrate significant differences in their kinetic properties except IC50. This study supports the recommendation of the use of these kinetic parameters as a tool for the analysis of the multiple forms of the various enzymes in the biological systems.

Investigation of the effect of anti-neoplastic drugs, cyclophosphamide, cisplatin and methotrexate on the turnover kinetics of human erythrocyte acetylcholinesterase

The present work addresses the effects of three antineoplastic drugs [cyclophosphamide (CP), cisplatin (CDDP) and methotrexate (MTX) which are in current use for the treatment of various tumors] on turnover kinetics of human erythrocyte membrane-bound acetylcholinesterase (AcChoEase, EC 3.1.1.7). It was found that CP and MTX significantly decreased kcat and ksp, whereas CDDP decreased only the kcat value, (ksp was non-significantly affected). In light of these findings, the CP, CDDP and MTX needs particular attention in tumor therapy and their effects on turnover number must be considered at the time of administering these drugs to cancer patients.

Optimization and kinetic studies of human erythrocyte membrane-bound acetylcholinesterase

The human erythrocyte membrane-bound acetylcholinesterase was characterized with respect to optimal assay conditions for its kinetic properties. It was found that 40.0 micrograms protein and 5.0 min incubation time were the suitable concentration of AChE protein and reaction time for the linearity of AChE activity at 25 degrees C. The Vmax for the AChE was 35% higher in the presence of 25 mM sodium phosphate buffer than 25 mM Tris-HCl buffer. The AChE activity was assayed at various strengths of sodium phosphate buffer (0.0125-0.20 M), and the optimum strength was found to be 0.05 M. The optimum substrate (ASCh) concentration was found to be 5.0 mM whereas at higher substrate concentrations, the AChE activity declined. The ASCh concentration ranges for different orders of the reactions were determined and kinetic parameters (Km, Vmax, Kcat and Ksp) were established at each order of the reaction.

AK-5 tumor-induced modulation of host immune function: upregulation of Th-1-type cytokine response mediates early tumor regression

AK-5, a rat histiocytoma, grows as ascites and undergoes spontaneous regression upon subcutaneous transplantation. Earlier studies from this laboratory have demonstrated that immunogenic rejection of AK-5 tumor is mediated through ADCC involving CD8+ NK cells and anti-AK-5 antibody. Upon subcutaneous transplantation, 55-60% of animals initiated tumor regression between 12-15 days after tumor transplantation (early rejectors), while 40-45% did not evoke regression up to 20-25 days (late rejectors). In order to delineate this differential response among syngeneic animals to the same tumor, we have evaluated the cytokine profiles in circulation of both early and late rejecting animals. Our results show that an increase in IL-2, IFN-gamma, IL-4, IL-12 and TNF-alpha contributed to early regression, suggesting a predominantly Th-1 type of cytokine function being evoked against AK-5 tumor. Hosts with lower circulating levels of these cytokines showed delayed tumor regression. In addition, administration of anti-IL-4/anti-IL-4 + anti-IL-10 lead to a decreased antibody response to AK-5 surface antigens in vivo. Neutralization of IFN-gamma in tumor-bearing animals resulted in inhibition of NK-cell-mediated cytotoxicity against AK-5 cells and delayed the regression process. The present study suggests that early regression of AK-5 tumor depends primarily on the higher levels of circulating Th-1-type cytokines; however, the role of IL-4 and anti-AK-5 antibody in tumor regression cannot be ruled out.