Kinetics of the inhibition of acetylcholinesterase from pigeon brain by procaine hydrochloride

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.

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.

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.

Purification and characterization of acetylcholinesterase from desert cobra (Walterinnesia aegyptia) venom

Acetylcholinesterase (AChE) has been identified and purified from the venom of desert cobra (W aegyptia) to apparent homogeneity using a TSK G 3000 SW gel filtration column and a Mono Q anion-exchange column. AChE was purified to homogeneity as established by sodium dodecylsulfate/polyacrylamide gel electrophoresis. The specific activity of AChE was 357 IU/mg with acetylthiocholine iodide as substrate. The denatured W aegyptia venom AChE displayed a molecular mass of 67000 +/- 3000 Da suggesting it was a single polypeptide. Isoelectric focusing of AChE revealed that the enzyme exists in different isoforms, with isoelectric points ranging between pH 7.4-7.9. The kinetic parameters (Km and Vmax) and IC50 of AChE inhibition by procaine, tetracaine and physostigmine were investigated in the present study.

Acetylcholinesterase from desert cobra (Walterinnesia aegyptia) venom: optimization and kinetics study

Acetylcholinesterase (AChE) was investigated in Walterinnesia aegyptia venom and characterized with respect to its kinetic properties. It was found that 4.0 micrograms of crude venom protein and an incubation time of 4.0 min were suitable conditions for linearity of AChE activity at 25 degrees C. The optimum strength of the sodium phosphate buffer was 0.05 M, and the optimum pH was 7.75. The optimum temperature was 30 degrees C. The activation energy and the heat of activation were observed to be 6510 and 5922 cal/mole. The AChE was specific for acetylthiocholine but it did not hydrolyse butyrylthiocholine. The optimum substrate concentration was 3.0 mM but 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.