The cholinesterases of human blood I. The specificity of the plasma enzyme and its relation to the erythrocyte cholinesterase

The contribution of charge to affinity at functional (M3) muscarinic receptors in guinea-pig ileum assessed from the effects of the carbon analogue of 4-DAMP methiodide

1. 4-Diphenylacetoxy-1:1-dimethyl cyclohexane (carbo-4-DAMP) is the carbon analogue of 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) methiodide. The compounds differ only in that the quaternary nitrogen atom in 4-DAMP methiodide is replaced by a quaternary carbon atom, which is uncharged. 2. Carbo-4-DAMP appears to act competitively at functional (M3) muscarinic receptors in guinea-pig ileum. Estimates of log affinity constant are 6.0 at 30 degrees C and 5.9 at 37 degrees C, i.e. the compound has 0.1% of the affinity of 4-DAMP methobromide. 3. The absence of charge makes little difference to the conformation as determined by X-ray crystallography. The bond lengths and angles are very similar, though the bonds in the cyclohexane ring of carbo-4-DAMP are consistently slightly longer than those in the piperidinium ring of 4-DAMP methiodide, and the presence of the charge slightly reduces the space between molecules. 4. The difference between the affinities of 4-DAMP methobromide and carbo-4-DAMP indicates that the contribution of coulombic forces to the binding between 4-DAMP methiodide and muscarinic (M3) receptors is at least 17 kJ mol-1 (4.1 kcal mol-1) at 37 degrees C. How much this is an underestimate depends upon how much hydrophobic binding is greater with the uncharged compound.

Physostigmine analogs anticholinesterases: effects of the lengthening of the N-carbamic chain on the inhibition kinetics

Data are presented about the inhibitor power of new carbamates against acetylcholinesterase. The study was carried out on two series of physostigmine analogs, N-alkyl and N-methyl,N-alkylphysostigmines. For these inhibitors, the second-order rate constants for inhibition, ki, and the first-order rate constants of reactivation, k3, have been determined. From the reported results, electronic, hydrophobic and steric effects, due to the enhancement of the alkyl chain, may have influenced all kinetics parameters discussed. In comparison to physostigmine, both the new N-methyl,N-alkylphysostigmines and the N-alkylphysostigmines showed a non-linear decrease in the values of ki and k3. This study presents the hydrophobic interactions as an important factor not only in determining carbamylation but also decarbamylation rates constants.

An automated method for the determination of acetyl and pseudo cholinesterase in hemolyzed whole blood

The aim of the present study was to develop a method which allows determination of pseudo (PsChE) and acetyl cholinesterase (AChE) activities in single hemolyzed blood samples of workers exposed to cholinesterase-inhibiting compounds, avoiding the time-consuming and laborious separation of plasma and erythrocytes. Two methods based on Ellman's colorimetric determination of cholinesterase activity were compared, and three different substrates were tested. The best results were obtained with the substrates butyrylthiocholine and acetyl(beta-methyl)thiocholine, both showing a substrate specificity of more than 97% with respect to PsChE and AChE, respectively. The method showed sensitivity to detect low levels of inhibition of AChE and PsChE in blood. The between-day precision was less than 4% for both cholinesterase activities. It was demonstrated with this method that hemolyzed blood can be stored at -20 degrees C at least 18 months without loss of cholinesterase activity. The method has been used for 18 months in a monitoring program for laboratory employees working with various cholinesterase-inhibiting compounds. The average co-efficients of intraindividual variation amounted to 6.8% (range 2.2-9.6%; 90 percentile, 8%) and 6.6% (range 2.9-9.9%; 90 percentile, 7.9%) for PsChE and AChE, respectively. In a group of non-exposed workers the average intraindividual variations were 4.0% (range 1.5-7.7%; 90 percentile, 7.6%) and 3.6% (range 0.6-6.6%; 90 percentile, 5.3%), respectively. Using the value of 4.0%, it appears possible to detect an individual decrease in cholinesterase activity of more than 8% below a baseline based on three determinations. The method can thus be used to detect relatively low levels of cholinesterase inhibition.

An inhibitory monoclonal antibody to human acetylcholinesterases

The monoclonal antibody AE-2 raised against acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) from human erythrocytes is shown to inhibit the enzyme activity. The reaction of the antibody with a structural epitope is investigated further. The epitope resides on monomeric, dimeric and tetrameric species of the enzyme. The rate of phosphorylation of the enzyme by diisopropylfluorophosphate was not affected by the antibody. On the other hand, inhibitors directed towards the anionic site(s) competed with antibody binding, suggesting that one of these is the epitope. The titration with antibody is biphasic and yields about 80% inhibition even in the presence of a large excess of antibody. Inhibition is fully reversible upon dilution, in a time-dependent manner. AE-2 also inhibited human adult and fetal brain acetylcholinesterase (to the same extent). However bovine brain acetylcholinesterase was inhibited to a lesser extent and rat brain acetylcholinesterase did not interact with the antibody. Butyrylcholinesterase (EC 3.1.1.8) also showed no reactivity towards the antibody.

Structure-activity relationships in acetylcholinesterase reactions. Hydrolysis of non-ionic acetic esters

The Michaelis-Menten parameters kcat, Ks(app) and the second-order rate constants kII = k2/Ks of acetylcholinesterase-catalyzed hydrolysis of 25 acetic esters with non-ionic leaving groups have been determined at 25 degree C and pH 7.5 in 0.15 M KCL. A linear relationship between the substrate noncovalent binding capacity and the leaving group hydrophobicity, and a multiparameter correlation of the acetylation reaction rate constant logarithm with the leaving group inductive effect, hydrophobicity, and steric effect, have been established. The acetyl-enzyme deacetylation rate constant has been calculated. Taken together, a fairly complete understanding of acetylcholinesterase specificity is possible. The data are consistent with a model of the acetylcholinesterase active site, in which the catalytically active groups are located at the bottom of a jaws-like slit with a limited range of hydrophobic walls that provide the sorption of the substrate leaving groups not longer than that in n-butyl acetate.