STUDIES ON CHOLINESTERASE

Assay of serum cholinesterase activity by an amperometric biosensor based on a co-crosslinked choline oxidase/overoxidized polypyrrole bilayer

Based on choline oxidase immobilized by co-crosslinking on an overoxidised polypyrrole modified platinum electrode, a novel electrochemical assay for cholinesterase activity in human serum was developed. The assay was performed by adding an aliquot of cholinesterase standard solution or serum sample to phosphate buffer containing choline or thiocholine ester and measuring the oxidation current of hydrogen peroxide at the rotating modified electrode polarized at +0.7 V vs. SCE. The influence of some experimental parameters such as pH of the assay, mass transport at the electrode, type and concentration of the cholinesterase substrate was studied and optimised. Reversible inhibition of choline oxidase from cholinesterase substrates was evidenced for the first time, which increases in the order of acetylcholine, butyrylcholine and s-butyrylthiocholine. Wide linear range, fast response time and appreciable long-term stability were assured for both acethyl- and butyrylcholinesterase assays. On allowing the polypyrrole layer to efficiently remove interferences from the electroactive compounds in the sample, the present method revealed to be suitable for the detection of butyrylcholinesterase in human serum at activities as low as 0.5 U L^-1. The validation with a reference spectrophotometric method showed no significant differences when human serum samples were analysed.

Characterization of butyrylcholinesterase from porcine milk

Human butyrylcholinesterase (HuBChE) is under development for use as a pretreatment antidote against nerve agent toxicity. Animals are used to evaluate the efficacy of HuBChE for protection against organophosphorus nerve agents. Pharmacokinetic studies of HuBChE in minipigs showed a mean residence time of 267 h, similar to the half-life of HuBChE in humans, suggesting a high degree of similarity between BChE from 2 sources. Our aim was to compare the biochemical properties of PoBChE purified from porcine milk to HuBChE purified from human plasma. PoBChE hydrolyzed acetylthiocholine slightly faster than butyrylthiocholine, but was sensitive to BChE-specific inhibitors. PoBChE was 50-fold less sensitive to inhibition by DFP than HuBChE and 5-fold slower to reactivate in the presence of 2-PAM. The amino acid sequence of PoBChE determined by liquid chromatography tandem mass spectrometry was 91% identical to HuBChE. Monoclonal antibodies 11D8, mAb2, and 3E8 (HAH 002) recognized both PoBChE and HuBChE. Assembly of 4 identical subunits into tetramers occurred by noncovalent interaction with polyproline-rich peptides in PoBChE as well as in HuBChE, though the set of polyproline-rich peptides in milk-derived PoBChE was different from the set in plasma-derived HuBChE tetramers. It was concluded that the esterase isolated from porcine milk is PoBChE.

Postnatal changes in the activity ratio of specific and nonspecific cholinesterases from neuronal perikarya

A soluble fraction from rat brain neuronal perikarya was shown to contain both the specific and nonspecific forms of the enzyme acetylcholinesterase (EC's 3.1.1.7. and 3.1.1.8., respectively). The ratio of the enzyme activities varied along the course of brain development: the nonspecific form being predominant from 1 to 15 days of age and the specific one showing the pattern of rising activity from day 15 onward. We suggest a possible relationship between this changing in cholinesterase activities and the establishment of synapses within the rat cerebral cortex.

The aryl acylamidases and their relationship to cholinesterases in human serum, erythrocyte and liver

Human serum aryl acylamidase associated with serum cholinesterase was purified to homogeneity. Evidence for the identity of the two enzymes was based on co-elution profiles, co-purification in the different steps including affinity chromatography with constant ratios of specific activity and percentage recoveries, co-migration on gel electrophoresis, parallel inhibition by typical cholinesterase inhibitors and co-precipitation by antibody raised against the purified enzyme. Human liver aryl acylamidase was partially purified. Based on the elution profiles, purification data, inhibitory characteristics and gel electrophoresis it was concluded that aryl acrylamidase of liver was not associated with liver cholinesterase. More conclusive evidence for the non-association of the liver aryl acylamidase and cholinesterase came from their clear-cut separation on procainamide-Sepharose affinity chromatography. Both the serum and liver aryl acylamidase were compared with the purified erythrocyte aryl acylamidase associated with acetylcholinesterase. While the erythrocyte and serum aryl acylamidases showed some similarities in their sensitivities to amines like serotonin or tryptamine and choline derivatives, the liver enzyme was unaffected by any of these compounds. A notable observation was the activation by tyramine of the serum aryl acylamidase but not the erythrocyte and liver aryl acylamidases. The liver aryl acylamidase also differed from the other two in its relative insensitivity to inhibition by eserine, neostygmine and other cholinesterase inhibitors. Immunodiffusion and immunoprecipitation studies showed that the aryl acylamidases from the liver and erythrocytes were immunologically non-identical with the serum enzyme.

Radiolabeled benzoylcholine derivatives as possible myocardial-imaging agents

Two radioiodinated analogs of benzoylcholine were investigated as possible myocardial-imaging agents. O-([2-125I]Iodobenzoyl)-choline and N-([2-125I]iodobenzoyl)cholamine were prepared by nucleophilic substitution of sodium [125I]iodide for stable iodine in O-(N,N]dimethylaminoethyl)-2-iodobenzoate and N-(N',N'-dimethylaminoethyl)-2-iodobenzamide, respectively, and by methylation with methyl iodide. The in vivo distribution of each compound in mice was determined as a function of time. Favorable heart-to-blood and heart-to-lung ratios were obtained with N-([2-125I]iodobenzoyl)cholamine.

Acetylcholinesterase kinetics

Three mechanisms have been suggested to describe the inhibition of acetylcholinesterase (EC. 3.1.1.7) by an excess of acetylcholine. (i) Substrate inhibition occurs through the reaction of acetylcholine with acetylated enzyme. The deacetylation of this ternary complex is supposed to be completely inhibited. (ii) A ternary complex is formed as in (i). However, the deacetylation is not completely inhibited. (iii) A two-site-mechanism is discussed. Acetylcholine binds either to the active site or to the modifier site. Binding to the latter changes the activity of the active site. Steady state treatment was applied to (i)-(iii). A least squares fit led to catalytic parameters. It is demonstrated that mechanism (ii) is the most simple one which can describe satisfactorily the experimental data. Limits for a set rate constants are derived from the catalytic parameters. A numerical integration shows that the steady state approximation may be used even when the mechanisms are rather complex.

Cerebrospinal fluid acetylcholinesterase in neuropsychiatric disorders

Acetylcholinesterase (AChE) was measured in the cerebrospinal fluid (CSF) of patients with a diagnosis of Huntington's disease, depression, schizophrenia, or mania and also in the CSF of normal subjects. No significant differences in CSF AChE were found between any diagnostic group and normal subjects. Furthermore, the administration of choline chloride, physostigmine, or probenecid did not significantly alter CSF AChE. No diurnal variation in CSF AChE activity was apparent. These findings, combined with the unclear relationship of brain AChE to CSF AChE, suggest that this measurement does not reflect the relative cholinergic underactivity presumed to exist in some neuropsychiatric conditions.

Cholinesterases from plant tissues. VI. Preliminary characterization of enzymes from Solanum melongena L. and Zea mays L

Enzymes capable of hydrolyzing esters of thiocholine have been assayed in extracts of Solanum melongena L. (eggplant) and Zea Mays L. (corn). The enzymes from both species are inhibited by the anti-cholinesterases neostigmine, physostigmine, and 284c51 and by AMO-1618, a plant growth retardant and they both have pH optima near pH 8.0. The enzyme from eggplant is maximally active at a substrate concentration of 0.15 mM acetylthiocholine and is inhibited at higher substrate concentrations. On the basis of this last property, the magnitude of inhibition by the various inhibitors, and the substrate specificity, we conclude that the enzyme from eggplant, but not that from corn, is a cholinesterase.

A study of acetylcholinesterase throughout the life cycle of Nippostrongylus brasiliensis

Acetylcholinesterase activity, measured per unit wet weight, is relatively low in the eggs and infective larvae ofN. brasiliensis. It increases rapidly during the parasitic phase, especially in the late 3rd- and early 4th-larval stages. Activity in normal adults is extremely high (× 15 egg activity) and this is doubled again in immune-damaged adults. Possible functions ofN. brasiliensisacetylcholinesterase are discussed.

It is a pleasure to acknowledge the technical assistance of Miss Anne Cronin and Miss Beryl Crooks.

Esters of choline and beta-trimethylammoniopropionic acid: geometrical isomerism and anti-acetylcholinesterase activity

The preparation of the choline esters of cis- and trans-4-t-butylcyclohexanecarboxylic acid is reported. The similar inhibitory potency displayed by these isomers towards the acetylcholinesterase catalysed hydrolysis of acetylcholine is explained on the basis of the binding of a thermodynamically unstable conformation of the cis-isomer to the active site. Similar studies employing the β-trimethylammoniopropionate esters of cis- and trans-4-t-butylcyclohexanol suggest that the “reverse esters” do not bind to the active site in an identical manner to the acylcholines.

Electrical phenomena associated with the activity of the membrane-bound acetylcholinesterase

Treatment of isolated electroplax with physiological solutions supplemented with either 1 molar sodium chloride, 2 molar urea, or 2 molar sucrose renders the cell insensitive to carbamylcholine, phenyltrimethylammonium, or decamethonium even at high concentrations. The treated cells have a residual resting potential of -20 +/- 10 millivolts (negative inside) and are depolarized by acetylcholine at concentrations larger than 10(-3) mole per liter. This response is not affected by d-tubocurarine but is blocked by physostigmine, diisopropylphosphorofluoridate, or strong buffers and thus depends on the catalytic activity of the membrane-bound acetylcholinesterase.

ACETYLCHOLINESTERASE

The general characteristics of acetylcholinesterase are described. The mechanism of action of the enzyme is considered primarily in relation to the two steps in the hydrolysis of acetylcholine, the acetylation and deacetylation reactions. Competitive and non-competitive inhibition, and inhibitions by hydrogen ion and by substrate, are explained in terms of this two-step process. On the basis of the available evidence, a suggestion is made regarding the configuration of groups in the active center.