Assessment of acetylcholinesterase activity using indoxylacetate and comparison with the standard Ellman's method

Assay of acetylcholinesterase (AChE) activity plays an important role in diagnostic, detection of pesticides and nerve agents, in vitro characterization of toxins and drugs including potential treatments for Alzheimer’s disease. These experiments were done in order to determine whether indoxylacetate could be an adequate chromogenic reactant for AChE assay evaluation. Moreover, the results were compared to the standard Ellman’s method. We calculated Michaelis constant Km (2.06 × 10⁻⁴ mol/L for acetylthiocholine and 3.21 × 10⁻³ mol/L for indoxylacetate) maximum reaction velocity V_max (4.97 × 10⁻⁷ kat for acetylcholine and 7.71 × 10⁻⁸ kat for indoxylacetate) for electric eel AChE. In a second part, inhibition values were plotted for paraoxon, and reactivation efficacy was measured for some standard oxime reactivators: obidoxime, pralidoxime (2-PAM) and HI-6. Though indoxylacetate is split with lower turnover rate, this compound appears as a very attractive reactant since it does not show any chemical reactivity with oxime antidots and thiol used for the Ellman’s method. Thus it can be advantageously used for accurate measurement of AChE activity. Suitability of assay for butyrylcholinesterase activity assessment is also discussed.

Active site mutant acetylcholinesterase interactions with 2-PAM, HI-6, and DDVP

We used mouse recombinant wild-type acetylcholinesterase (AChE; EC 3.1.1.7), butyrylcholinesterase (BChE; EC 3.1.1.8), and AChE mutants with mutations (Y337A, F295L, F297I, Y72N, Y124Q, and W286A) that resemble residues found at structurally equivalent positions in BChE, to find the basis for divergence between AChE and BChE in following reactions: reversible inhibition by two oximes, progressive inhibition by the organophosphorus compound DDVP, and oxime-assisted reactivation of the phosphorylated enzymes. The inhibition enzyme-oxime dissociation constants of AChE w.t. were 150 and 46 microM, of BChE 340 and 27 microM for 2-PAM and HI-6, respectively. Introduced mutations lowered oxime binding affinities for both oximes. DDVP progressively inhibited cholinesterases yielding symmetrical dimethylphosphorylated enzyme conjugates at rates between 104 and 105/min/M. A high extent of oxime-assisted reactivation of all conjugates was achieved, but rates by both oximes were up to 10 times slower for phosphorylated mutants than for AChE w.t.

In vitro reactivation of sarin-inhibited brain acetylcholinesterase from different species by various oximes

In vitro as well as in vivo evaluation of the reactivating efficacy of various oximes against nerve agent-inhibited acetylcholinesterase has been usually done with the help of animal experiments. Nevertheless, previously published data indicate that the reactivation potency of oximes may be different in human and animal species, which may hamper the extrapolation of animal data to human data. Therefore, to better evaluate the efficacy of various oximes (pralidoxime, obidoxime, HI-6, K033) to reactivate brain acetylcholinesterase inhibited by sarin by in vitro methods, human, rat and pig brain acetylcholinesterase were used to calculate kinetic parameters for the reactivation. Our results show differences among the species, depending on the type of oxime, and indicate that data from animal experiments needs to be carefully evaluated before extrapolation to humans.

Inhibition of plasma butyrylcholinesterase activity in the lizard Gallotia galloti palmae by pesticides: a field study

A field study was performed to evaluate the effect of exposure to organophosphorus (OP) and carbamate (CB) pesticides on the lizard Gallotia galloti palmae. Butyrylcholinesterase (BChE) activity was measured in the plasma of 420 lizards collected from agricultural and reference areas on the Island of La Palma (Canary Islands, Spain) in two sampling periods. Exposure to cholinesterase-inhibiting pesticides was evaluated by a statistical criterion based on a threshold value (two standard deviations below the mean enzyme activity) calculated for the reference group, and a chemical criterion based on the in vitro reactivation of BChE activity using pyridine-2-aldoxime methochloride (2-PAM) or after water dilution of the sample. Mean (+/- SD) BChE activity for lizards from agricultural areas was significantly lower (Fuencaliente site = 2.00 +/- 0.98 micromol min(-1) ml(-1), Tazacorte site = 2.88 +/- 1.08) than that for lizards from the reference areas (Los Llanos site = 3.06 +/- 1.17 micromol min(-1) ml(-1), Tigalate site = 3.96 +/- 1.62). According to the statistical criterion, the number of lizards with BChE depressed was higher at Fuencaliente (22% of males and 25.4% of females) than that sampled at Tazacorte (7.8% of males and 6.2% of females). According to the chemical criterion, Fuencaliente also yielded a higher number of individuals (112 males and 47 females) with BChE activity inhibited by both OP and CB pesticides. CBs appeared to be the pesticides most responsible for BChE inhibition because most of the samples showed reactivation of BChE activity after water treatment (63.3% from Fuencaliente and 29% from Tazacorte). We concluded that the use of reactivation techniques on plasma BChE activity is a better and more accurate method for assessing field exposure to OP/CB pesticides in this lizard species than making direct comparisons of enzyme activity levels between sampling areas.

O-Substituted derivatives of pralidoxime: muscarinic properties and protection against soman effects in rats

O-Substituted aldoximes of the cholinesterase reactivator pralidoxime (O-methyl 1, O-benzyl 2, O-propynyl 3 and O-butynyl 4 derivatives) were synthesized and found to exhibit strong binding affinities for muscarinic receptors in rat brain, heart and submandibulary glands. The aldoximes were noncompetitive antagonists of acetylcholine-induced contraction of the guinea pig ileum. A good correlation was observed between binding affinity and pK(B). Weak anticholinesterase activities were observed for these compounds. When given intracerebroventricularly into conscious rats before soman administration (0.9 LD(50), subcutaneously), the aldoximes, like atropine but not pralidoxime, protected against respiratory depression (3,4) and bradycardia (2). No protection against soman-induced pressor effects was noted. The protective effects of these aldoximes may be the outcome of compensatory mechanisms, of which the cholinergic receptor agonist and antagonist properties of these compounds may be important.

Accidental injection of a U.S. Air Force aviator by a pralidoxime chloride auto-injector: a case report

To counter the threat of organophosphate nerve agents, military personnel may be issued auto-injectors containing pralidoxime chloride. This drug helps to dephosphorylate the nerve agent-acetylcholinesterase complex and, thus, regenerate the enzyme. In non-poisoned persons, pralidoxime chloride is rapidly excreted by the kidneys and is fairly well tolerated. We present the first reported case of an accidental injection of an Air Force aviator by an auto-injector. The patient recovered well with no specific treatment needed. The pharmacology and toxicology of pralidoxime chloride are discussed.

Reactivating potency of obidoxime, pralidoxime, HI 6 and HLö 7 in human erythrocyte acetylcholinesterase inhibited by highly toxic organophosphorus compounds

The treatment of poisoning by highly toxic organophosphorus compounds (nerve agents) is unsatisfactory. Until now, the efficacy of new potential antidotes has primarily been evaluated in animals. However, the extrapolation of these results to humans is hampered by species differences. Since oximes are believed to act primarily through reactivation of inhibited acetylcholinesterase (AChE) and erythrocyte AChE is regarded to be a good marker for the synaptic enzyme, the reactivating potency can be investigated with human erythrocyte AChE in vitro. The present study was undertaken to evaluate the ability of various oximes at concentrations therapeutically relevant in humans to reactivate human erythrocyte AChE inhibited by different nerve agents. Isolated human erythrocyte AChE was inhibited with soman, sarin, cyclosarin, tabun or VX for 30 min and reactivated in the absence of inhibitory activity over 5-60 min by obidoxime, pralidoxime, HI 6 or HLö 7 (10 and 30 microM). The AChE activity was determined photometrically. The reactivation of human AChE by oximes was dependent on the organophosphate used. After soman, sarin, cyclosarin, or VX the reactivating potency decreased in the order HLö 7 > HI 6 > obidoxime > pralidoxime. Obidoxime and pralidoxime were weak reactivators of cyclosarin-inhibited AChE. Only obidoxime and HLö 7 reactivated tabun-inhibited AChE partially (20%), while pralidoxime and HI 6 were almost ineffective (5%). Therefore, HLö 7 may serve as a broad-spectrum reactivator in nerve agent poisoning at doses therapeutically relevant in humans.

Acetylcholinesterase inhibition in the crayfish Procambarus clarkii exposed to fenitrothion

Laboratory and field studies were conducted to evaluate the effects of fenitrothion (O,O-dimethyl O-4-nitro-m-tolyl phosphorothioate) on the crayfish Procambarus clarkii. Acetyl- and butyryl-cholinesterase activities were measured in the muscle of P. clarkii exposed to different doses of fenitrothion (4, 20, and 100 microg/liter) for different times (up to 48 hr). A positive correlation was found between both cholinesterases, and acetylcholinesterase (AChE) was selected as a biomarker of exposure to this compound. Significant AChE depression (20%) was detected 2 hr after exposure to 20 microg/liter of fenitrothion, reaching a maximum at 48 hr (47%), followed by a slow recovery. Reactivation techniques using the nucleophilic reagent pyridine 2-aldoxime methiodide were assayed in fenitrothion-poisoned specimens, and the results suggested the utility of this method to diagnose exposure, particularly when control animals are not available. Finally, AChE inhibition was used to test a field population of P. clarkii potentially exposed to high concentrations of the organophosphorus pesticide fenitrothion, and a 55% inhibition was detected.

In vitro binding of oxime acetylcholinesterase reactivators to proteoglycans synthesized by cultured chondrocytes and fibroblasts

The incorporation of the 14C-labelled acetylcholinesterase reactivators 1-(methyl-imidazolium)-3 (4-carbaldoxime-pyridinium) propane dibromide (pyrimidoxime) and N,N'-trimethylene bis(pyridinium-4-aldoxime) dibromide (TMB4) into cultured chondrocytes and fibroblasts was measured and their binding to macromolecules synthesized by these cells studied. The results showed that these drugs concentrated slowly and poorly into these cells, but bound firmly to high molecular mass materials in the culture supernatants. The chromatographic properties of these macromolecules on Sepharose CL-2B in non-dissociative or dissociative conditions were similar to those of the proteoglycans synthesized by these cells. Dialysis of the macromolecule-bound drugs against increasing pH buffers showed half-dissociation pH > 8, identical to those for chondroitin sulphate. These results suggest strongly that pyrimidoxime and TMB4 are bound to proteoglycans by ionic interactions, and this together with their poor lipophilicity can explain their high selectivity for the cartilaginous tissues as opposed to other proteoglycan-containing structures such as skin.

Therapeutic dosing of pralidoxime chloride

Pralidoxime chloride is a useful agent in the treatment of organophosphate poisoning. Poisindex, a widely used poisoning treatment resource, recommends dosing pralidoxime chloride as an intermittent iv infusion every 8-12 hours, whereas other authors have used continuous iv infusion with good results. Available animal data suggest that a serum concentration of 4 micrograms/ml may be a minimal level to protect against the toxic effects of organophosphates. Pharmacokinetic simulations, based on parameters obtained from healthy nonpoisoned subjects, show that pralidoxime levels fall rapidly to less than 4 micrograms/ml within 1.5-2 hours after a 1-g iv bolus. Continuous iv infusion (0.5 g/h) maintains pralidoxime levels greater than 4 micrograms/ml throughout the length of infusion. We conclude that continuous iv infusion of pralidoxime chloride may be the preferred method of administration in patients with acute organophosphate poisoning. Clinical trials will be necessary to document the effectiveness of this regimen.

Reactivation kinetics of diethylphosphoryl acetylcholine esterase

The kinetics of reactivation of diethylphosphorylated acetylcholine esterase by pyridine-2-aldoxime methochloride has been studied using the approach of following the course of the hydrolysis of acetylcholine during the reactivation of the phosphorylated enzyme by the reactivator [Tsou, C.-L. (1965) Acta Biochem. Biophys. Sin. 5, 398-417]. Equations are derived based on the scheme of the formation of a complex between the phosphorylated enzyme and the reactivator and the rate of dissociation of this complex is not necessarily faster than the dephosphorylation and regeneration of the active enzyme. The regenerated enzyme then reacts with the substrate through an acetyl-enzyme intermediate as generally depicted. The equation obtained for product formation during the course of reactivation contains two exponential terms and this is in accord with the experimentally observed biphasic reaction. By making the assumption that the dissociation of the phosphorylated enzyme-reactivator complex is much faster than the dephosphorylation reaction, the above equation can be simplified to a form containing only one exponential term. By following the course of the reactivation reaction with the conventional approach of taking aliquots and assaying for enzyme activity recovery, it would appear likely that one would miss the initial stage of this biphasic reaction.

Influence of various anions on intestinal disappearance of hexamethonium chloride and pralidoxime chloride in rats

The intestinal transfer of two poorly absorbed quaternary ammonium drugs, hexamethonium chloride (I) and pralidoxime chloride (II), in the presence of various organic and inorganic anions was investigated in the rat using a modified in situ gut technique. The results were in agreement with those of the conventional in situ loop and plasms drug level techniques. Of the anions investigated, cholate, desoxycholate, taurocholate, phoscholate, dehydrocholate, and hyodesoxycholate had the greatest effect on increasing the amount and rate of disappearance of I. Similarly, the amount and rate of disappearance of II were enhanced markedly in the presence of phoscholate and trichloroacetate. The effect of cholate and phoscholate was investigated in detail. The membrane permeability and histological studies indicated that these anions may compromise the structural integrity of the membrane tissue, thus enhancing drug transfer.

Aqueous humour pralidoxime mesylate (P2S) concentrations and intraocular tension following intramuscular P2S

Small transient increases in rabbit intraocular tension were measured following intramuscular pralidoxime mesylate (P2S) at doses of 10, 40 or 100 mg/kg. Peak increases in tension showed some dose dependency, and although not statistically correlated with plasma or aqueous humour P2S concentrations the changes in tension followed the general time-concentration profile for P2S in these fluids. P2S was not detected in aqueous humour following 10 mg/kg, but at 40 and 100 mg/kg dose dependent concentrations were detected in aqueous humour. From experiments involving multiple injections of P2S there was no evidence for a cumulative effect of P2S on the eye. The results indicate that P2S can enter aqueous humour following its parenteral administration and is associated with changes in intraocular fluid dynamics. Whilst they also suggest that some ophthalmic side-effects of oxime therapy may be a consequence of a direct action of the oxime on the eye, they do not exclude contributions from actions at other sites.

Expression of simian virus 40 gene A affects tubulin stability

The stability of tubulins present in crude extracts of untransformed BALB/c-3T3 mouse fibroblasts, Chinese hamster lung cells, and various of their simian virus 40 transformants was assessed by measurement of their individual colchicine-binding decay rates. In all cases studied the decays followed the kinetics of first-order reactions, and rates were reduced at low temperatures and by vinblastine sulfate. Under all assay conditions, including different temperatures and protein concentrations, tubulins of normal cells decayed considerably faster than those of simian virus 40-transformed cells. Experiments performed with a number of Chinese hamster lung cell clones transformed with temperature-sensitive simian virus 40 gene A mutants showed a clear correlation between increased tubulin stability and the expression of gene A function. These results suggest that it is T-antigen, the viral gene A product, that affects tubulin.

Kinetics of absorption and elimination of pralidoxime chloride in dogs

The kinetics of the absorption and elimination of pralidoxime chloride were investigated in the dog. Similar apparent elimination rate constants were obtained after intravenous, intramuscular, and oral administration. Although oral absorption occurred slowly, intramuscular absorption proceeded rapidly. With in situ techniques, it was found that no absorption occurred from the isolated stomach and duodenum but that absorption did take place from the jejunum and ileum.

Drug uptake into everted intestinal sacs. II. Inhibition of secretion by hypertonicity

Mucosal hypertonicity, metabolic inhibitors, or absence of glucose and oxygen enhance mucosal-to-serosal influx of the cationic drug, pralidoxime (PAM), into sacs of everted rat jejunum in vitro. Conversely, efflux of PAM, which is twice the influx rate, is inhibited by mucosal hypertonicity or cyanide and iodoacetate. When sacs containing PAM, 0.87 mM, and glucose, 10 mM, were placed in identical drug- and sugar-containing mediums, the inside (serosal) concentration of PAM fell by over half in 120 min, whereas that of glucose more than doubled. Mucosal hypertonicity depressed PAM efflux and glucose influx regardless of serosal osmolarity. Although azide and mucosal hypertonicity each depressed glucose uptake and oxygen consumption while accelerating net PAM influx, azide more effectively depressed glucose and oxygen uptake, whereas hypertonicity caused greater acceleration of PAM uptake. Hypertonicity did not affect PAM binding to intestinal tissue. Varying mucosal pH did not change PAM or glucose uptake. Thus, mucosal hypertonicity apparently enhances net mucosal-to-serosal transfer of PAM by blocking its active secretion from serosa to mucosa.

Drug uptake into everted intestinal sacs. I. Enhancement by hypertonicity

The transfer of the cationic drugs, pralidoxime (PAM) and tetraethylammonium, and anionic ampicillin from the mucosal-to-serosal sides of everted rat jejunal sacs is enhanced by mucosal hypertonicity. PAM uptake, which is proportional to initial mucosal concentrations up to 2.3 mM, is enhanced by mucosal hypertonicity due to addition of sodium, potassium, lithium and choline chloride, sodium sulfate, and the nonionic solutes, urea, sucrose, and mannitol. Bicarbonate, Tris, or phosphate buffer and the presence of magnesium and calcium do not affect this hypertonicity-induced acceleration of PAM passage. Serosal osmolality has no effect on transfer and mucosal hypertonicity is equally effective in the presence and absence of a transmural osmotic gradient. This observation and minimal changes in the concentration of inulin placed in the sacs suggest that fluid shifts and solvent drag are not responsible for the enhanced mucosal-to-serosal transfer of PAM from hypertonic buffer. Mucosal hypertonicity at 450 mosmol/kg causes reversible enhancement of PAM transfer, whereas the effect of 600 mosmol/kg cannot be reversed by replacing the tissue in isotonic buffer. The effect of osmotic manipulation on PAM transfer across the intestine thus differs from its effect on the passage of other ionized species and drugs across other epithelia.

Effect of intravenous thiamine on pralidoxime kinetics

Subjects were given pralidoxime chloride (5 mg/kg, intravenously) alone and again while they were receiving an infusion of thiamine hydrochloride. After the addition of thiamine: (1) overall, the urinary excretion of oxime was the same but the amount excreted in the first three hours was smaller; (2) the plasma half-life of oxime lengthened; (3) the plasma concentrations of oxime rose; and (4) the intercompartmental clearances and rate constant for elimination for oxime fell. These changes suggest that thiamine and oxime compete for a common renal secretory mechanism or that thiamine alters the membrane transport of oxime.

Increased intestinal absorption in the rat caused by sodium lauryl sulphate, and its possible relation to the cAMP system

The increases in the absorption of ouabain, phenolsulphonphthalein and pralidoxime caused by 17 mM sodium lauryl sulphate (SLS) from jejunal loops of anaesthetized rats were significantly reduced if sodium and chloride (Briseid et al., 1974) or chloride and bicarbonate were replaced by other ions in the loop fluid. Separate substitutions of sodium, chloride of bicarbonate did not significantly alter the SLS-caused absorption, except that the substitution of choline for sodium reduced the absorption of pralidoxime, both in the presence and in the absence of SLS. The increases in the absorption of phenolsulphonphthalein and pralidoxime caused by SLS were potentiated by theophylline (25 mM) and reduced by imidazole (25 mM). The addition of dibutyryl cyclic AMP (2.5 mM) to the loop fluid increased this absorption of the test substances. This effect was reduced by imidazole, but under the experimental conditions it was not potentiated by theophylline. Determinations of cyclic AMP in the rat intestinal mucosa showed that the level of this substance was significantly higher in the presence than in the absence of SLS. The experimental conditions were as described for the absorption experiments. It is concluded that the data obtained support the idea of an increased level of cyclic AMP as the main basis for the effect of SLS on the absorption.

Improved delivery through biological membranes. 3. Delivery of N-methylpyridinium-2-carbaldoxime chloride through the blood-brain barrier in its dihydropyridine pro-drug form

Administration of N-methyl-1,6-dihydropyridine-2-carbaldoxime hydrochloride, the pro-drug form of 2-PAM, resulted in an average of 13-fold increase in the amount of 2-PAM delivered into the brain of mice as compared to the administration of 2-PAM. The pro-drug which crossed the BBB resulted in a dramatic increase in the reactivation of AChE blocked by DFP. In vivo studies of the "aging" of the phosphorylated AChE in the brain of mice could also be studied using pro-2-PAM.

Improved delivery through biological membranes. 1. Synthesis and properties of 1-methyl-1,6-dihydropyridine-2-carbaldoxime, a pro-drug of N-methylpyridinium-2-carbaldoxime chloride

A dihydropyridine-pyridine type redox pro-drug system was developed for delivering quaternary pyridinium salts through biological membranes. As a first application, the dihydropyridine derivative of N-methylpyridinium-2-carbaldoxime chloride (2-PAM) was synthesized using a reduction-addition-elimination sequence. The dihydro-2-PAM obtained has all the required properties for an effective transport through lipoidal barriers and it reverts easily back to 2-PAM as a result of a chemical or enzymatic oxidation process.

Improved delivery through biological membranes. 2. Distribution, excretion, and metabolism of N-methyl-1,6-dihydropyridine-2-carbaldoxime hydrochloride, a pro-drug of N-methylpyridinium-2-carbaldoxime chloride

N-Methyl-1,6-dihydropyridine-2-carbaldoxime hydrochloride, the pro-drug of 2-PAM, was found to be converted in vivo to 2-PAM, rapidly and quantitatively. The significantly changed properties of the pro-2-PAM resulted in a longer biological half-life and a favorable distribution of 2-PAM formed upon its oxidation. No new metabolite was found when pro-2-PAM was administered intravenously; however, a new metabolic product was formed when the pro-drug was given by oral route.

Delivery of a quaternary pyridinium salt across the blood-brain barrier by its dihydropyridine derivative

A dihydropyridine-pyridine type redox system was successfully applied for delivering a quaternary pyridinium salt, N-methylpyridinium-2-aldoxime chloride (2-PAM), through the blood-brain barrier. The dihydropyridine derivative of 2-PAM was quickly oxidized to 2-PAM after crossing the blood-brain barrier. As a result of this approach, the brain cholinesterase blocked by organophosphates could be reactivated. The new method should be useful in delivering numerous drugs which are otherwise inaccessible to the brain because of their polar ionic character.

Pralidoxime mesylate absorption and heart rate response to atropine sulphate following intramuscular administration of solution mixtures

1. The oxime pralidoxime mesylate (P2S) and atropine sulphate, alone and mixed, have been administered intramuscularly to forty-four human subjects. Doses of 750 mg and 500 mg P2S and 2.0 mg atropine sulphate were used. 2. The presence of P2S had no significant influence, as judged by effect on heart rate, on the absorption of atropine although there was a tendency for atropine to exert its effects more rapidly when administered mixed with P2S. 3. No significant difference in the rate of uptake of P2S as judged by plasma levels following injection, between a combined (plus atropines) and single (P2S alone) intramuscular injection was found.

An in vitro study of the intestinal absorption of pyridinium aldoximes

1. The transfer rates of three pyridinium aldoximes, a non-quaternary pyridine aldoxime and choline across the wall of sacs from the jejunum of rats were measured in vitro.2. The transfer rates observed for any one of the quaternary compounds could be inversely correlated with the transmural potential of the particular sac studied, but there was no correlation with the rates of water or glucose transfer.3. 2-hydroxyiminomethyl-N-methylpyridinium iodide (PAM) had a transfer rate seven times less than that of its non-quaternary analogue, 2-hydroxyiminomethyl pyridine.4. Neither neostigmine nor EDTA affected the transfer rate of PAM in the conditions used.5. It was concluded that the transfer of the quaternary compounds could be explained by diffusion through aqueous pores.