What happens if you inhibit acetylcholinesterase

The cause of autoimmune response is unknown and only symptomatic therapies for MG are currently available. Clinically relevant treatments of MG include immunosuppressive drugs, plasmapheresis, thymectomy and inhibitors of cholinesterases ChEs 4. All treatments suffer from a variety of side effects.

For daily pharmacological correction of muscle weakness, the most frequently used drugs cause partial inhibition of AChE and butyrylcholinesterase BChE. These enzymes catalyze hydrolysis of ACh, thus terminating its action on ACh receptors 5.

Extension of duration of ACh action at partial inhibition of ChEs is able to compensate for autoimmune decrease in nAChRs density and, thus, rescues muscle contractions. However, inhibition of ChEs in other tissues also occurs resulting in adverse effects. Significant bulk of side effects is associated with hyperactivation of muscarinic acetylcholine receptors mAChRs in vegetative nerve system, primarily in smooth muscles and, to a lesser extent, in myocardium 6 , 7.

Previously we have described a series of cholinesterase inhibitors based on 1,3-bis[5- o -nitrobenzylethylammonium pentyl]methyluracilic unit with selectivity towards mammalian AChE vs.

These inhibitors were found to be efficacious in an animal model of MG and can be considered as potentially valuable candidates for treatment of pathological muscle weakness syndromes in humans. Kinetic analysis of inhibition showed that C is a slow-binding inhibitor of type B, i. Thus, C was found to be one of the most potent and selective reversible inhibitors of AChE discovered so far.

In this study, we decided to compare effects of C and clinically used non-selective ChEs inhibitor, pyridostigmine, on contractility of rat and human urinary bladder muscle preparations. We found that C, in the doses effectively controling muscle weakness in in vivo experiments, did not affect activity of rat bladder muscles.

In contrast, the dose of pyridostigmine required to alleviate MG symptoms enhanced the tonus of urinary bladder and significantly amplified the force of its spontaneous contractions.

We assume, that the difference in the effectivness of inhibitors is due to higher selectivity of C with respect to AChE as compared to BChE. Our experiments allow us to suggest that, after partial and selective inhibition of AChE, remaining activity of BChE in the urinary bladder is sufficient to prevent development of significant side effects. We also made an important finding that sensitivity of human urinary bladder preparations to AChE and BChE inhibition is similar to that of the rat bladder.

This observation provides reasonable bases to hypothesize that remaining activity of BChE in urinary bladder in humans can also be sufficient to reduce side effects when selective AChE inhibitors are used for MG treatment. Experimental autoimmune myasthenia gravis EAMG induced in rats, in its chronic phase, resembles human myasthenia. As it has been shown earlier 14 , 15 , this type of rat EAMG resembles human myasthenia in the following aspects: a blood serum of affected animals contains antibodies toward muscle type nAChR; b there is a characteristic decrement in the amplitude of compound muscle AP as evidenced by EMG upon repetitive nerve stimulation as compared to normal animals.

In addition to this, myasthenia should manifest itself by a decrease in amplitude and duration of miniature end-plate currents mEPCs as a result of the drop in the density of expressed and functional muscle nAChRs. We decided to check-up whether these changes could be also observed in the EAMG model chosen. For this purpose we have compared mEPC recordings in normal animals and rats immunized with the peptide and developing characteristic decrement in the amplitude of compound muscle AP.

We found that in normal muscle, the mean amplitude of mEPCs was equal to 4. This observation is consistent with what one should expect from inhibition of synaptic ChEs: an increase in amplitude and duration of synaptic currents due to repetitive activation of nAChRs Previously we have tested C and pyridostigmine in the peptide-induced EAMG model in rat to find doses which reduce the decrement in the amplitude of compound muscle AP to control level.

In this study, we decided to evaluate the effects of ChEs inhibitors on urinary bladder contractions in diseased animals. Then we measured tonus and amplitude of spontaneous contractions in urinary bladders in diseased rats. Co-inhibition of acetylcholinesterase and butyrylcholinesterase increases urinary bladder contractions recorded in vivo in rats affected by EAMG. A Representative individual urinary bladder basal contractions recorded in the same rat with intact cholinesterases left and after IP injection of non-specific cholinesterase inhibitor pyridostigmine at the dose of 0.

B Mean amplitude of urinary bladder basal contractions recorded in diseased animals following inhibition of cholinesterase activity: acetylcholinesterase - by C 0. C Mean basal tonus recorded in the same animals as amplitude of urinary bladder basal contraction. It was previously shown that both AChE and BChE are involved in cholinergic modulation of rat urinary bladder contractions 18 and guinea pig detrusor muscle tonus As a first step, we decided to check-up how bambuterol affects the activity of BChE and AChE in homogenates of rat urinary bladder muscle.

Bambuterol specifically blocks BChE in urinary bladder. Residual activities of acetylcholinesterase AChE and butyrylcholinesterase BChE were estimated in homogenates of rat urinary bladders after single IP injection of bambuterol at the dose of 0. However, administration of bambuterol, after pre-treatment with C, affected basal tonus and amplitude of contractions similarly to pyridostigmine, i.

Thus, higher selectivity of C to AChE vs. BChE, could account for its weaker effects in smooth muscle in vivo as compared to pyridostigmine. Assuming, that unsuppressed BChE activity in urinary bladder can play a critical role in reducing side effects during pharmacological MG treatment, we decided to compare effects of AChE and BChE inhibition in rat and human urinary bladder preparations ex vivo.

We found that both pyridostigmine and C were able to increase the amplitude of ACh-induced contractions of rat urinary bladder strips in a dose-dependent manner. Each concentration for each drug was tested in 10 animals. Augmentation of contractions in rat urinary bladder strips by pyridostigmine. C also caused a significant increase in contractions starting at 0. Maximum effect of C on muscle contraction was significantly lower than maximum effect of pyridostigmine.

Thus, one could hypothesize that it was just inhibition of BChE that accounts for the difference seen in the maximum effects of C and pyridostigmine. Next, we have studied effects of C and pyridostigmine on the amplitude of ACh-induced contractions in human urinary bladder strips. We found, that treatment with pyridostigmine markedly augmented the amplitude of ACh-induced contractions Fig.

C was also found to increase contractions of human bladder strips in a dose-dependent manner. Concentration of 0. Effects of inhibitors of cholinesterase activity on contractility of human urinary bladder strips. In the next series of experiments, we studied the role of BChE inhibition in potentiation of contractions in human bladder strips.

Iso-OMPA by itself significantly increased bladder strips contractions to Application of iso-OMPA after pre-treatment with C caused a further increase in contraction in an additive manner, and combined maximum effect reached the level of maximum effect caused by pyridostigmine Fig.

It is worth noting that sensitivity of human bladder strips to BChE inhibition was found to be similar, if not identical, to sensitivity of rat bladder preparation. This is a very encouraging result and it would suggest that in human bladder, when treated with selective AChE inhibitors, the residual activity of BChE could be also sufficient to guarantee a significant reduction in side effects related to hyperactivation of smooth muscles.

For symptomatic treatment of muscle weakness, partial inhibition of ChE activity is used in clinic, when the disease is accompanied by reduction in the density of functional nAChRs in NMJs. One of the main objectives of this study was to compare effects of C, a novel, selective inhibitor of AChE, and pyridostigmine, a traditional, clinically used non-selective inhibitor of ChEs, on hyperactivation of smooth musculature in urine bladder of EAMG rats.

Hyperactivation represents one of the major side effects when ChEs inhibitors are used for treatment of MG in human patients. To accomplish this, we used the method developed to measure urinary bladder contractile force in vivo In the EAMG model used, administration of pyridostigmine at a dose alleviating MG symptoms caused significant increase in the force of spontaneous contractions and enhancement of bladder tonus. At the same time, equi-potent dose of C did not affect the force of spontaneous contractions and basal tonus of rat urinary bladder.

Since the most obvious pharmacological difference between pyridostigmine and C is the higher AChE selectivity of the latter, we suggested that background activity of BChE in the bladder could be sufficient to compensate for partial inhibition of AChE, in contrast to skeletal muscle. In these conditions, almost complete inhibition of BChE did not affect either the force of contractions or tonus of EAMG rat bladders in vivo. These data suggest that under normal conditions BChE probably does not contribute significantly to ACh hydrolysis in rat urinary bladder.

In contrast, one can assume that AChE is the main enzyme, controlling lifetime of ACh in urinary bladder of normal rats. Following C pre-administration, bambuterol significantly increased the force of contractions and tonus of rat bladder to the extent similar to pyridostigmine.

This fact implies that, in spite of its secondary role, the activity of BChE in the bladder could be sufficient to compensate for partial AChE inhibition, when C is used at the doses effective in alleviation of MG symptoms. Functional role of BChE could be different in the urinary bladder of rat and human. Thus, one can assume that preservation of certain level of BChE activity in human bladder can also help to decrease hyperactivation of smooth muscle and reduce related side effects during MG therapy.

On the scientific venue, the results of this study warrant further investigation of the effects of inhibitors of AChE and BChE in human smooth muscles. For example, it would be interesting to compare activities of BChE at therapeutically effective doses of C and pyridostigmine in human bladder preparations. We would like to emphasize some other important aspects of using AChE-selective inhibitors to treat MG. Pharmacokinetics of specific AChE inhibitors should be obviously distinct from that of non-specific inhibitors because BChE is highly abundant in serum and other tissues The levels of BChE vary in human populations due to genetic polymorphism and that is why the use of non-specific inhibitors is more difficult since they bind to variable levels of BChE as compared to specific inhibitors of AChE.

The repertoire of pharmacological inhibitors of ChEs for treatment of muscle weakness in MG patients remains poorly developed. Currently, the two most frequently used inhibitors are pyridostigmine and neostigmine.

They both suffer from side effects stemmed from hyperactivation of smooth muscles in urinary bladder and intestine. Pyridostigmine is a pseudo-reversible carbamylating agent, its selectivity for human AChE vs. BChE is very poor Neostigmine dimethyl carbamate, analog of pyridostigmine is also pseudo-reversible non-specific ChEs inhibitor. Neostigmine is used less frequently since it has stronger muscarinic side effects than pyridostigmine Thus, there is a clinical need in a new generation of ChEs inhibitors and an expansion of the assortment of selective AChE vs.

BChE inhibitors designed for MG treatment is of undisputed interest. Natural carbamate derivate physostigmine Fig. As a potent AChE inhibitor, this therapeutic agent reduces ACh hydrolysis rate, and thereby increases its level in damaged neurosynaptic clefts improving nerve impulse transmission.

Besides, pyridostigmine Fig. Consequently, it is applied as a prophylactic against nerve agent intoxication [ 77 - 79 ]. Furthermore, rivastigmine Fig. OPs are esters or thiols derived from phosphoric, phosphonic, phosphinic or phosphoramidic acid Fig.

R 1 and R 2 are aryl or alkyl groups that are bonded to the phosphorus atom either directly forming phosphinates , or through an oxygen or sulphur atom forming phosphates or phosphothioates. In some cases, R 1 is directly bonded to the phosphorus atom, and R 2 is bonded to an oxygen or sulphur atom forming phosphonates or thiophosphonates.

In phosphoramidates, at least one of these groups is —NH 2 un-, mono- or bi-substituted , and the atom double-bonded with phosphorus is either oxygen or sulphur. The —X group, also binding to the phosphorus atom through oxygen or sulphur atom, may belong to a wide range of halogen, aliphatic, aromatic or heterocyclic groups. The OPs exert their main toxicological effects through non-reversible phosphorylation of esterases in the central nervous system [ 81 , 82 ].

The acute toxic effects are related to irreversible inactivation of AChE [ 82 ]. Actually, OPs are substrate analogues to ACh, and like natural substrate enter the active site covalently binding to serine —OH group. As in acetylation, OP is split and the enzyme is phosphorylated Fig. While the acyl enzyme is quickly hydrolyzed to regenerate the free enzyme, dephosphorylation is very slow on the order of days , and phosphorylated enzyme cannot hydrolyze the neurotransmitter [ 83 ].

The inhibition of the enzyme leads to accumulation of ACh in the synaptic cleft resulting in over-stimulation of nicotinic and muscarinic ACh receptors and impeded neurotransmission. The typical symptoms of acute poisoning are agitation, muscle weakness, muscle fasciculations, miosis, hypersalivation, sweating.

Irreversible inhibition occurs in two steps; the first one is fast, short term reversible enzyme inactivation, and its influence is dominant in the begining of the inhibition. The next step is slow irreversible inhibition producing a very stable enzyme-inhibitor complex phosphorylated enzyme -inhibitor is covalently bonded to the enzyme [ 88 ].

Time dependent irreversible inhibition can be described by the equation:. Progressive development of inhibition produced by reaction of AChE with different concentrations of diazoxon plotted as semi logarithmic curve in accordance with Equation 1.

Reproduced from [ 90 ]. The dependence of kapp upon the concentration of diazoxon 1 , chlorpyrifos-oxon 2 and chlorpyrifos 3 , inset plotted as reciprocals in accordance with Equation 2. Effective OPs have the following structural features: a terminal oxygen connected to phosphorus by a double bond oxo form , two lipophilic groups —R 1 , —R 2 bonded to the phosphorus, and a good leaving group —X bonded to the phosphorus Fig.

OPs can produce delayed neurotoxic effect in humans and chickens, called OP induced delayed neuropathy. It is associated with phosphorylation and further dealkylation aging Fig.

The symptoms of this neuropathy are paralysis and ataxia, and appear between 14 and 24 days after the poisoning [ 67 , 70 , 71 ].

The majority of OPs have been commonly used as nonspecific insecticides for over fifty years, to control a variety of insects in agriculture and the household environment. The synthesis of OP pesticides in large quantities started after World War II, and parathion was among the first marketed, followed by malathion and azinphosmethyl.

Commonly used OP insecticides have included ethyl parathion, malathion, methyl parathion, chlorpyrifos, diazinon, dichlorvos, phosmet, fenitrothion, tetrachlorvinphos, azinphos methyl, pirimiphos-methyl, dimethoate, phosalone Fig. In the s organochlorine insecticides DDT, dieldrin, aheptachlor were banned because of their persistence and accumulation in the environment, and replaced by more degradable OPs. Actually, OP insecticides in the environment undergo the natural degradation pathway including mainly homogeneous and heterogeneous hydrolysis especially at high pH enhanced by the presence of dissolved metals, humic substances, microorganisms and other compounds present in soil [ 94 - 96 ].

OP degradation processes also occur in chemical treatments for purification of polluted waters, generally referred as advanced oxidation processes, as well as throughout the enzymatic reactions in birds, fish, insects and mammals. Degradation studies revealed different kinetics, mechanisms and transformation products, suggesting complete mineralization of the starting compound usually thio form , but forming toxic break down products as well [ 89 , 97 - ]. Actually, oxidation and isomerisation reaction products were reported as much more potent AChE inhibitors compared to the starting thio OPs, while hydrolysis products do not noticeably affect the enzyme activity.

Although OPs insecticides degrade rapidly, that made them an attractive alternative to the organochloride pesticides, they have greater acute toxicity, posing risks to people who may be exposed to large amounts - workers employed in the manufacture and application of these pesticides.

OPs are one of the most common causes of poisoning worldwide occurring as a result of agricultural use, suicide or accidental exposure. OP pesticides can be absorbed by all routes, including inhalation, ingestion, and dermal absorption [ ]. Their toxicity is not limited to the acute phase, but chronic effects have long been noted. Actually, repeated or prolonged exposure to OPs may result in the same effects as acute exposure including the delayed symptoms.

The effects, reported in workers repeatedly exposed, include impaired memory and concentration, disorientation, severe depressions, irritability, confusion, headache, speech difficulties, delayed reaction times, nightmares, sleepwalking and drowsiness or insomnia. Influenza-like condition with headache, nausea, weakness, loss of appetite, and malaise has also been reported [ ].

Neurotransmitters such as ACh are profoundly important in the brain's development, and many OPs have neurotoxic effects on developing organisms, even from low levels of exposure, causing various diseases of nervous and immune system [ 85 , ]. Oxo forms of OP insecticides,are highly, approximately equally toxic to warm-blooded as well as cold-blooded organisms.

On the other hand, thio forms are converted into the oxo forms by mixed function oxidases. The activation proceeds in cold-blooded organisms but this is not common in warm-blooded organisms where dealkylation into non toxic compounds takes place [ 51 , ]. Thus, numerous derivatives of highly toxic insecticides have been prepared to reduce the toxicity towards warm-blooded organisms and retain toxicity to insects, thereby enhancing their specificity. The examples of effective, commonly used OP insecticides, and relative safe for warm-blooded organisms are: malathion, chlorpyrifos, fenitrothion, pirimiphos-methyl, dimethoate, phosalone [ 51 ].

Nowadays, the common use of OP insecticides results in their accumulation, environmental pollution and acute and chronic poisoning events [ ]. For this reason, the use of OP insecticides has to be strictly controlled and restricted. Accordingly, the majority of countries have strong regulations on the application of pesticides; e. Also, the applied insecticides and their by-products in the environment, water and food are monitored applying different bioanalytical techniques [ ].

Nerve agents of OP group include tabun, sarin, soman, cyclosarin and VX. Sarin, soman and cyclosarin are phosphonofluoridates, and VX is a phosphonothioate Fig. Soman has four, while sarin and VX have two isoforms, which significantly differ in toxicity and irreversible AChE inactivation rate. Based on the acute toxicity, VX is the most toxic compound among all the nerve agents [ 67 ].

The developing and production of these extremely toxic nerve agents started in the s, and later used in wars and by terrorists on several occasions. As chemical weapons, they are classified as weapons of mass destruction by the United Nations, and their production and stockpiling was outlawed by the Chemical Weapons Convention.

Acute poisoning by a nerve agent leads to contraction of pupils, profuse salivation, convulsions, involuntary urination and defecation, and eventual death by asphyxiation as control is lost over respiratory muscles. Some nerve agents are readily vaporized or aerosolized and the primary portal of entry into the body is the respiratory system.

Nerve agents can also be absorbed through the skin, requiring that those exposed to such agents wear a full body suit in addition to a respirator [ ]. Moreover, the effects of nerve agents are very long lasting and cumulative increased by successive exposures , and survivors of nerve agent poisoning usually suffer chronic neurological damage that can lead to continuing psychiatric effects [ ]. OPs, except their use as toxic compounds, have been applied in ophthalmology as therapeutic agents in the treatment of chronic glaucoma, an eye disease in which the optic nerve is damaged in a characteristic pattern.

The disease is associated with increased fluid pressure in the eye, and can permanently damage vision in the affected eye s and lead to blindness if left untreated [ ]. These medical useful OPs include diisopropyl fluorophosphate and echothiophate. It is known as fluostigmine and dyflos in such uses. It exerts ocular side effects mainly associated with its AChE inhibitory properties, and ability to induce delayed peripheral neuropathy [ 67 , ].

Echothiophate phospholine Fig. It is used as an ocular antihypertensive in the treatment of chronic glaucoma and, in some cases, accommodative esotropia. Its application is local eye drops , and the effects can last a week or more.

The drug is available under several trade names such as phospholine iodide. Adverse effects include muscle spasm and other systemic effects [ ].

OP compounds may be used in the therapy of neurological damages such as AD and Parkinson's disease. The example is trichlorfon metrifonate Fig. The primary target of OP action is AChE, and the main mechanism of toxicity in acute OP exposure involves the specific irreversible inhibition of this enzyme activity in the nervous system and blood, manifesting as a cholinergic crisis with excessive glandular secretions and weakness, miosis and fasciculation of muscle, which may lead to death [ , , ].

Additionally, many studies suggest that both acute and chronic intoxication disturb the redox processes changing the activities of antioxidative enzymes and causing enhancement of lipid peroxidation in many organs, and there is little correlation between organ damage and the degree of OP induced AChE inhibition [ - ].

Indeed, in acute, and rather subchronic or chronic OP exposition, induction of oxidative stress has been reported as the main mechanism of its toxicity [ ]. Oxidative stress is defined as an imbalance between the production of free radicals — reactive oxygen species ROS and the antioxidant defense system — enzymatic and non-enzymatic.

The ROS may be generated as the result of the metabolism of OPs by cytochrome Ps, monooxygenases that catalyze oxidation by addition of one atom of molecular oxygen into the substrate OP by electron transport pathway [ ]. There is some evidence that OPs may affect liver, kidney, muscles, immune, and hematological system, causing many human body disorders [ - ].

Also, some findings indicate oxidative stress as an important pathomechanism of neurological disorders such as AD and Parkinson's disease, as well as of cardiovascular diseases [ , , ]. The highly reactive free radicals attack DNA resulting in single and double strand breaks, as well as oxidative damage to sugar and base residues that can later be converted to strand breaks [ ].

On the other hand, phosphorus moiety in the OPs appears to be a good substrate for nucleophilic attack leading to phosphorylation of DNA which is an instance of DNA damage [ ]. Some reported studies indicate the increase in chromosomal aberrations CA , micronuclei MN and sister chromatid exchanges SCE , as the markers of cytogenetic damage, in cultured lymphocytes isolated from peripheral blood taken from exposed individuals.

Thus, cytogenetic damage in circulating lymphocytes has been widely used as a biomarker of exposure and effects of pesticides [ , ]. It has been reported that AChE non-inhibiting OP decomposition products exert stronger genotoxic potency compared to the parent compound [ 99 ], suggesting that the risk of genotoxicity from some insecticides might be appreciably greater than that predicted from standard toxicity tests [ ].

Moreover, DNA damage leads to genomic instability that may result in mutagenesis and carcinogenesis [ ]. Some epidemiological studies demonstrate cancer risk due to pesticides exposure [ - ], while The United States Environmental Protection Agency lists parathion as a possible human carcinogen [ ]. The mechanism of OP insecticides action is based on the irreversible inhibition of AChE in an insect body, resulting in the disrupted neuronal transmission and the consequent death.

However, the OPs are not selective for insect species, but they have the same mechanism of action for the warm-blooded organisms including humans that may be also intoxicated. Actually, OPs irreversibly inhibit human AChE in Ser leading to the cholinergic crisis which manifests as the muscarinic lacrimation, salivation, miosis , nicotinic neuromuscular blockade or central breath depression symptoms, and the organism death in the case of untreated OP intoxication [ 52 ].

OP poisoning can be treated non-pharmacologically and pharmacologically [ ]. The non-pharmacologic treatment includes resuscitation, oxygen supply or decontamination depending on the OP entrance to the human body e. Parasympatolytics usually atropine, Fig. The causal treatment comprises AChE reactivators that, unlike the symptomatic drugs, regenerate the enzyme native function by cleaving OP moiety from AChE serine active site.

The mechanism of AChE reactivation is based on the nucleophilic attack of the reactivator hydroxyiminomethyl oxime moiety towards the OP moiety of the phosphorylated AChE. However, the inactivated phosphorylated enzyme can be dealkylated Fig. Therefore, the oxime reactivators should be administered rapidly after the OP poisoning [ ]. The commercially available oxime reactivators - pralidoxime, methoxime, trimedoxime, obidoxime, asoxime Fig. Additionally, some of them exerted good effects in the restoration of inhibited cholinesterases by OP insecticides as well [ ].

Although pralidoxime was the first synthesized and most common used causal drug, its capability to regenerate the inhibited enzyme activity is not satisfactory. On the contrary, bisquaternary oximes, trimedoxime and obidoxime exhibited very good abilities in the treatment of OP insecticides poisoning, whereas asoxime was found to be effective for nerve agent induced intoxication and without the meaningful reactivation of the inhibited AChE by OP insecticides.

Moreover, obidoxime, combined with atropine and diazepam, has demonstrated positive results in the clinical trials [ , ]. Furthermore, among synthesized and evaluated oxime reactivators, some novel compounds e. K, BTM, TMB-4, BT possess, related to the commercially available reactivators, both better reactivation capabilities against different OP insecticides and significantly decreased toxicity tested using in vitro and in vivo animal models [ - ].

Carbamates and OPs are apolar compounds accumulating in fatty tissues, and can be eliminated by their conversion to water soluble compounds. The most effective way to increase the water solubility of these toxic compounds is hydrolysis to much more water soluble metabolites that may be removed in urine.

Although these insecticides are able to hydrolyze spontaneously especially at high pH, the main route of their detoxification is enzymatic degradation by hydrolases generating less toxic metabolites. Carbamates are most effectively decomposed by carboxylesterases CESs , the esterases capable to hydrolyze carboxyl esters [ ].

The mechanism of the CESs catalyzed hydrolysis of carboxyl esters Fig. Subsequently, the acylated intermediate is decomposed by nucleophilic water attack to the corresponding carboxylic acid and the free active CES participating in the new catalytic cycle. Highest CESs concentrations have been found in serum and liver of mammals [ ]. Since the carbamates are structurally similar to carboxyl esters, these insecticides are susceptive to CESs catalyzed hydrolysis. The mechanism of carbamates induced inhibition of CESs is similar to the mechanism of hydrolysis of the enzyme natural substrates.

The additional step is the final rapid degradation of carbamic acid to carbon dioxide and the corresponding amine Fig. Moreover, oxidized metabolites of the parent carbamate insecticides can also be decomposed by CESs due to the intact carbamic ester bond [ 71 ]. The metabolism of carbamates depends on their chemical structure and animal species. So, the hydrolysis of carbaryl in rabbit serum is significantly more efficient than chicken and human serum activity.

Also, CESs with the ability to hydrolyze carbamates have been found in some bacteria Blastobacter, Arthrobacter, Pseudomonas, Achromobacter, Micrococcus [ 71 , ]. Additionally, some findings have demonstrated the capacity of serum albumin to decompose N -methylcarbamates and some OPs, and its hydrolyzing potency differs among species.

Generally, mammals exert higher hydrolyzing activity than birds [ 71 , , ]. OPs are detoxified through oxidation and hydrolysis. Actually, in mammals especially in liver and serum there is a pool of CESs of unknown physiological role, named B-esterases that both hydrolyze carboxyl esters and being inhibited by OPs. Different from A-esterases being capable to hydrolyze carboxyl esters but not inhibited by OPs, B-esterases may be involved in the detoxification of OPs, and carbamates as well [ ].

While OP and carbamate insecticides exhibit major toxic effects through phosphorylation and carbamylation, respectively, of AChE and neuropathy target esterase inactivating these CESs, the inhibition of B-esterases is not associated with evident toxic effects. The mechanism of B-esterases inhibition by OPs is analogous to that induced by carbamates Fig. The difference is in the final step; the phosphorylated enzyme cannot be reactivated by water, and cannot release the free active enzyme Fig.

In this detoxification system that removes the insecticides from the media, each molecule of CESs scavenges at least one molecule of the toxic compound before this reaches targets in the nervous system [ 71 , ]. However, much more efficient route of OPs detoxification is hydrolysis by PTEs, esterases of an unknown physiological role. The resulting metabolites are less toxic and more polar than the parent OP, and therefore do not accumulate in fatty tissues and are eliminated in urine.

PTEs have been found in various biological tissues of mammals, fish, birds, molluscs and bacteria. Although these detoxification enzymes are located in a few tissues of mammals, higher PTEs activity levels have been detected in serum and liver [ , ]. Some studies indicate correlation between PTEs presence in some species and their susceptibility to toxic effects of OPs.

So, insects and birds lack paraoxonase, whereas mammals contain high concentrations of this PTE making them more resistant to paraoxone toxicity [ ]. For the strong potency to degrade the toxic insecticides, exogenous purified PTEs exert protective effects against OPs poisoning and can be used as prophylactics and antidotes in the therapeutic treatment of OPs intoxication [ - ].

Besides, these decomposing enzymes exhibit promising applications in other biotechnology fields: bioremediation of wasted materials, biodegradation of insecticide residues, detoxifying warfare arsenals, development of biosensors for OPs [ , ]. Reversible, competitive or noncompetitive, inhibitors donepezil, rivastigmine, galantamine are protagonists in the pharmacotherapy of AD symptoms. Their therapeutic effect is based on maintaining ACh level through slowing down its hydrolysis rate.

Hence, these generally well tolerated drugs enhance cholinergic neurotransmission in forebrain regions and compensate for the loss of functioning brain cells. On the other hand, carbamate reversible ACh inhibitors also exhibit toxic mode of action, and are used as insecticides, fungicides and herbicides.

Different from reversible inhibitors, irreversible AChE inactivators including OPs have toxicological relevance accompanied with ACh accumulation in the synaptic cleft and disrupted neurotransmission. Accordingly, these toxic compounds are applied as nerve gases, and insecticides being most frequently used in last two decades. OPs, as well as carbamate pesticides, can be detoxified through enzymatic hydrolysis in mammals. Carbamates, as structural analogs to carboxyl esters, mainly undergo CESs catalyzed degradation, whereas the most efficient detoxification route of OPs is the hydrolysis catalyzed by different PTEs.

Besides, OPs induced poisoning can be pharmacologically treated by simptomatic atropine, diazepam , and causal drugs such as oxime reactivators. Except their use as toxic compounds, some OPs demonstrate pharmacological significance. For instance, diisopropyl fluorophosphate and echothiophate have been administered in the treatment of chronic glaucoma.

Although the use of AChE inhibitors relies on the interaction with AChE as their primary target, a number of cholinesterase inhibitors such as OPs have additional sites of action that may have toxicological relevance.

Actually, acute, and rather subchronic or chronic OP exposition results in elevated ROS level attacking lipids, proteins, DNA, and consequently causes membrane damage, enzyme inactivation, DNA damage and cell death. National Center for Biotechnology Information , U. Journal List Curr Neuropharmacol v. Curr Neuropharmacol. Published online May. Author information Article notes Copyright and License information Disclaimer.

This article has been cited by other articles in PMC. Abstract Acetylcholinesterase is involved in the termination of impulse transmission by rapid hydrolysis of the neurotransmitter acetylcholine in numerous cholinergic pathways in the central and peripheral nervous systems. Acetylcholine as Neurotransmitter The first neurotransmitter discovered ACh is neurotransmitter at all autonomic ganglia, at many autonomically innervated organs, at the neuromuscular junction, and at many synapses in the central nervous system.

Open in a separate window. Acetylcholinesterase — Structure and Catalytical Function AChE is a serine hydrolase mainly found at neuromuscular junctions and cholinergic brain synapses. Reversible Acetylcholinesterase Inhibitors Reversible AChE inhibitors play an important role in pharmacological manipulation of the enzyme activity.

Donepezil Fig. Rivastigmine Fig. Galantamine trade name Razadyne, Nivalin is an alkaloid Fig. General chemical structure of biologically active carbamates. Selected carbamates being applied as herbicides and fungicides.

Irreversible Acetylcholinesterase Inhibitors — Organophosphorus Compounds OPs are esters or thiols derived from phosphoric, phosphonic, phosphinic or phosphoramidic acid Fig. Organophosphorus Insecticides The majority of OPs have been commonly used as nonspecific insecticides for over fifty years, to control a variety of insects in agriculture and the household environment.

Irreversible Acetylcholinesterase Inhibitors as Therapeutic Agents OPs, except their use as toxic compounds, have been applied in ophthalmology as therapeutic agents in the treatment of chronic glaucoma, an eye disease in which the optic nerve is damaged in a characteristic pattern.

Nonspecific Toxic Effects of Organophosphates The primary target of OP action is AChE, and the main mechanism of toxicity in acute OP exposure involves the specific irreversible inhibition of this enzyme activity in the nervous system and blood, manifesting as a cholinergic crisis with excessive glandular secretions and weakness, miosis and fasciculation of muscle, which may lead to death [ , , ]. Pharmacological Treatment of Organophosphates Induced Intoxication The mechanism of OP insecticides action is based on the irreversible inhibition of AChE in an insect body, resulting in the disrupted neuronal transmission and the consequent death.

Frequently used drugs for symptomatic treatment of OP intoxication. Regeneration of inhibited AChE activity by oxime reactivators. Detoxification of Organophosphorus and Carbamate Insecticides Through Enzymatic Hydrolysis Carbamates and OPs are apolar compounds accumulating in fatty tissues, and can be eliminated by their conversion to water soluble compounds. Molecular and cellular biology of cholinesterases. Histochemical localization of cholinesterase in the amphibian spinal cord and alterations following ventral root section.

Nerve impulses become blocked, causing muscles to become weak and easily tired. Symptoms fluctuate in severity. Acetylcholine is a chemical messenger that carries signals between nerve and muscle. An enzyme called acetylcholinesterase breaks down acetylcholine. Some drugs that are used to treat myasthenia gravis act on acetylcholinesterase to stop the breakdown of acetylcholine.

These acetylcholinesterase inhibitors increase the amount of acetylcholine available and so help muscle activation and contraction. We only included evidence from randomised controlled trials RCTs in the review. In RCTs, participants are assigned to groups by chance. This makes it more likely that any changes seen can be attributed to the treatments under study rather than to other possible causes.

We found only one RCT for the treatment of myasthenia gravis. The participants received either the study drug or placebo for the first period of the trial. They then received the other treatment for the second period of the trial. For example, if a person had study drug in the first period they received placebo for the second period.

If they had placebo for the first period, they received study drug for the second period. This type of study is called a 'cross-over' trial. The trial included 10 people with myasthenia gravis. In three people the condition affected only their eyes.

In seven people it affected the body more widely. The trial compared neostigmine an acetylcholinesterase inhibitor given via the nose, with placebo. Each treatment was given for two weeks.