Steady-state kinetic analysis of human cholinesterases over wide concentration ranges of competing substrates

Substrate competition for human acetylcholinesterase (AChE) and human butyrylcholinesterase (BChE) was studies under steady-state conditions using wide range of substrate concentrations. Competing couples of substates were acetyl-(thio)esters. Phenyl acetate (PhA) was the reporter substrate and competitor were either acetylcholine (ACh) or acetylthiocholine (ATC). The common point between investigated substrates is that the acyl moiety is acetate, i.e. same deacylation rate constant for reporter and competitor substrate.Steady-state kinetics of cholinesterase-catalyzed hydrolysis of PhA in the presence of ACh or ATC revealed 3 phases of inhibition as concentration of competitor increased: a) competitive inhibition, b) partially mixed inhibition, c) partially uncompetitive inhibition for AChE and partially uncompetitive activation for BChE. This sequence reflects binding of competitor in the active centrer at low concentration and on the peripheral anionic site (PAS) at high concentration. In particular, it showed that binding of a competing ligand on PAS may affect the catalytic behavior of AChE and BChE in an opposite way, i.e. inhibition of AChE and activation of BChE, regardless the nature of the reporter substrate.For both enzymes, progress curves for hydrolysis of PhA at very low concentration (?K_m) in the presence of increasing concentration of ATC showed that: a) the competing substrate and the reporter substrate are hydrolyzed at the same time, b) complete hydrolysis of PhA cannot be reached above 1 mM competing substrate. This likely results from accumulation of hydrolysis products (P) of competing substrate and/or accumulation of acetylated enzyme·P complex that inhibit hydrolysis of the reporter substrate.     

Effects of LF-14, THA and physostigmine in rat hippocampus and cerebral cortex

The effects of physostigmine, tetrahydroaminoacridine (THA) and LF-14 [3,3-dimethyl-1(4- amino-3-pyridyl)urea], a 3,4-diaminopyridine derivative, were compared on inhibition of acetyl- cholinesterase (AChE) activity, and release of [³H]acetylcholine (ACh) from rat brain cortical and hippocampal slices. All three compounds caused a concentration dependent inhibition of AChE, with an order of potency physostigmine > THA > LF-14. The electrically stimulated release of ACh from hippocampal and cortical slices was decreased by 10⁻⁵M physostigmine, although the effect was significant only in cortex. THA (5 × 10⁵M) caused a slight, but not significant, decrease in ACh release from both tissues. In contrast, LF-14 (5 × 10⁻⁵ M) caused an approx. 3-fold enhancement of stimulated release. When AChE was inhibited by prior addition of physostigmine, THA caused only a slight enhancement of ACh release, whereas LF-14 greatly increased release. ACh release was also reduced by stimulation of presynaptic muscarinic receptors with oxotremorine. In this case, THA had no effect on ACh release, while LF-14 was able to reverse the inhibition. This study suggests that LF-14 acts to promote ACh release through blocking K⁺ channels, and has a less potent AChE inhibitory effect. It is possible that a compound like LF-14 could be useful in treating diseases of cholinergic dysfunction such as Alzheimer's disease, by both promoting the release of ACh and inhibiting its breakdown.     

MEASUREMENT OF THE RATE OF ACETYLCHOLINE DIFFUSION THROUGH A BRAIN SLICE AND ITS SIGNIFICANCE IN STUDIES OF THE CELLULAR DISTRIBUTION OF ACETYLCHOLINESTERASE

The rate of ACh diffusion through a 0·8 mm thick slice from the surface of the rat cerebral cortex, under aerobic conditions at 37°C, was determined by bathing the intact surface of the slice (compartment A) with ACh containing buffer and determining the concentration of ACh in buffer bathing the cut surface of the slice (compartment B). With 1 or 5 mM-ACh in compartment A no ACh was detectable in compartment B within 3 h unless at least 95 per cent of the AChE, as assessed on homogenates, was inhibited. With a given level of AChE inhibition, the rate of ACh diffusion was dependent on its concentration in compartment A. With 1 mM-ACh in compartment A the difference between the rates of hydrolysis of ACh during diffusion through slices with an AChE inhibition of 98·3 and 99·4 per cent, as assessed by AChE assays of homogenates made from the slices, was only 6 per cent of the difference between the rates of hydrolysis of 1 m-ACh by the homogenates of that part of the slices through which diffusion took place. For 5 mM-ACh and levels of 95 and 99·2 per cent inhibition the corresponding value was 10-3 per cent. Since the concentration of ACh must fall across the slice it is not possible to calculate from these figures the number of enzyme sites involved in the hydrolysis during diffusion, i.e. the concentration of extracellular AChE. The implications of these observations are discussed, particularly in relation to studies of the release of ACh from the cerebral cortex in vivo     

PROPERTIES OF THE EXTERNAL ACETYLCHOLINESTERASE IN GUINEA-PIG IRIS

Abstract— The acetylcholinesterase (AChE) of intact iris, the so-called external AChE, differs in several respects from the AChE in an homogenate of iris, called the total AChE. Maximum enzyme activities of the external and total AChE were obtained with an ACh concentration of 10 and 1.3 m m , respectively. The total AChE exhibited substrate inhibition at high substrate concentrations, whereas the external enzyme did not exhibit substrate inhibition in the range studied. The external AChE activity, when measured at 1.3 m m -ACh. accounted only for 12% of the total enzyme activity. After irreversible inhibition of AChE with diisopropylfluorophosphate (DFP) or methylisocyclopentylfluorophosphate (soman) the external AChE recovered to almost normal values after 48 h, whereas only 30% of the total AChE recovered during this period. Pupillographic studies after inhibition with DFP demonstrated that pupillary diameter had reached normal size after 24 h.
Destruction of the cholinergic input to iris reduced the total AChE activity by 40%, but did not alter the external AChE activity nor its rate of recovery after DFP inhibition. The specific activities of total AChE and total choline acetyltransferase were significantly higher in the sphincter than in the dilator muscle. After such denervation of iris the greatest reduction in total AChE and choline acetyltransferase were found in the sphincter region. After treatment with DFP the total AChE was inhibited to the same extent and recovered at the same rate in both regions.
After extraction of AChE from iris with various salt solutions and detergents, the particulate enzyme recovered faster than the soluble enzyme from DFP inhibition.     

Effect of local acetylcholinesterase inhibition on sweat rate in humans.

ACh is the neurotransmitter responsible for increasing sweat rate (SR) in humans. Because ACh is rapidly hydrolyzed by acetylcholinesterase (AChE), it is possible that AChE contributes to the modulation of SR. Thus the primary purpose of this project was to identify whether AChE around human sweat glands is capable of modulating SR during local application of various concentrations of ACh in vivo, as well as during a heat stress. In seven subjects, two microdialysis probes were placed in the intradermal space of the forearm. One probe was perfused with the AChE inhibitor neostigmine (10 microM); the adjacent membrane was perfused with the vehicle (Ringer solution). SR over both membranes was monitored via capacitance hygrometry during microdialysis administration of various concentrations of ACh (1 x 10(-7)-2 M) and during whole body heating. SR was significantly greater at the neostigmine-treated site than at the control site during administration of lower concentrations of ACh (1 x 10(-7)-1 x 10(-3) M, P < 0.05), but not during administration of higher concentrations of ACh (1 x 10(-2)-2 M, P > 0.05). Moreover, the core temperature threshold for the onset of sweating at the neostigmine-treated site was significantly reduced relative to that at the control site. However, no differences in SR were observed between sites after 35 min of whole body heating. These results suggest that AChE is capable of modulating SR when ACh concentrations are low to moderate (i.e., when sudomotor activity is low) but is less effective in governing SR after SR has increased substantially.     

Acetylcholinesterase and the ADH-dependent transport of water in the amphibian bladder]

It was found that acetylcholine (ACh) at the concentration of 10(-3) M inhibited ADH-stimulated water transport through the wall of amphibian urinary bladder. This effect was suggested to be caused by an interaction of ACh with acetylcholinesterase (AChE) rather than by a stimulation of the M- or N-cholinoreceptor. The inhibitory action of ACh was completely suppressed in the presence of various AChE inhibitors (physostigmine, proserine, armine, Gd-42, acridine-iodmethylate), while an inhibitor of butyrylcholinesterase (BuChE), AD-4, failed to affect it. In accord with this observation the activity of AChE (but not of BuChE) was demonstrated in the urinary bladder epithelium. Since, in addition to the hydrosmotic effects of pituitrine, 8-arginine-vasopressin or oxytocin, ACh blocked also effects of forskolin or cyclic AMP, one may conclude that it acts at some post-cyclic AMP production stage. AChE-dependent inhibition of the ADH-stimulated water transport decreased significantly when the serosal pH was raising from 7.2 to 8.0, but was augmented by serosal acidification (pH 6.8), whereas such pH alterations did not affect the activity of the epithelium AChE. The effect of ACh under consideration was suppressed by adding amiloride (10(-4) M) to the serosal solution. Similarly, the ACh effect was blocked by an inhibitor of Ca-dependent K+ channels, 4-aminopyrdine, which in addition prevented the inhibition of the ADH-stimulated water transport by the serosal acidification. It was noteworthy that some other K+ channel blockers (Ba2+, Cs+, tetraethylammonium, apamine, quinine) did not affect either the water transport or the antipituitrine effect of ACh. In conclusion, we suggest that the inhibitory action of ACh on the ADH-stimulated water transport in the urinary bladder is mediated through the intracellular acidification resulting from ACh interaction with AChE. It is unlikely that the acidification is merely a consequence of the ACh hydrolysis, rather the ACh-AChE interaction induces directly an increase in the proton conductivity of the basolateral membrane of the urinary bladder epithelium.     

Acetylcholinesterase and butyrylcholinesterase activity in the atria of the heart of adult albino rats

In experiments on adult albino rats the authors used the substances BW 284 C51 (1.5-bis(allyldimethylammoniumphenyl)-pentane-3-one-dibromide) as a specific inhibitor of acetylcholinesterase (AChE) and ethopropazine (10-(2-diethylaminopropyl) phenothiazine hydrochloride) as a specific inhibitor of butyrylcholinesterase (BuChE) to determine the two enzyme activities in atrial homogenates and to investigate changes after AChE or BuChE inhibition of the negative chronotropic effect of acetylcholine (ACh) on atria incubated in vitro. AChE accounted for only 12% and BuChE for 88% of the total ability of atrial homogenates to hydrolyse acetylcholine. The concentration of exogenous ACh needed to reduce the spontaneous frequency of contractions of the isolated right atrium by 30, 60, or 90/min fell by 78%, 79% and 84% respectively after BW 284 C51 inhibition of AChE and by 95%, 94% and 94% after simultaneous inhibition of AChE and BuChE. The significance of AChE in control of the negative chronotropic effect of ACh is thus evidently significantly greater than would correspond to the percentual proportion of AChE in cholinesterase activities in the atria of the rat heart. In can be assumed that AChE is functionally associated with parasympathetic innervation of the heart and that it is probably present in a high concentration in the primary pacemaker region.     

Structural insights into substrate traffic and inhibition in acetylcholinesterase

Acetylcholinesterase (AChE) terminates nerve-impulse transmission at cholinergic synapses by rapid hydrolysis of the neurotransmitter, acetylcholine. Substrate traffic in AChE involves at least two binding sites, the catalytic and peripheral anionic sites, which have been suggested to be allosterically related and involved in substrate inhibition. Here, we present the crystal structures of Torpedo californica AChE complexed with the substrate acetylthiocholine, the product thiocholine and a nonhydrolysable substrate analogue. These structures provide a series of static snapshots of the substrate en route to the active site and identify, for the first time, binding of substrate and product at both the peripheral and active sites. Furthermore, they provide structural insight into substrate inhibition in AChE at two different substrate concentrations. Our structural data indicate that substrate inhibition at moderate substrate concentration is due to choline exit being hindered by a substrate molecule bound at the peripheral site. At the higher concentration, substrate inhibition arises from prevention of exit of acetate due to binding of two substrate molecules within the active-site gorge.     

Biochemical studies of the actions of ethanol on acetylcholinesterase activity: ethanol-enzyme-solvent interaction

1. Biochemical studies of the actions of ethanol on the activity of acetylcholinesterase (AChE), isolated from electric eel (Electrophorus electricus) and purified by affinity chromatography, were performed to elucidate ethanol-enzyme-solvent interactions. 2. Ethanol at a low concentration [( EtOH] = 2.7-200 mM) was found to enhance AChE activity slightly and systematically. 3. This observation was consistent with the result from enzyme-kinetic studies that ethanol might noncompetitively activate AChE activity at this lower concentration range. 4. If ethanol alters the hydrophobic site interaction on the enzyme and subsequently induces a favorable conformation for the active center of the enzyme, then a slight increase in the AChE activity in the presence of a low concentration of ethanol will be observed. 5. This speculation was supported by the finding of ethanol's ability to perturb the inhibition of AChE activity by tetrabutylammonium bromide and to affect hydrophobic interaction between this salt and AChE, as investigated by enzyme activity and microcalorimetric measurements. 6. The ethanol effect on the activity of this soluble AChE was found to be distinguishable from that on a membrane-bound AChE. 7. Furthermore, to elucidate the effect of ethanol-solvent interaction on AChE activity, enzyme activity in the presence of much higher concentrations of ethanol was also examined. 8. At [EtOH] greater than 800 mM, ethanol can perturb the structure of water around hydrophobic areas of AChE, causing an instability in the enzyme conformation and subsequently decreasing AChE activity.     

Non-quantal secretion of a mediator as factor determining consequences of acetylcholinesterase inhibition]

The mechanism of shortening MEPC decay phase after initial prolongation due to acetylcholinesterase inhibition by armine and neostigmine was studied by use of two-electrode voltage-clamp at the mice diaphragm Factors which switch off non-quantal secretion of acetylcholine from the nerve (acute denervation in vitro, ouabain, high concentration of magnesium ions) only slightly reduced the prolongation of MEPC caused by AChE inhibition. So, postsynaptic potentiation of MEPC by nonquantal ACh is not significant immediately after AChE inhibition. At the same time these factors abolished the process of shortening MEPC decay phase. It is concluded, that desensitization of the postsynaptic membrane induced by nonquantal ACh is the main mechanism of the MEPC shortening and that this mechanism can compensate insufficient AChE activity.     

Substrate activation in acetylcholinesterase induced by low pH or mutation in the π-cation subsite

Substrate inhibition is considered a defining property of acetylcholinesterase (AChE), whereas substrate activation is characteristic of butyrylcholinesterase (BuChE). To understand the mechanism of substrate inhibition, the pH dependence of acetylthiocholine hydrolysis by AChE was studied between pH 5 and 8. Wild-type human AChE and its mutants Y337G and Y337W, as well as wild-type Bungarus fasciatus AChE and its mutants Y333G, Y333A and Y333W were studied. The pH profile results were unexpected. Instead of substrate inhibition, wild-type AChE and all mutants showed substrate activation at low pH. At high pH, there was substrate inhibition for wild-type AChE and for the mutant with tryptophan in the π-cation subsite, but substrate activation for mutants containing small residues, glycine or alanine. This is particularly apparent in the B. fasciatus AChE. Thus a single amino acid substitution in the π-cation site, from the aromatic tyrosine of B. fasciatus AChE to the alanine of BuChE, caused AChE to behave like BuChE. Excess substrate binds to the peripheral anionic site (PAS) of AChE. The finding that AChE is activated by excess substrate supports the idea that binding of a second substrate molecule to the PAS induces a conformational change that reorganizes the active site.     

Characterization and distribution of cholinesterase activity in mouse uterine horns: changes in estrous cycle

1. Both butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) are present in the mouse uterus, BChE being more abundant.2. Their molecular forms were sequentially solubilized by different extraction media obtaining three ChE fractions whose specific activity was different, depending on the stage of the estrous cycle: hydrosoluble (estrous: 75.5 ± 6.6 and diestrous: 47.9 ± 8.7 μU/mg prot); detergent-soluble or amphiphilic (estrous 26.6 ± 2.4 and diestrous 14.7 ± 3.3 mU/mg prot.), and high ionic strength-soluble (estrous: 18.7 ±4.2 and diestrous 12.8 ± 1.2 mU/mg prot.).3. Histochemical procedures demonstrated a different distribution for both ChE activities. AChE was found in nerves next to smooth muscle cells of the circular layer and blood vessels, while BChE was concentrated in the longitudinal stratum surrounding the smooth muscle cells. Under the predominance of progesterone, BChE was also found in the endometrial glands.4. Maximal contractions evoked by the addition of ACh to the isolated organ bath were concentration dependent and greater in estrous than in diestrous. Nevertheless the difference at the two stages of the estrous cycle disappeared when contractions were normalized to smooth muscle cross-sectional area.5. BChE but not AChE inhibition augmented maximal contractions elicited by ACh in longitudinal but not in circular smooth muscle.6. The effect of BChE inhibition on the contractile force developed was greater at lower concentrations of ACh and did not depend on the stage of the estrous cycle.     

Interaction of an organophosphate with a peripheral site on acetylcholinesterase

O-Ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (MPT) is an active site directed inhibitor of acetylcholinesterase (AChE). Inhibition of the Electrophorus electricus (G4) enzyme follows classical second-order kinetics. However, inhibition of total mouse skeletal muscle AChE and inhibition of the individual molecular forms from muscle, including the monomeric species, do not proceed as simple irreversible bimolecular reactions. Similarly, complex inhibition kinetics are observed for the purified enzyme from Torpedo californica. AChE can be cross-linked with glutaraldehyde into a semisolid matrix. Under these conditions the abnormal concentration dependence for MPT inhibition is accentuated, and a range of MPT concentrations can be found where inhibition of polymerized AChE is far less than that observed at lower concentrations. Inhibition in certain concentration ranges is partially reversible after removal of all unbound ligand. Thus, there are two different modes of organophosphorus inhibition by MPT: the classical irreversible phosphorylation of the active site and a reversible interaction at a site peripheral to the active center. Propidium, a well-studied peripheral site ligand, can prevent the later interaction. Hence, the second site of MPT interaction with AChE may overlap or be linked to the peripheral anionic site of AChE characterized by the binding of propidium and other peripheral site inhibitors.     

Acetylthiocholine binds to asp74 at the peripheral site of human acetylcholinesterase as the first step in the catalytic pathway

Studies of ligand binding to acetylcholinesterase (AChE) have demonstrated two sites of interaction. An acyl-enzyme intermediate is formed at the acylation site, and catalytic activity can be inhibited by ligand binding to a peripheral site. The three-dimensional structures of AChE-ligand complexes reveal a narrow and deep active site gorge and indicate that ligands specific for the acylation site at the base of the gorge must first traverse the peripheral site near the gorge entrance. In recent studies attempting to clarify the role of the peripheral site in the catalytic pathway for AChE, we showed that ligands which bind specifically to the peripheral site can slow the rates at which other ligands enter and exit the acylation site, a feature we called steric blockade [Szegletes, T., Mallender, W. D., and Rosenberry, T. L. (1998) Biochemistry 37, 4206-4216]. We also demonstrated that cationic substrates can form a low-affinity complex at the peripheral site that accelerates catalytic hydrolysis at low substrate concentrations but results in substrate inhibition at high concentrations because of steric blockade of product release [Szegletes, T., Mallender, W. D., Thomas, P. J., and Rosenberry, T. L. (1999) Biochemistry 38, 122-133]. In this report, we demonstrate that a key residue in the human AChE peripheral site with which the substrate acetylthiocholine interacts is D74. We extend our kinetic model to evaluate the substrate affinity for the peripheral site, indicated by the equilibrium dissociation constant K(S), from the dependence of the substrate hydrolysis rate on substrate concentration. For human AChE, a K(S) of 1.9+/-0.7 mM obtained by fitting this substrate inhibition curve agreed with a K(S) of 1.3+/-1.0 mM measured directly from acetylthiocholine inhibition of the binding of the neurotoxin fasciculin to the peripheral site. For Torpedo AChE, a K(S) of 0.5+/- 0.2 mM obtained from substrate inhibition agreed with a K(S) of 0.4+/- 0.2 mM measured with fasciculin. Introduction of the D72G mutation (corresponding to D74G in human AChE) increased the K(S) to 4-10 mM in the Torpedo enzyme and to about 33 mM in the human enzyme. While the turnover number k(cat) was unchanged in the human D74G mutant, the roughly 20-fold decrease in acetylthiocholine affinity for the peripheral site in D74G resulted in a corresponding decrease in k(cat)/K(app), the second-order hydrolysis rate constant, in the mutant. In addition, we show that D74 is important in conveying to the acylation site an inhibitory conformational effect induced by the binding of fasciculin to the peripheral site. This inhibitory effect, measured by the relative decrease in the first-order phosphorylation rate constant k(OP) for the neutral organophosphate 7-[(methylethoxyphosphonyl)oxy]-4-methylcoumarin (EMPC) that resulted from fasciculin binding, decreased from 0.002 in wild-type human AChE to 0.24 in the D74G mutant.     

Recovery of acetylcholinesterase activity after irreversible inhibition by organophosphorous compounds in embryonic development

1. Recovery of acetylcholinesterase (AChE) activity was studied using the embryos of sea urchins Strongylocentrotus intermedius and S. nudus, embryos of axolotl Ambystoma mexicanum and in the chick embryo muscle culture treated by "irreversible" organophosphorous inhibitors (OPI). 2. AChE activity was assayed by a modified Ellman's procedure. 3. It follows from the data obtained that, unlike the plutei of sea urchins and the monolayer culture of chick embryo muscle cells, the embryos of axolotl show a compensatory increase in AChE biosynthesis after inhibition by OPI. 4. This mechanism is assumed to be related to the presence of a well developed neuromuscular system in the A. mexicanum embryos. 5. It is possible that acetylcholine accumulated as a result of partial AChE inhibition is responsible for the compensatory increase in AChE biosynthesis.     

The effect of heptyl-physostigmine,a new cholinesterase inhibitor,on the central cholinergic system of the rat

Heptyl-physostigmine (Heptyl-Phy; MF-201) is a new carbamate derivative of physostigmine (Phy) with greater lipophilicity and longer inhibitory action on cholinesterase (ChE) activity than the parent compound. Following single dose administration of 5 mg/kg heptyl-Phy i.m., maximal whole brain acetylcholinesterase (AChE) inhibition (82%) if reached at 60 min. Inhibition of plasma BuChE butyrylcholinesterase (BuChE) remains close to the steady state level (60%) between 120 and 360 min. At 360 min, whole brain AChE activity is still 67% inhibited compared to controls. Inhibition of AChE activity displays brain regional differences which are more significant at 360 min. At this time point, AChe activity in cerebellum is only 40% inhibited while frontal cortex and medial septum are still 80% inhibited. Increases in acetycholine (ACh) levels also show regional differences, however, there is no direct relationship between AChE inhibition and ACh increase. The electrically evoked [³H]ACh release in cortical slices was inhibited only by the highest concentration of heptyl-Phy tested (10^–4M). At this concentration ChE activity was 97% inhibited in vitro. In conclusion, our results demonstrate that heptyl-Phy compares favorably to other reversible cholinesterase inhibitors (ChEI), particularly to Phy as far as producing a more long-lasting inhibition of AChE and a more prolonged increase of ACh in brain with less severe side effects. Therefore, it represents an interesting candidate for cholinomimetic therapy of Alzheimer disease (AD).Dept. of Pharmacology, Shanghai Institute of Materia Medica Chinese Academy of Sciences Shanghai 20031 China.Special issue dedicated to Dr. Paola S. Timiras     

Cholinesterases of aphids—III. Sensitivity of acetylcholinesterases to several inhibitors as a possible phylogenetic character

The influence of 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB), three organophosphorous inhibitors (OPI) and physostigmine on the velocity of the acetylthiocholine enzymatic hydrolysis by the homogenates of 35 species of insects of the order Homoptera including 30 species of aphids (Aphidinea) was studied. Acetylcholinesterase (AChE) of the representatives of the suborders psylla (Psyllinea), coccids (Coccinea), whitefly (Aleyrodinea) and leafhoppers (Cicadinea) is not inhibited by DTNB and has lower sensitivity to OPI and higher sensitivity to physostigmine as compared to the AChE of most species of aphids which have been investigated. It was found that only aphid AChE is sensitive to DTNB, this property being less pronounced for AChE of the aphid families Phylloxeridae and Lachnidae than for AChE of the aphid families Callaphididae, Pemphigidae, Chaitophoridae, Anoeciidae, Thelaxidae and Aphididae. AChE of all the aphids under investigation had very similar sensitivity to the OPIs studied and to physostigmine with the exception of AChE of the grape phylloxera Viteus vitifolii Fitch (Phylloxeridae) whose sensitivity to OPI was extremely low. It was concluded that the Lachnidae family is more ancient compared to other contemporary families of viviparous aphids.     

Effect of acetylcholine and acetylcholinesterase on the activity of contractile vacuole of <Emphasis Type="Italic">Amoeba proteus</Emphasis>

Acetylcholine (ACh, 1 μM) stimulates the activity of contractile vacuole of the amoeba Amoeba proteus. The ACh action is not reproduced by ACh analogs carbacholine and 5-methylfurmethide that are not hydrolized by acetylcholinesterase (AChE). The ACh effect is not blocked by M-cholinolytics (atropine and metylone), but is suppressed by the N-cholynolytic tubocurarine (0.01 μM). The AChE inhibitors eserine (0.001 μM) and armine (0.01 μM) suppress action of ACh on the amoeba contractile vacuole. ACh does not affect the contractile vacuole activation produced by arginine-vasopressin (AVP, 1 μM), but blocks the contractile vacuole activation caused by the ligand of opioid receptors dynorphin A (1–13) at a concentration of 0.1 μM. Based on comparison of the obtained results with literature data, the conclusion is drawn that, in the described ACh effects, the enzyme AChE plays the role of synergist, but not of antagonist. Regulation of the contractile vacuole activity and, hence, the water-salt homeostasis of A. proteus is provided by three independent mechanisms through receptors of the AVP, ACh, and opioid systems.     

Substrate activation in acetylcholinesterase induced by low pH or mutation in the pi-cation subsite

Substrate inhibition is considered a defining property of acetylcholinesterase (AChE), whereas substrate activation is characteristic of butyrylcholinesterase (BuChE). To understand the mechanism of substrate inhibition, the pH dependence of acetylthiocholine hydrolysis by AChE was studied between pH 5 and 8. Wild-type human AChE and its mutants Y337G and Y337W, as well as wild-type Bungarus fasciatus AChE and its mutants Y333G, Y333A and Y333W were studied. The pH profile results were unexpected. Instead of substrate inhibition, wild-type AChE and all mutants showed substrate activation at low pH. At high pH, there was substrate inhibition for wild-type AChE and for the mutant with tryptophan in the pi-cation subsite, but substrate activation for mutants containing small residues, glycine or alanine. This is particularly apparent in the B. fasciatus AChE. Thus a single amino acid substitution in the pi-cation site, from the aromatic tyrosine of B. fasciatus AChE to the alanine of BuChE, caused AChE to behave like BuChE. Excess substrate binds to the peripheral anionic site (PAS) of AChE. The finding that AChE is activated by excess substrate supports the idea that binding of a second substrate molecule to the PAS induces a conformational change that reorganizes the active site.     

Effect of exogenous acetylcholine on potassium currents in the frog motor nerve ending during acetylcholinesterase inhibition

The effect of exogenous acetylcholine (ACh) on potassium currents in the motor nerve ending (NE) has been studied in neuromuscular preparations of the frog cutaneous-sternal muscle by extracellular recording of evoked electrical potentials from the NE. The investigation was performed during inhibition of acetylcholinesterase (AChE) activity by specific inhibitors and AChE removal from the synaptic cleft by collagenase. After AChE inhibition by either armine or proserine, or after treatment of the preparation with collagenase, no effect of exogenous ACh in concentrations of 1·10^–4–6·^–4 mole/liter was observed, in contrast to results from preparations with intact AChE. However, under the same conditions, as in the case of active AChE, ACh in concentrations of 7·10^–4–2·10^–3 mole/liter inhibited Ca-activated potassium current of the NE membrane. Experiments with dipyroxim, a synaptic AChE reactivator, have shown that the ACh effect on the potential-dependent potassium current is mediated by specific AChE. The role of AChE is discussed in respect to its significance for realization of the ACh action on potential-dependent potassium current in NE.Translated from Neirofiziologiya, Vol. 25, No. 2, pp. 146–149, March–April, 1993.