Acetylcholinesterase from human erythrocytes membranes: dimers as functional units

Synthesis, pharmacological assessment, and molecular modeling of acetylcholinesterase/butyrylcholinesterase inhibitors: effect against amyloid-β-induced neurotoxicity

The synthesis, molecular modeling, and pharmacological analysis of phenoxyalkylamino-4-phenylnicotinates (2-7), phenoxyalkoxybenzylidenemalononitriles (12, 13), pyridonepezils (14-18), and quinolinodonepezils (19-21) are described. Pyridonepezils 15-18 were found to be selective and moderately potent regarding the inhibition of hAChE, whereas quinolinodonepezils 19-21 were found to be poor inhibitors of hAChE. The most potent and selective hAChE inhibitor was ethyl 6-(4-(1-benzylpiperidin-4-yl)butylamino)-5-cyano-2-methyl-4-phenylnicotinate (18) [IC(50) (hAChE) = 0.25 ± 0.02 μM]. Pyridonepezils 15-18 and quinolinodonepezils 20-21 are more potent selective inhibitors of EeAChE than hAChE. The most potent and selective EeAChE inhibitor was ethyl 6-(2-(1-benzylpiperidin-4-yl)ethylamino)-5-cyano-2-methyl-4-phenylnicotinate (16) [IC(50) (EeAChE) = 0.0167 ± 0.0002 μM], which exhibits the same inhibitory potency as donepezil against hAChE. Compounds 2, 7, 13, 17, 18, 35, and 36 significantly prevented the decrease in cell viability caused by Aβ(1-42). All compounds were effective in preventing the enhancement of AChE activity induced by Aβ(1-42). Compounds 2-7 caused a significant reduction whereas pyridonepezils 17 and 18, and compound 16 also showed some activity. The pyrazolo[3,4-b]quinolines 36 and 38 also prevented the upregulation of AChE induced by Aβ(1-42). Compounds 2, 7, 12, 13, 17, 18, and 36 may act as antagonists of voltage sensitive calcium channels, since they significantly prevented the Ca(2+) influx evoked by KCl depolarization. Docking studies show that compounds 16 and 18 adopted different orientations and conformations inside the active-site gorges of hAChE and hBuChE. The structural and energetic features of the 16-AChE and 18-AChE complexes compared to the 16-BuChE and 18-BuChE complexes account for a higher affinity of the ligand toward AChE. The present data indicate that compounds 2, 7, 17, 18, and 36 may represent attractive multipotent molecules for the potential treatment of Alzheimer's disease.

Cholinesterase inhibitors: a patent review (2007 - 2011)

INTRODUCTION

Cholinesterase inhibitors participate in the maintenance of the levels of the neurotransmitter acetylcholine by inhibiting the enzymes implicated in its degradation, namely, butyrylcholinesterase and acetylcholinesterase. This pharmacological action has an important role in several diseases, including neurodegenerative diseases such as Alzheimer's.

AREAS COVERED

This article reviews recent advances in the development of cholinesterase enzyme inhibitors, covering the development of new chemical entities, new pharmaceutical formulations with known inhibitors or treatments in combination with other drug families.

EXPERT OPINION

The development of cholinesterase inhibitors has to face several issues, including the fact that the principal indication for these drugs, Alzheimer's disease, is not currently believed to derivate from a cholinergic deficiency, although most of the drugs clinically used for these disease are cholinesterase inhibitors. Moreover, the adverse effects found when administering cholinesterase inhibitors limit their use in other diseases, such as gastrointestinal diseases, glaucoma, or analgesia.

Cholinergic and neuroprotective drugs for the treatment of Alzheimer and neuronal vascular diseases. II. Synthesis, biological assessment, and molecular modelling of new tacrine analogues from highly substituted 2-aminopyridine-3-carbonitriles

The synthesis, biological assessment, and molecular modelling of new tacrine analogues 11-22 is described. Compounds 11-22 have been obtained by Friedländer-type reaction of 2-aminopyridine-3-carbonitriles 1-10 with cyclohexanone or 1-benzyl-4-piperidone. The biological evaluation showed that some of these molecules were good AChE inhibitors, in the nanomolar range, and quite selective regarding the inhibition of BuChE, the most potent being 5-amino-2-(dimethylamino)-6,7,8,9-tetrahydrobenzo[1,8-b]-naphthyridine-3-carbonitrile (11) [IC(50) (EeAChE: 14nM); IC(50) (eqBuChE: 5.2μM]. Kinetic studies on the easily available and potent anticholinesterasic compound 5-amino-2-(methoxy)-6,7,8,9-tetrahydrobenzo[1,8-b]-naphthyridine-3-carbonitrile (16) [IC(50) (EeAChE: 64nM); IC(50) (eqBuChE: 9.6μM] showed that this compound is a mixed-type inhibitor (K(i)=69.2nM) of EeAChE. Molecular modelling on inhibitor 16 confirms that this compound, as expected and similarly to tacrine, binds at the catalytic active site of EeAChE. The neuroprotective profile of molecules 11-22 has been investigated in SH-SY5Y neuroblastoma cells stressed with a mixture of oligomycin-A/rotenone. Compound 16 was also able to rescue by 50% cell death induced by okadaic acid in SH-SY5Y cells. From these results we conclude that the neuroprotective profile of these molecules is moderate, the most potent being compounds 12 and 17 which reduced cell death by 29%. Compound 16 does not affect ACh- nor K(+)-induced calcium signals in bovine chromaffin cells. Consequently, tacrine analogues 11-22 can be considered attractive therapeutic molecules on two key pharmacological targets playing key roles in the progression of Alzheimer, that is, cholinergic dysfunction and oxidative stress, as well as in neuronal cerebrovascular diseases.

Synthesis, inhibitory activity of cholinesterases, and neuroprotective profile of novel 1,8-naphthyridine derivatives

1,8-Naphthyridine derivatives related to 17 (ITH4012), a neuroprotective compound reported by our research group, have been synthesized. In general, they have shown better inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) than most tacrine derivatives previously synthesized in our laboratory. The compounds presented an interesting neuroprotective profile in SH-SY5Y neuroblastoma cells stressed with rotenone/oligomycin A. Moreover, compound 14 (ethyl 5-amino-2-methyl-6,7,8,9-tetrahydrobenzo[b][1,8]naphthyridine-3-carboxylate) also caused protection in cells stressed with okadaic acid (OA) or amyloid beta 1-42 peptide (Abeta(1-42)). Interestingly, compound 14 prevented the OA-induced PP2A inhibition, one of the enzymes implicated in tau dephosphorylation. This compound also exhibited neuroprotection against neurotoxicity elicited by oxygen and glucose deprivation in hippocampal slices. Because these stressors caused neuronal damage related to physiopathological hallmarks found in the brain of Alzheimer's disease (AD) patients, we conclude that compound 14 deserves further in vivo studies in AD models to test its therapeutic potential in this disease.

Approaches to determining membrane protein structures to high resolution: do selections of subpopulations occur?

Three different methods are currently used for the study of high-resolution structures of membrane proteins: X-ray crystallography, electron crystallography, and nuclear magnetic resonance (NMR) spectroscopy. Thus far, all methods combined have yielded a rather modest number of crystal structures that have been solved at the atomic level. It is hypothesized here that different methods may select different populations of proteins on the basis of various properties. Thus, protein stability may be a significant factor in the formation of three-dimensional (3D) crystals from detergent solutions, since exposure of hydrophobic protein zones to water may cause structural perturbation or denaturation in conformationally labile proteins. This is different in the formation of two-dimensional (2D) crystals where a protein remains protected in its native membrane environment. A biological selection mechanism may therefore be operative in that highly ordered lattices may form only if strong protein-protein interactions are relevant in vivo, thereby limiting the number of proteins that are amenable to electron crystallography. Keeping a protein in a bilayer environment throughout 3D crystallization maintains the lateral pressure existing in native membranes. This can be accomplished by using lipidic cubic phases. Alternatively, the hydrophobic interface of a membrane protein may be spared from contact with water by crystallization from organic solvents where the polar caps are protected in reverse micelles by using appropriate detergents. Some of the criteria that are useful in optimizing the various approaches are given. While the usefulness of complementary methods seems obvious, the results presented may be particularly critical in recognizing key problems in other structural approaches.

Kinetics of human acetylcholinesterase inhibition by the novel experimental Alzheimer therapeutic agent, tolserine

Characterization of the kinetic parameters of tolserine, a novel acetylcholinesterase (AChE) inhibitor of potential in the therapy of Alzheimer's disease, to inhibit purified human erythrocyte AChE was undertaken for the first time. An IC(50) value was estimated by three methods. Its mean value was found to be 8.13 nM, whereas the IC(100) was observed to be 25.5 nM as calculated by single graphical method. The Michaelis-Menten constant (K(m)) for the hydrolysis of the substrate acetylthiocholine iodide was found to be 0.08 mM. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the partial non-competitive type. The value of K(i) was estimated as 4.69 nM by the primary and secondary replots of the Dixon as well as secondary replots of the Lineweaver-Burk plot. Four new kinetic constants were also investigated by polynomial regression analysis of the relationship between the apparent K(i) (K(Iapp)) and substrate concentration, which may open new avenues for the kinetic study of the inhibition of several enzymes by a wide variety of inhibitors in vitro. Tolserine proved to be a highly potent inhibitor of human AChE compared to its structural analogues physostigmine and phenserine.

Kinetics of human erythrocyte acetylcholinesterase inhibition by a novel derivative of physostigmine: phenserine

The effect of phenserine, a novel cholinesterase inhibitor, was assessed for the first time on kinetic parameters of human erythrocyte acetylcholinesterase (AChE). Phenserine (0.025-0.40 microM) inhibited the activity of human erythrocyte AChE in a concentration-dependent fashion, the IC50 was 0.0453 microM. The Michaelis-Menten constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.124 mM and the Vmax was 0.980 mumol/min/mg protein. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the noncompetitive type. The value of Ki was estimated as 0.048 microM by the primary and secondary replots of the Dixon as well as secondary replots of the Lineweaver-Burk plot. A novel relationship between Ki and substrate concentration was also identified which permits more precise prediction of the specific type of noncompetitive inhibition of various enzymes by a wide variety of drugs, chemicals and, in some circumstances, by their own substrates.

Increased expression of acetylcholinesterase T and R transcripts during hematopoietic differentiation is accompanied by parallel elevations in the levels of their respective molecular forms

Differentiation of hematopoietic cells is known to be accompanied by profound changes in acetylcholinesterase (AChE) enzyme activity, yet the basic mechanisms underlying this developmental regulation remain unknown. We initiated a series of experiments to examine the molecular mechanisms involved in regulating AChE expression during hematopoiesis. Differentiation of murine erythroleukemia (MEL) cells using dimethyl sulfoxide resulted in a 5- and 10-fold increase in intracellular and secreted AChE enzyme activity, respectively. Interestingly, these increases resulted from a preferential induction of the globular molecular form G1 and a slight increase in G4 instead of an increase in the levels of the G2 membrane-bound form, a molecular form expressed in mature erythrocytes. Concomitantly, expression of the two predominant AChE transcripts (R and T, for read-through and tail, respectively) in MEL cells was induced to a similar extent with differentiation. Nuclear run-on assays performed with nuclei isolated from induced versus uninduced MEL cells revealed that in contrast to the large increases seen in the transcription of the beta-globin gene, the transcriptional activity of the AChE gene remained largely unaffected after differentiation. Determination of the half-lives of the R and T transcripts demonstrated that they both exhibited an increase in stability in induced MEL cells. Taken together, results from these studies indicate that post-transcriptional regulatory mechanisms account for the increased expression of AChE in differentiated hematopoietic cells.

Dimerization regulates the enzymatic activity of Escherichia coli outer membrane phospholipase A

The outer membrane phospholipase A (OMPLA) of Escherichia coli is present in a dormant state in the cell envelope. The enzyme is activated by various processes, which have in common that they perturb the outer membrane. Kinetic experiments, chemical cross-linking, and analytical ultracentrifugation were carried out with purified, detergent-solubilized OMPLA to understand the underlying mechanism that results in activation. Under conditions in which the enzyme displayed full activity, OMPLA was dimeric. High detergent concentrations or very dilute protein concentrations resulted in low specific activity of the enzyme, and under those conditions the enzyme was monomeric. The cofactor Ca2+ was required for dimerization. Covalent modification of the active site serine with hexadecylsulfonylfluoride resulted in stabilization of the dimeric form and a loss of the absolute calcium requirement for dimerization. The results of these experiments provide evidence for dimerization as the molecular mechanism by which the enzymatic activity of OMPLA is regulated. This dimerization probably plays a role in vivo as well. Data from chemical cross-linking on whole cells indicate that OMPLA is present in the outer membrane as a monomer and that activation of the enzyme induces dimerization concurrent with the appearance of enzymatic activity.

Genetic predisposition to adverse consequences of anti-cholinesterases in 'atypical' BCHE carriers

Normal butyrylcholinesterase (BuChE), but not several of its common genetic variants, serves as a scavenger for certain anti-cholinesterases (anti-ChEs). Consideration of this phenomenon becomes urgent in view of the large-scale prophylactic use of the anti-ChE, pyridostigmine, during the 1991 Persian Gulf War, in anticipation of nerve gas attack and of the anti-ChE, tacrine, for improving residual cholinergic neurotransmission in Alzheimer's disease patients. Adverse symptoms were reported for subjects in both groups, but have not been attributed to specific causes. Here, we report on an Israeli soldier, homozygous for 'atypical' BuChE, who suffered severe symptoms following pyridostigmine prophylaxis during the Persian Gulf War. His serum BuChE and recombinant 'atypical' BuChE were far less sensitive than normal BuChE to inhibition by pyridostigmine and several other carbamate anti-ChEs. Moreover, atypical BuChE demonstrated 1/200th the affinity for tacrine of normal BuChE or the related enzyme acetylcholinesterase (AChE). Genetic differences among BuChE variants may thus explain at least some of the adverse responses to anti-ChE therapies.

Total synthesis and functional properties of the membrane-intrinsic protein phospholamban

The membrane-intrinsic protein phospholamban (PLN), the regulatory protein of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase, was chemically synthesized. The synthesis was accomplished by double couplings and efficient capping procedures, thus eliminating hydrophobic failure sequences. The crude peptide was purified by high-performance liquid chromatographic ion exchange and gel permeation chromatography in chloroform-methanol mixtures. Ion spray mass spectroscopy showed that the product had the correct molecular mass. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis runs produced the typical monomer-pentamer structural pattern. A predominantly helical CD spectrum was obtained in 0.075% C12E8 (67.9% helix, 1.8% beta, 12.2% turn, 18.1% random coil). Synthetic PLN was phosphorylated in detergent solutions by protein kinase A with a stoichiometry close to 1:1 (Pi to PLN monomer). Reconstitution of the isolated skeletal muscle SR Ca2+ ATPase in phosphatidylcholine membranes in the presence of PLN using the freezing and thawing technique yielded a preparation with lower Ca(2+)-dependent ATPase activity. The inhibition was mainly due to a decrease in the affinity (Km(Ca)) of the ATPase for Ca2+ and was partially reversed by PLN phosphorylation with protein kinase A. By contrast, addition of PLN to diluted intact SR vesicles uncoupled the Ca(2+)-transport reaction, suggesting an ionophoric effect of PLN. Because this effect was observed at very high PLN-to-SR vesicle ratios and was not influenced by PLN phosphorylation, its biological function is doubtful.

Secreted acetylcholinesterase: non-classical aspects of a classical enzyme

Recent evidence suggests that termination of cholinergic transmission is just one of the many ways in which acetylcholinesterase (AChE) could influence neuronal function. Neuronal AChE can be secreted from several brain regions, while purified AChE possesses several properties (in addition to its cholinesterase activity) that can affect neuronal function, including the abilities to influence certain membrane conductances, enhance excitatory amino acid transmission and hydrolyse peptides. Loss of AChE and its non-classical actions would have a profound effect on brain function in neurodegenerative diseases such as Alzheimer's disease where there is widespread loss of AChE-containing neurons.

Glycolipid-anchored acetylcholinesterases from rabbit lymphocytes and erythrocytes differ in their sensitivity to phosphatidylinositol-specific phospholipase C

The type of membrane association of acetylcholinesterase (AChE, EC 3.1.1.7) was studied in rabbit lymphocytes and erythrocytes. In both cases, the unique AChE molecular form was an amphiphilic dimer (referred to as G2a) anchored in the membrane by a glycosylphosphatidylinositol. In lymphocytes, G2a AChE was directly converted into its hydrophilic G2h counterpart by a treatment with Bacillus thuringiensis phosphatidylinositol-phospholipase C (PI-PLC, EC 3.1.4.10). In erythrocytes, AChE was resistant to PI-PLC but was rendered sensitive by a prior deacylation with alkaline hydroxylamine. This observation suggests that, as previously reported for human erythrocyte AChE, an acylation of the inositol ring in the glycolipid anchor of rabbit erythrocyte AChE (that does not occur in lymphocytes) prevents the cleavage.

Alkylacyl glycerophosphoinositol in human and bovine erythrocytes. Molecular species composition and comparison with glycosyl-inositolphospholipid anchors of erythrocyte acetylcholinesterases

Glycosyl-inositolphospholipid (glycosyl-PtdIns) anchors of proteins in mammalian cells which have been analyzed so far are exclusively of the alkylacyl type. However, little is known about the putative precursor of glycosyl-PtdIns, the alkylacyl derivative of glycerophosphoinositol (GroPIns), in these cells since it is generally believed that cellular GroPIns consists of diacyl-type molecular species only. In this report, we describe the isolation and identification of alkylacyl GroPIns molecular species in both human and bovine erythrocytes, and compare it with the molecular species compositions of the glycosyl-PtdIns anchors of human and bovine erythrocyte acetylcholinesterase. Diradyl GroPIns was isolated from lipid extracts of ghost membranes and treated with phospholipase C. Diradylglycerols of the glycosyl-PtdIns anchors of affinity-purified human and bovine erythrocyte acetylcholinesterase were generated by sequential treatment with glycoprotein phospholipase D and acidic phosphatase and by PtdIns-specific phospholipase C, respectively. Diradylglycerols were subsequently converted into benzoate derivatives and separated into diacyl, alkylacyl, and alkenylacylglycerol subclasses. The molecular species compositions were quantitated and determined by combined HPLC/mass spectrometry. We found that human and bovine erythrocyte membrane diradyl GroPIns consist of 1.5-4.8% alkylacyl GroPIns. Molecular species analysis showed a heterogeneous species composition for both human and bovine erythrocyte alkylacyl GroPIns. Their compositions are distinctly different from those of human and bovine erythrocyte acetylcholinesterase glycosyl-PtdIns anchors. The number of alkylacyl GroPIns molecules/cell is roughly equal with the number of glycosyl-PtdIns-anchored proteins in human erythrocytes.

Non-cholinergic actions of acetylcholinesterases: proteases regulating cell growth and development?

The enzyme acetylcholinesterase has a well-established function in limiting the duration of acetylcholine's action at cholinergic synapses. Until recently, the function of this enzyme in non-cholinergic tissues has been a mystery. Recent evidence suggests that some forms of acetylcholinesterase act as proteases to regulate cell growth and development.

Molecular forms of acetylcholinesterase in two sublines of human erythroleukemia K562 cells. Sensitivity or resistance to phosphatidylinositol-specific phospholipase C and biosynthesis

Acetylcholinesterase (AChE) in K562 cells exists in two molecular forms. The major form, an amphiphilic dimer (G2a) which sediments at 5.3 S, and the minor form, an amphiphilic monomer (G1a) which sediments at 3.5 S. Extraction in the presence of the sulfhydryl alkylating agent N-ethylmaleimide was required to preserve the G2a form. In Triton X-100 extracts of the subline K562-243, phosphatidylinositol-specific phospholipase C (PtdIns-PLC) from Bacillus thuringiensis converted most of the G2a AChE into a hydrophilic dimer (G2h), indicating that the G2a form possessed a hydrophobic glycoinositol phospholipid that mediated its attachment to the membrane. Treatment of intact K562-243 cells with PtdIns-PLC released approximately 60% of the total AChE activity and provided an estimate of the externally exposed AChE. The direct conversion from an amphiphilic to a hydrophilic dimeric form by PtdIns-PLC was not obtained in extracts or intact cells of the subline K562-48. Instead, pretreatment with alkaline hydroxylamine was necessary to render the amphiphilic G2 form of this subline susceptible to digestion by the phospholipase. In this respect, the amphiphilic dimer of K562-48 AChE resembles the G2a form of human erythrocyte AChE, which is resistant to PtdIns-PLC because of the direct palmitoylation of an inositol hydroxyl group in the anchor [Roberts et al. (1988) J. Biol. Chem. 263, 18766-18775]. Release of this acyl chain by hydroxylamine renders the enzyme susceptible to PtdIns-PLC [Toutant et al. (1989) Eur. J. Biochem. 180, 503-508]. In both K562 sublines, sialidase decreased the migration of the G2a form but not of the G1a form of AChE. G1a forms thus appear to represent an intracellular pool of newly synthesized molecules residing in a compartment proximal to the trans-Golgi apparatus. The sialidase-resistant G1a molecules were also resistant to PtdIns-PLC digestion; possible explanations for this resistance are presented.

Immunoglobulins from patients with amyotrophic lateral sclerosis affect human erythrocyte acetylcholinesterase

Human erythrocyte acetylcholinesterase (AChE) solubilized with Triton X-100 and obtained as a complex with micelles containing Triton and membrane phospholipids was incubated with immunoglobulins (Igs) from patients with amyotrophic lateral sclerosis (ALS) and from normal individuals. The temperature dependence of the AChE activity was determined. Biphasic (broken) Arrhenius plots were obtained with control Igs with the break point at 32.8 +/- 0.3 degrees C (SD, n = 18) indicating that the enzyme changes its conformation at this temperature. With ALS-Igs monophasic (linear) plots were observed in 14 cases and a biphasic in one case. ALS-Igs prevent the conformational change occurring at the break point temperature. The activation energy at physiological temperature increased by 60% from 2.4 to 3.8 kcal/mol (10.0-15.9 kJ/mol) which implies that ALS-Igs inhibit AChE. Thus, ALS-patients have autoantibodies that change the normal behaviour of erythrocyte AChE and which bind to the enzyme molecule or/and to phospholipids associated with the enzyme. At least part of the autoantibodies should be directed against the enzyme molecule, since a change in the Arrhenius plot was also observed in a control experiment with AChE which probably had micelles without any phospholipids. This enzyme was isolated by affinity chromatography and was washed with a buffer containing Triton X-100 before desorption from the affinity column, a treatment known to remove all phospholipids from erythrocyte AChE.

Acetylcholinesterase in the human erythron. II. Biochemical assay

Acetylcholinesterase (AChE) is an integral erythrocyte membrane protein. A role for the enzyme in the developing human erythron is being explored. Assays of AchE by the standard Ellman technique overestimate the amount of enzyme by failing to account for the contribution of hemoglobin to the optical density of the reaction mixture. Furthermore, reliance on substrate selection alone for specificity is unsatisfactory. Incorporation of inhibitors of "true" AchE and of pseudocholinesterase confer greater ability to distinguish one enzyme from the other. In our experience, the inhibitor constant (Kl) for edrophonium, which is highly specific for AChE, is approximately 5 x 10(-5) M against adult human erythrocytes that contain significantly more total cholinesterase activity than do erythrocytes from umbilical cord blood. This consists of both "true" and "pseudo" enzyme, the former predominating and accounting for 0.75-1.65 (mean 1.02, median 0.87) femtomoles of substrate hydrolysed per min per cell in adult blood, with values of 0.15-1.04 (mean 0.71, median 0.73) obtained on cord blood. Moreover, the enzyme activity in neonatal erythrocytes has a rather different inhibitor profile from that of adult cells. AChE was also demonstrated in fresh (ALL) and cultured (K562 and HL60) human leukemic cells, as well as in primitive granulocyte-macrophage and erythroid cells cloned from normal human bone marrow. In the erythroid colonies the enzyme activity was 0-3.76 (mean 1.20, median 0.76) femtomoles per min per cell, apparently the first successful measurement of AChE in such cells.

Molecular form of human lymphocyte membrane-bound acetylcholinesterase

The membrane-bound acetylcholinesterase (AchE) from human peripheral blood lymphocyte gives only one symmetrical peak on sucrose density gradient centrifugation in the presence of Triton X-100 detergent, with the calculated sedimentation coefficient of 6.5 S. However, this dimeric form of AchE was converted to a monomeric 3.8 S form when treated with 2-mercaptoethanol and iodoacetic acid. The results are consistent with studies which have shown by sodium dodecyl sulfate gel electrophoresis that the enzyme is built up of two identical monomers inter-linked by disulfide bond(s). Under reducing conditions, revealed a single species of 70,000 molecular weight, whereas under non-reducing conditions, another species of 140,000 molecular weight of the AchE was found. Polyacrylamide gel electrophoresis indicated a single band with AchE activity in the presence of Triton X-100. In contrast, in the absence of the same detergent multiple band pattern could be observed. These results suggest that membrane-bound AchE enzyme is present in homogenous dimeric form on human lymphocyte membrane.

Erythrocyte acetylcholinesterase in psychiatric disorders and controls

Acetylcholinesterase (AChE) from erythrocytes was solubilized by Triton X-100. Size and charge heterogeneity of AChE was investigated by polyacrylamide gel electrophoresis (PAGE) and isoelectric focusing (IEF) in polyacrylamide gels in the presence of 0.5% (v/v) Triton X-100. There were no interindividual differences in these parameters in 46 psychiatric patients (schizophrenia, major affective disorder, personality disorder, dependency, dementia) and controls. The specific activity of solubilized AChE did not discriminate between controls and patients or between the diagnostic subgroups.

Molecular forms of acetylcholinesterase and butyrylcholinesterase in human plasma and cerebrospinal fluid

The measurement of cholinesterase activities in either plasma or cerebrospinal fluid (CSF) may ultimately prove to be relevant in the diagnosis of neurological and neuropsychiatric disorders. However, studies to date have examined only total enzyme activities. Therefore in the present study we have examined the distribution of the individual molecular forms of both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in plasma and CSF using sucrose density gradient centrifugation. Although the total activities of AChE were of the same order of magnitude in plasma and CSF, there was a considerable difference (120-500-fold) between total BChE activity in the CSF and the BChE-rich plasma. The analysis of the individual molecular forms revealed that the predominant molecular species of AChE and BChE in the CSF--both lumbar and ventricular--was the G4 form. The G4 form also constituted the majority of the plasma BChE activity and, on average, over half (56%) of the plasma AChE activity. The significance of the AChE and BChE molecular form compositions of both plasma and CSF and their possible relationship to pathological states are discussed.

Neocortical cholinergic enzyme and receptor activities in the human fetal brain

In the human fetus, obtained postmortem at estimated gestational ages of 8-22 weeks, biochemical activities of cortical choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) were comparable to those of adult brain tissue. In contrast cholinergic receptor binding, including muscarinic M1 and M2 subtypes (measured by displacement of [3H]N-methylscopolamine with, respectively, pirenzepine and carbachol) and [3H]nicotine (putative nicotinic) binding were undetectable before 13-14 weeks and even at 22 weeks were substantially (three- to fourfold) below the respective adult values. Cortical ChAT activity decreased significantly with gestational age whereas binding to the three receptors, including the proportion M1/M2, increased significantly. AChE was present at all ages investigated as the two molecular monomeric (G1) and tetrameric (G4) forms. The proportion of G4, which was much more soluble in fetal compared with adult cortex, increased approximately threefold. Histochemically AChE, although intense in the nucleus of Meynert, was generally confined to subcortical white matter at early fetal developmental periods, appearing later in the cortex localized to nerve fibres and occasional cell bodies. These observations suggest that during the second trimester of human fetal development, cortical cholinergic function may be preceded by relatively high ChAT activity and paralleled not only by increasing receptor binding but also by a proportional increase in the tetrameric form and histochemical reactivity of AChE.

Blood acetyl- and butyrylcholinesterases in senile dementia of Alzheimer type

The major neurochemical abnormality described to date in senile dementia of the Alzheimer type (SDAT) is a central cholinergic deficit. To determine whether this central deficit is reflected by changes in the levels of blood cholinesterases, plasma and erythrocyte acetylcholinesterase (AChE) and plasma butyrylcholinesterase (BChE), were measured in SDAT and other psychiatric disorders. Plasma AChE, which has only recently been described in human blood, was significantly elevated (P less than 0.01) in the SDAT group compared with the control and other clinical groups investigated. In contrast there were no significant differences in the activities of either erythrocyte AChE or plasma BChE between any of the clinical groups. Although the source of plasma AChE is unknown the possibility that some portion originates from the central nervous system and that the elevated AChE levels in SDAT reflect increased release from degenerating cholinergic neurons is discussed.

Human acetylcholinesterase. Immunochemical studies with monoclonal antibodies

Monoclonal antibodies were used to investigate the immunochemistry of human erythrocyte acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7). A series of experiments on the sedimentation velocity and Stokes radius of acetylcholinesterase and its immune complexes indicated that each antibody recognized a single high-affinity binding site (epitope) on the monomeric enzyme. Further analysis suggested that the antibody-binding sites were replicated on multimeric enzyme forms but were subject to steric hindrance between nearby IgG molecules or adjacent enzyme subunits. The cellular localization of the epitopes was studied by measuring the binding of monoclonal antibodies to the cholinesterase of intact erythrocytes. The results implied that most of the epitopes are exposed to the external media. However, one antibody failed to bind to intact cells, despite a relatively high affinity for detergent-solubilized antigen, possibly because its epitope is buried in the lipid bilayer.

Characterization of an active hydrophilic erythrocyte membrane acetylcholinesterase obtained by limited proteolysis of the purified enzyme

Purified human erythrocyte membrane acetylcholinesterase was subjected to limited proteolysis with papain. This treatment generated a hydrophilic form of the enzyme as determined by charge-shift crossed immunoelectrophoresis and by binding to phenyl-Sepharose. The hydrophilic enzyme was stable and its activity was independent of the presence of amphiphiles. Electroimmunochemical analysis showed no antigenic difference between the two enzyme forms. Although the proteolytic treatment only brought about a small change in molecular weight, marked differences in the hydrodynamic properties were encountered. The Stokes radius decreased from 8.2 to 5.9 nm and the sedimentation coefficient increased from 6.3 to 7.0 S. The results are consistent with the view that a short hydrophobic peptide responsible for the amphipatic character of acetylcholinesterase is removed by the treatment with papain.

Amphiphile dependency of the monomeric and dimeric forms of acetylcholinesterase from human erythrocyte membrane

Human erythrocyte membrane-bound acetylcholinesterase was converted to a monomeric species by treatment of ghosts with 2-mercaptoethanol and iodoacetic acid. After solubilization with Triton X-100, the reduced and alkylated enzyme was partially purified by affinity chromatography and separated from residual dimeric enzyme by sucrose density gradient centrifugation in a zonal rotor. Monomeric and dimeric acetylcholinesterase showed full enzymatic activity in presence of Triton X-100 whereas in the absence of detergent, activity was decreased to approx. 20% and 15%, respectively. Preformed egg phosphatidylcholine vesicles fully sustained activity of the monomeric species whereas the dimer was only 80% active. The results suggest that a dimeric structure is not required for manifestation of amphiphile dependency of membrane-bound acetylcholinesterase from human erythrocytes. Furthermore, monomeric enzyme appears to be more easily inserted into phospholipid bilayers than the dimeric species.

Stereospecific reactivation by some Hagedorn-oximes of acetylcholinesterases from various species including man, inhibited by soman

Reactivation by bispyridinium mono-oximes (Hagedorn-oximes) and some classical oximes (0.03 or 1mM) was studied in vitro of rat, bovine and human erythrocyte acetylcholinesterase and of electric eel acetylcholinesterase inhibited by soman. Relative reactivating potencies of the oximes are similar for the three inhibited erythrocyte enzymes. In general, Hagedorn-oximes are more potent than the classical oximes. Among the Hagedorn-oximes, HI-6 is the most potent reactivator for the three inhibited enzymes. Relative reactivating potencies for the inhibited erythrocyte acetylcholinesterases and electric eel acetylcholinesterase, however, clearly differ. Since the reactivation experiments were carried out with racemic soman, a mixture of the two inhibited enzymes may be formed, which may cause additional problems in the comparison of various results. In order to get more detailed information on differences between human erythrocyte and electric eel acetylcholinesterase, reactivation of these enzymes inhibited with the P(-)-isomers of C(+)- and C(-)-soman were studied separately. Reactivation appeared to be dependent on the chirality of the alpha-carbon atom in the pinacolyl group. HI-6 is by far the most potent reactivator for the human enzyme inhibited by the two P(-)-isomers. It is suggested that electric eel acetylcholinesterase is not a reliable model for in vitro testing of therapeutic potencies of oximes against soman intoxication in mammals. Rate constants of aging of the four acetylcholinesterases inhibited with racemic soman and of the human and eel enzyme inhibited by the P(-)-isomers of C(+)- and C(-)-soman were also determined. The aging of the inhibited rat enzymes proceeds remarkably slowly (t1/2 = 21 min). The rate of aging is not affected by the chirality on the alpha-carbon atom in the pinacolyl group. Consequences of the present results are discussed in view of extrapolation of reactivation data of a series of reactivators to their relative therapeutic effect, ultimately in man. It is speculated that the more rapid aging of the human inhibited enzyme may hamper oxime-therapy in man more seriously than in rat.

[Properties and characterization of soluble forms of lymphocyte acetylcholinesterase from an ox]

Two soluble forms of AChE from lymphocyte membrane have been obtained, the Triton solubilized Sd form and the high molar salt solubilized Ss form. They present similar Km (0.10 mM). Hydrodynamic properties of these forms have been studied on saccharose gradients with and without detergent or salt. A similar sedimentation coefficient has been found for these two forms (5.7 S). Lymphocyte plasma membrane AChE is a dimeric form (G2). Without detergent, the Sd form shows multiple secondary forms due to main form polymerization. Increase of NaCl concentration (2M) gives rise to a partial dissociation of these polymers. In the same conditions, the Ss form is not affected. The Ss form centrifugated on cesium chloride gradient has a higher density than the Sd form. These two forms have been treated by HPLC: the Stokes radii are respectively 7.1 nm for the Sd form and 4.5 nm for the Ss form. The molecular weights have been estimated at 175 000 for the Sd form and 105 000 for the Ss form. Pronase enzymatic digestion shows that the Ss form is more rapidly inactivated than the Sd form. Phospholipase C inhibits the Ss form and indicates that this form is a lipid-enzyme complex. The Sd form presents a different behaviour: this form is first activated, and afterwards inhibited by phospholipase C. This behaviour could be due to a more preponderant lipidic environment for the Sd form. The Sd form is probably a detergent-lipid-enzyme complex with an important hydrophobocity. These two forms can be explained by a different association between the enzyme and the phospholipids at the plasma membrane.

A monomeric form of human erythrocyte membrane acetylcholinesterase

Purified detergent solubilized dimeric human erythrocyte acetylcholinesterase (6.3 S form) was converted to a stable monomeric 3.9 S species when treated with 2-mercaptoethanol and iodoacetic acid. More than 60% of the enzymatic activity were recovered after this treatment. A decreased susceptibility to reduction and alkylation was observed with purified, detergent depleted acetylcholinesterase aggregates. When erythrocyte membranes (ghosts) were subjected to the same treatment, acetylcholinesterase could subsequently be solubilized as monomeric 3.9 S form and and more than 90% of the activity were recovered. Monomeric acetylcholinesterase was less reactive towards antibodies raised against (dimeric) human erythrocyte membrane acetylcholinesterase and towards antibodies against human erythrocyte membranes. The results suggest that acetylcholinesterase is present as dimeric species in human erythrocyte membranes despite the fact that fully active monomers can be obtained.