p.D70G Asp70Gly c.293A>G (p.D98G Asp98Gly in primary sequence with 28 amino-acids signal peptide) rs1799807, Atypical variant, CHE1*A, Dibucaine-resistant I CHE*70G, the classic deficiency variant described by Kalow and Gunn (1959), Kalow and Staron (1957) , has a homozygote frequency of about 1:3,000 in white North Americans. (from OMIM) McGuire et al. (1989) found that a mutation in nucleotide 209, which changes codon 70 from GAT to GGT, was the abnormality in all 5 atypical cholinesterase families examined. The mutation caused the loss of a Sau3A1 restriction site. The gene change results in a substitution of glycine for aspartic acid as amino acid 70. This is an acidic to neutral amino acid change which accounts for the reduced affinity of atypical cholinesterase for choline esters. Aspartic acid must be an important component of the anionic site. Atypical BCHE, the classic deficiency variant described by Kalow (1962), Kalow and Gunn (1959), Kalow and Staron (1957), has a homozygote frequency of about 1:3,000 in white North Americans. In the nomenclature system of La Du et al. (1991), this allelic variant is referred to as CHE*70G. Reduced enzyme activity with butyrylthiocholine as substrate; ; 2-fold lower affinity for butyrylthiocholine; 10-fold lower affinity for butyrylthiocholine at homozygosity rs1799807
Succinylcholine is a neuromuscular block whose duration of action depends on rapid hydrolysis by butyrylcholinesterase (BChE). In patients with common BChE activities, succinylcholine duration of action is short (10min). BChE deficiency induces a slower hydrolysis of the drug and consequently prolonged neuromuscular block, leading to apnea. We report a case of prolonged neuromuscular block after administration of succinylcholine in a 14-year-old boy. Biological investigations revealed a marked BChE deficiency (1099U/L) related to the presence of three point mutations in the BCHE gene in a compound heterozygous state: p.Asp70Gly (rs1799807), p.Ala539Tyr (rs1803274), and p.Phe118Valfs*12 (rs398124632). The diagnosis of genetic BChE deficiency (OMIM 177400) was retained. This case is intended to present the pathophysiology of genetic BChE deficiency, its management, and the diagnostic strategy to be implemented.
Butyrylcholinesterase (BChE) deficiency is characterized by prolonged apnea after the use of muscle relaxants (suxamethonium or mivacurium). Although many acquired conditions may affect BChE activity, BChE deficiency is mainly due to mutations in the BCHE gene (MIM 177400). Though close to 70 natural mutations have been documented in human BCHE, the atypical variant (rs1799807) is the most frequently involved in prolonged apnea. We describe an HRM method for the detection of this variant. Thirty-four patients with known genotype [5 wild-type (U/U), 12 heterozygous (U/A), 17 homozygous (A/A) - A: atypical allele of BCHE, U: usual allele of BCHE -] were screened with the HRM analysis. Within and between-run precision were also evaluated. In silico prediction of HRM curves was performed in order to evaluate the potential impact of the other SNPs described within the PCR product on the HRM diagnostic accuracy. HRM analysis for the BCHE atypical variant genotyping is a simple, rapid, sensitive and low cost method.
Acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) catalyze the hydrolysis of the neurotransmitter acetylcholine and, thereby, function as coregulators of cholinergic neurotransmission. For both enzymes, hydrolysis takes place near the bottom of a 20 deep active site gorge. A number of amino acid residues within the gorge have been identified as important in facilitating efficient catalysis and inhibitor binding. Of particular interest is the catalytic triad, consisting of serine, histidine, and glutamate residues, that mediates hydrolysis. Another site influencing the catalytic process is located above the catalytic triad toward the periphery of the active site gorge. This peripheral site (P-site) contains a number of aromatic amino acid residues as well as an aspartate residue that is able to interact with cationic substrates and guide them down the gorge to the catalytic triad. In human AChE, certain aryl residues in the vicinity of the anionic aspartate residue (D74), such as W286, have been implicated in ligand binding and have therefore been considered part of the P-site of the enzyme. The present study was undertaken to explore the P-site of human BuChE and determine whether, like AChE, aromatic side chains near the peripheral aspartate (D70) of this enzyme contribute to ligand binding. Results obtained, utilizing inhibitor competition studies and BuChE mutant species, indicate the participation of aryl residues (F329 and Y332) in the E-helix component of the BuChE active site gorge, along with the anionic aspartate residue (D70), in binding ligands to the P-site of the enzyme.
Title: Butyrylcholinesterase gene mutations in patients with prolonged apnea after succinylcholine for electroconvulsive therapy Mollerup HM, Gatke MR Ref: Acta Anaesthesiologica Scandinavica, 55:82, 2011 : PubMed
BACKGROUND: patients undergoing electroconvulsive therapy (ECT) often receive succinylcholine as part of the anesthetic procedure. The duration of action may be prolonged in patients with genetic variants of the butyrylcholinesterase enzyme (BChE), the most common being the K- and the A-variants. The aim of the study was to assess the clinical significance of genetic variants in butyrylcholinesterase gene (BCHE) in patients with a suspected prolonged duration of action of succinylcholine after ECT. METHODS: a total of 13 patients were referred to the Danish Cholinesterase Research Unit after ECT during 38 months. We determined the BChE activity and the BCHE genotype using molecular genetic methods, the duration of apnea, time to sufficient spontaneous ventilation and whether neuromuscular monitoring was used. The duration of apnea was compared with published data on normal subjects. RESULTS: in 11 patients, mutations were found in the BCHE gene, the K-variant being the most frequent. The duration of apnea was 5-15 min compared with 3-5.3 min from the literature. Severe distress was noted in the recovery phase in two patients. Neuromuscular monitoring was used in two patients. CONCLUSION: eleven of 13 patients with a prolonged duration of action of succinylcholine had mutations in BCHE, indicating that this is the possible reason for a prolonged period of apnea. We recommend objective neuromuscular monitoring during the first ECT.
Title: Molecular basis of succinylcholine sensitivity in a prairie Hutterite kindred and genetic characterization of the region containing the BCHE gene Zelinski T, Coghlan G, Mauthe J, Triggs-Raine B Ref: Mol Genet Metab, 90:210, 2007 : PubMed
The tetrameric glycoprotein butyrylcholinesterase (BChE; EC 3.1.1.8) is one of two enzymes that hydrolyze choline esters. The controlling gene (BCHE) is comprised of four coding exons and is located on chromosome 3q26. Based on BChE activity measurements in the presence and absence of dibucaine, usual (designated U) and atypical (designated A) gene products have been distinguished. Homozygotes for the A gene product are at risk for prolonged apnea following exposure to the surgical anesthetics succinylcholine or mivacurium. In this report, we detail biochemical and molecular investigations of succinylcholine sensitivity in a prairie Hutterite kindred. Our results establish that BChE activities in the family members are impacted by two distinct BCHE mutations, namely, c.209A>G p. D70G and c.1615G>A p. A539T. However, homozygotes for the c.209A>G mutation (i.e., atypical or A) are the only individuals whose BChE activity could lead to adverse reactions to succinylcholine. Interestingly, haplotype analysis of the chromosomal region containing BCHE indicates that the c.209A>G mutation is carried on a unique haplotype, suggesting that it was likely introduced into the population only once. Conversely, the c.1615G>A mutation is carried on various haplotypes and was likely introduced into the population more than once.
Title: Naturally occurring mutation, Asp70his, in human butyrylcholinesterase Boeck AT, Fry DL, Sastre A, Lockridge O Ref: Annals of Clinical Biochemistry, 39:154, 2002 : PubMed
BACKGROUND: People with genetic variants of butyrylcholinesterase can have hours of prolonged apnoea after a normal dose of succinylcholine or mivacurium. METHODS: Serum samples from 308 persons living in mid-USA were phenotyped to identify the atypical and fluoride variants. 308 samples were analysed for the K variant by DNA amplification, digestion with Mae III and gel electrophoresis. Amplified DNA from 16 samples was sequenced to identify the D70G, T243M and D70H mutations. Values for kcat and Km were determined for the D70H mutant BChE expressed in 293T cells. RESULTS: A new mutation, Asp70His, was identified. This mutation is located in the peripheral anionic site of butyrylcholinesterase, where it causes a 10-fold decrease in binding affinity for positively charged substrates. CONCLUSION: People homozygous for the Asp70His mutation are expected to have prolonged apnoea in response to succinylcholine or mivacurium, similar to people with the Asp70Gly mutation.
Title: Rapid simultaneous genotyping of the frequent butyrylcholinesterase variants Asp70Gly and Ala539Thr with fluorescent hybridization probes Gatke MR, Viby-Mogensen J, Bundgaard JR Ref: Scand J Clin Lab Invest, 62:375, 2002 : PubMed
BACKGROUND: The clinically important variants of butyrylcholinesterase (BChE) are the A- (Asp70Gly) and K-variants (Ala539Thr), which are common among Caucasians. These variants are associated with abnormal drug metabolism during anaesthesia, which leads to a prolonged neuromuscular block following administration of the neuromuscular blocking agents, mivacurium and succinylcholine. In addition, the K-variant has been proposed to be associated with Alzheimer's disease together with apolipoprotein E epsilon4. To facilitate diagnostics, we set out to establish a rapid and simple method for simultaneous genotyping of the A- and K-variants. METHODS: Using the LightCycler, a rapid-cycle duplex PCR is combined with generation of allele-specific fluorescent probe melting profiles. This allows simultaneous detection of both of the mutations in the BChE gene. The results were compared with direct sequencing and phenotyping results. RESULTS: Samples from 80 subjects were genotyped. The genotypes determined using the LightCycler were identical to those obtained by direct sequencing of conventional polymerase chain reaction products and was more accurate than phenotyping based on biochemical assays. CONCLUSIONS: A high-speed and easy to perform mutation detection assay has been established for the two most common mutations, Asp70Gly and Ala539Thr, in BChE, using the LightCycler technology and melting curves.
Title: Analysis of Mutations in the Plasma Cholinesterase Gene of Patients with a History of Prolonged Neuromuscular Block during Anesthesia Barta C, Sasvari-Szekely M, Devai A, Kovacs E, Staub M, Enyedi P Ref: Mol Genet Metab, 74:484, 2001 : PubMed
Decreased activity of plasma cholinesterase is responsible for prolonged apnea during anesthesia using neuromuscular blockers such as suxamethonium and mivacurium. More than 20 mutations have been identified so far in the BCHE gene resulting in impaired plasma cholinesterase activity. Biochemical tests are not always able to differentiate between pathological and normal sera; hence in some cases unanticipated complications can still occur during anesthesia even after measurements of enzyme activity and dibucaine numbers within the normal range. Therefore, molecular genetic testing is required for the accurate diagnosis of this deficiency. Here we present a study of plasma cholinesterase activity and BCHE genotyping of patients with a history of prolonged neuromuscular block and most of their pedigrees. All four exons of the BCHE gene were directly sequenced from samples and a number of mutations responsible for the reduction of plasma cholinesterase activity were identified. In most cases the atypical mutation in exon 2 (nt 209A --> G, Asp70 --> Gly) was found together with the K-variant mutation in exon 4 (nt 1615G --> A, Ala539 --> Thr), which is in good agreement with previous data suggesting that these mutations along with two others (at nt -116 and nt 1914) are in linkage disequilibrium.
Title: Interaction between the peripheral site residues of human butyrylcholinesterase, D70 and Y332, in binding and hydrolysis of substrates Masson P, Xie W, Froment MT, Levitsky V, Fortier PL, Albaret C, Lockridge O Ref: Biochimica & Biophysica Acta, 1433:281, 1999 : PubMed
Human butyrylcholinesterase displays substrate activation with positively charged butyrylthiocholine (BTC) as the substrate. Peripheral anionic site (PAS) residues D70 and Y332 appear to be involved in the initial binding of charged substrates and in activation control. To determine the contribution of PAS residues to binding and hydrolysis of quaternary substrates and activation control, the single mutants D70G/Y and Y332F/A/D and the double mutants Y332A/D70G and Y332D/D70Y were studied. Steady-state hydrolysis of the charged substrates, BTC and succinyldithiocholine, and the neutral ester o-nitrophenyl butyrate was measured. In addition, inhibition of wild-type and mutant enzymes by tetramethylammonium was investigated, at low concentrations of BTC. Single and double mutants of D70 and Y332 showed little or no substrate activation, suggesting that both residues were important for activation control. The effects of double mutations on D70 and Y332 were complex. Double-mutant cycle analysis provided evidence for interaction between these residues. The category of interaction (either synergistic, additive, partially additive or antagonistic) was found to depend on the nature of the substrate and on measured binding or kinetic parameters. This complexity reflects both the cross-talk between residues involved in the sequential formation of productive Michaelian complexes and the effect of peripheral site residues on catalysis. It is concluded that double mutations on the PAS induce a conformational change in the active site gorge of butyrylcholinesterase that can alter both substrate binding and enzyme acylation.
Title: Structural and hydration changes in the active site gorge of phosporhylated butyrylcholinesterase accompanying the aging process Masson P, Fortier PL, Albaret C, Clery C, Guerra P, Lockridge O Ref: Chemico-Biological Interactions, 119-120:17, 1999 : PubMed
Wild-type (wt) butyrylcholinesterase (BuChE) and the E197D and D70G mutants were inhibited by diisopropylfluorophosphate (DFP) or soman under standard conditions of pH, temperature and pressure. The effect of hydrostatic and osmotic pressures on the aging process of DFP-phosphorylated enzymes (diisopropylphosphoryl-BuChE (DIP-BuChE)) was investigated. Hydrostatic pressure strongly increased the rate of aging of wt enzyme. The activation volumes (deltaV*) for the dealkylation reaction was -150 ml/mol for DIP-wtBuChE. On the other hand, pressure had little effect on the aging of the DIP-E197D mutant and no effect on the DIP-D70G mutant, indicating that the transition state of the aging reaction (dealkylation of an isoproxy chain) was associated with an extended conformation/hydration change in wtBuChE but not in mutants. The rate of aging decreased with osmotic pressure, supporting the idea that water is important for stabilizing the transition state. Molecular dynamics simulations were performed on the wtDIP adduct to relate the kinetic data to hydration changes in the enzyme active site gorge. The pH dependence of the melting temperature (Tm) of native and soman-wtBuChE, as determined by differential scanning calorimetry (DSC), indicated that the stabilization energy of aged BuChE is mainly due to the salt bridge between protonated H438 and PO-, with pK(H438) = 8.3. Electrophoresis under high pressure up to 2.5 kbar showed that aged wtBuChE did not undergo pressure-induced molten globule transition unlike the native enzyme. This transition was not seen for the mutant enzymes, indicating that mutants are resistant to the penetration of water into their structure. Our results support the conclusion that D70 and E197 are major residues for the water/H-bond network dynamics in the active site gorge of BuChE, both residues acting like valves. In mutant enzymes, mutated residues function like check valves: forced penetration of water in the gorge is difficult, release of water is facilitated.
Title: An explanation for the different inhibitory characteristics of human serum butyrylcholinesterase phenotypes deriving from inhibition of atypical heterozygotes Simeon-Rudolf V, Kovarik Z, Skrinjaric-Spoljar M, Evans RT Ref: Chemico-Biological Interactions, 119-120:159, 1999 : PubMed
The time course of inhibition of butyrylcholinesterase (EC 3.1.1.8) by the dimethylcarbamate Ro 02-0683 in sera taken from patients heterozygous for the usual (U), atypical (A), K or J variants was followed using propionylthiocholine as substrate. Data obtained were used to determine rate constants of inhibition together with the contribution made by each variant to total enzyme activity. The findings substantiate earlier reports that J and K mutations lead to quantitative changes in the concentration of usual enzyme in contrast to the qualitative changes of the atypical variant. The contribution of the atypical enzyme to the total activity in serum from UA, AK and AJ heterozygotes was respectively 17-20, 24-31 and 34-53%. The altered ratios of atypical to usual, K or J enzyme in UA, AK and AJ together with the constants on the usual enzyme alone, explain the differences in observed inhibitor numbers which enable these heterozygotes to be identified.
Title: Catalytic parameters for the hydrolysis of butyrylthiocholine by human serum butyrylcholinesterase variants Simeon-Rudolf V, Reiner E, Evans RT, George PM, Potter HC Ref: Chemico-Biological Interactions, 119-120:165, 1999 : PubMed
Catalysed hydrolysis of butyrylthiocholine (BTCh) by the usual (UU), fluoride-resistant (FS), AK, AJ and atypical (AA) human serum butyrylcholinesterase (EC 3.1.1.8) variants was measured in phosphate buffer pH 7.4 at 25 degrees C. pS-curves for all phenotypes were S-shaped; the activities rose to a plateau with increasing substrate concentration except at 100 mM where there was a small decrease. To obtain the catalytic constants, three equations were applied: Michaelis-Menten equation (Eq. 1), Hill equation (Eq. 2) and an equation which assumes simultaneous binding of the substrate to the catalytic site and to a peripheral site on the enzyme (Eq. 3). Over a range from 0.01 to 50 mM BTCh, the activity versus substrate concentration relationship deviated from Michaelis-Menten kinetics (Eq. 1) while data fitted well with Eqs. 2 and 3. The Michaelis-Menten equation was applied separately to two BTCh concentration ranges: the corresponding Km constants for the UU, FS, AK, AJ and AA phenotypes ranged from 0.1 to 0.2 mM (at 0.01-1.0 mM BTCh) and from 0.3 to 2.0 mM (at 1.0-50 mM BTCh). Hill coefficients (nH) calculated from Eq. 2 were similar for all phenotypes (nH approximately 0.5). The dissociation constants K1 and K2 calculated from Eq. 3 for two sites on the enzyme fell between 0.02 and 0.12 mM (K1) and 0.89 and 4.9 mM (K2) for the five phenotypes. Experimental data support the assumption that the phenotypes studied have two substrate binding sites.
Although aspirin (acetylsalicylic acid) is negatively charged, it is hydrolysed by butyrylcholinesterase (BCHE). Catalytic parameters were determined in 100 mM Tris buffer, pH 7.4, in the presence and absence of metal cations. The presence of Ca2+ or Mg2+ (<100 mM) in buffer did not change the Km, but accelerated the rate of hydrolysis of aspirin by wild-type or D70G mutant BCHE by 5-fold. Turnover numbers were of the order of 5000-12000 min-1 for the wild-type enzyme and the D70G and D70K enzymes in 100 mM Tris, pH 7.4, containing 50 mM CaCl2 at 25 degreesC; Km values were 6 mM for wild-type, 16 mM for D70G and 38 mM for D70K. People with 'atypical' BCHE have the D70G mutation. The apparent inhibition seen at high aspirin concentration was not due to inhibition by excess substrate but to spontaneous hydrolysis of aspirin, causing inhibition by salicylate. The wild-type and D70G enzymes were competitively inhibited by salicylic acid; the D70K enzyme showed a complex parabolic inhibition, suggesting multiple binding. The effect of salicylate was substrate-dependent, the D70K mutant being activated by salicylate with butyrylthiocholine as substrate. Km value for wild-type enzyme was lower than for D70 mutants, suggesting that residue 70 located at the rim of the active site gorge was not the major site for the initial encounter aspirin-BCHE complex. On the other hand, the virtual absence of affinity of the W82A mutant for aspirin indicated that W82 was the major residue involved in formation of the Michaelis complex. Molecular modelling of aspirin binding to BCHE indicated perpendicular interactions between the aromatic rings of W82 and aspirin. Kinetic study of BCHE-catalysed hydrolysis of different acetyl esters showed that the rate limiting step was acetylation. The bimolecular rate constants for hydrolysis of aspirin by wild-type, D70G and D70K enzymes were found to be close to 1x106 M-1 min-1. These results support the contention that the electrostatic steering due to the negative electrostatic field of the enzyme plays a role in substrate binding, but plays no role in the catalytic steps, i.e. in the enzyme acetylation.
Organophosphate-inhibited cholinesterases can be reactivated by nucleophilic compounds. Sometimes phosphylated (phosphorylated or phosphonylated) cholinesterases become progressively refractory to reactivation; this can result from different reactions. The most frequent process, termed 'aging', involves the dealkylation of an alkoxy group on the phosphyl moiety through a carbocation mechanism. In attempting to determine the amino acid residues involved in the aging of butyrylcholinesterase (BuChE), the human BuChE gene was mutated at several positions corresponding to residues located at the rim of the active site gorge and in the vicinity of the active site. Mutant enzymes were expressed in Chinese hamster ovary cells. Wild-type BuChE and mutants were inhibited by di-isopropylfluorophosphate at pH 8.0 and 25 degrees C. Di-isopropyl-phosphorylated enzymes were incubated with the nucleophilic oxime 2-pyridine aldoxime methiodide and their reactivatability was determined. Reactivatability was expressed by the first-order rate constant of aging and/or the half-life of aging (t12). The t12 was found to be of the order of 60 min for wild-type BuChE. Mutations on Glu-197 increased t12 60-fold. Mutation W82A increased t12 13-fold. Mutation D70G increased t12 8-fold. Mutations in the vicinity of the active site serine residue had either moderate or no effect on aging; t12 was doubled for F329C and F329A, increased only 4-fold for the double mutant A328G+F329S, and no change was observed for the A328G mutant, indicating that the isopropoxy chain to be dealkylated does not directly interact with Ala-328 and Phe-329. These results were interpreted by molecular modelling of di-isopropylphosphorylated wild-type and mutant enzymes. Molecular dynamics simulations indicated that the isopropyl chain that is lost interacted with Trp-82, suggesting that Trp-82 has a role in stabilizing the carbonium ion that is released in the dealkylation step. This study emphasized the important role of the Glu-197 carboxylate in stabilizing the developing carbocation, and the allosteric control of the dealkylation reaction by Asp-70. Indeed, although Asp-70 does not interact with the phosphoryl moiety, mutation D70G affects the rate of aging. This indirect control was interpreted in terms of change in the conformational state of Trp-82 owing to internal motions of the Omega loop (Cys-65-Cys-92) in the mutant enzyme.
The atypical variant of human butyrylcholinesterase has Gly in place of Asp 70. Patients with this D70G mutation respond abnormally to the muscle relaxant succinyldicholine, experiencing hours of apnea rather than the intended 3 min. Asp 70 is at the rim of the active site gorge 12 A from the active site Ser 198. An unanswered question in the literature is why the atypical variant has a 10-fold increase in Km for compounds with a single positive charge but a 100-fold increase in Km for compounds with two positive charges. We mutated residues Asp 70, Trp 82, Trp 231, Glu 197, and Tyr 332 and expressed mutant enzymes in mammalian cells. Steady-state kinetic parameters for hydrolysis of butyrylthiocholine, benzoylcholine, succinyldithiocholine, and o-nitrophenyl butyrate were determined. The wild type and the D70G mutant had identical k(cat) values for all substrates. Molecular modeling and molecular dynamics suggested that succinyldicholine could bind in two consecutive orientations in the active site gorge; formation of one complex caused a conformational change in the omega loop involving Asp 70 and Trp 82. We propose the formation of three enzyme-substrate intermediates preceding the acyl-enzyme intermediate; kinetic data support this contention. Substrates with a single positive charge interact with Asp 70 just once, whereas substrates with two positive charges, for example succinyldithiocholine, interact with Asp 70 in two complexes, thus explaining the 10- and 100-fold increases in Km in the D70G mutant.
Title: Importance of aspartate-70 in organophosphate inhibition, oxime re-activation and aging of human butyrylcholinesterase Masson P, Froment MT, Bartels CF, Lockridge O Ref: Biochemical Journal, 325:53, 1997 : PubMed
Asp-70 is the defining amino acid in the peripheral anionic site of human butyrylcholinesterase (BCHE), whereas acetylcholinesterase has several additional amino acids, the most important one being Trp-277 (Trp-279 in Torpedo AChE). We studied mutants D70G, D70K and A277W to evaluate the role of Asp-70 and Trp-277 in reactions with organophosphates. We found that Asp-70 was important for binding positively charged echothiophate, but not neutral paraoxon and iso-OMPA. Asp-70 was also important for binding of positively charged pralidoxime (2-PAM) and for activation of re-activation by excess 2-PAM. Excess 2-PAM had an effect similar to substrate activation, suggesting the binding of 2 mol of 2-PAM to wild-type but not to the D70G mutant. A surprising result was that Asp-70 was important for irreversible aging, the D70G mutant having a 3- and 8-fold lower rate of aging for paraoxon-inhibited and di-isopropyl fluorophosphate-inhibited BCHE. Mutants of Asp-70 had the same rate constants for phosphorylation and re-activation by 2-PAM as wild-type. The A277W mutant behaved like wild-type in all assays. Our results predict that people with the atypical (D70G) variant of BCHE will be more sensitive to the toxic effects of echothiophate, but will be equally sensitive to paraoxon and di-isopropyl fluorophosphate. People with the D70G mutation will be resistant to re-activation of their inhibited BCHE by 2-PAM, but this will be offset by the lower rate of irreversible aging of inhibited BCHE, allowing some regeneration by spontaneous hydrolysis.
Butyrylcholinesterase [BCHE (acylcholine acyl hydrolase); EC 3.1.1.8] limits the access of drugs, including tacrine, to other proteins. The "atypical" BCHE variant, in which Asp70 at the rim of the active site gorge is substituted by glycine, displayed a more drastically weakened interaction with tacrine than with cocaine, dibucaine, succinylcholine, BW284c51 [1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide], or alpha-solanine. To delineate the protein domains that are responsible for this phenomenon, we mutated residues within the rim of the active site gorge, the region parallel to the peripheral site in the homologous enzyme acetylcholinesterase [AChE (acetylcholine acetyl hydrolase); EC 3.1.1.7], the oxyanion hole, and the choline-binding site. When expressed in microinjected Xenopus laevis oocytes, all mutant DNAs yielded comparable amounts of immunoreactive protein products. Most mutants retained catalytic activity close to that of wild-type BCHE and were capable of binding ligands. However, certain modifications in and around the oxyanion hole caused a dramatic loss in activity. The affinities for tacrine were reduced more dramatically than for all other ligands, including cocaine, in both oxyanion hole and choline-binding site mutants. Modified ligand affinities further demonstrated a peripheral site in residues homologous with those of AChE. BCHE mutations that prevented tacrine interactions also hampered its ability to bind other drugs and inhibitors, which suggests a partial overlap of the binding sites. This predicts that in addition to their genetic predisposition to adverse responses to tacrine, homozygous carriers of "atypical" BCHE will be overly sensitive to additional anticholinesterases and especially so when exposed to several anticholinesterases in combination.
Title: Asp7O in the peripheral anionic site of human butyrylcholinesterase Masson P, Froment MT, Bartels CF, Lockridge O Ref: European Journal of Biochemistry, 235:36, 1996 : PubMed
The goal of this work was to determine what amino acids at the mouth of the active-site gorge are important for the function of human butyrylcholinesterase. Mutants D70G, Q119Y, G283D, A277W, A277H and A277W/G283D were expressed in human embryonal kidney cells and the secreted enzymes were assayed by steady-state kinetics. The result was that only one amino acid, D70 was found to be important for function. When D70 was mutated to G, the same mutation as in the naturally occurring atypical butyrylcholinesterase, the affinity for positively charged substrates and positively charged inhibitors decreased 5-30-fold. The D70G mutant had another striking abnormality in that it was virtually devoid of the phenomenon of substrate activation by excess butyrylthiocholine. Thus, though kcat was the same for wild-type and D70G mutant, being 24000 min(-1) at low butyrylthiocholine concentrations (0.01-0.1 mM), it failed to increase for the D70G mutant at 40 mM butyrylthiocholine, whereas it increased threefold for wild type. The D70G mutant was more sensitive to changes in salt concentration, its catalytic rate decreasing more than that of the wild type. The D70G mutant appeared to have a greater surface negative charge than wild type suggesting that the D70G mutant had a conformation different from that of the wild type. That D70 affects the function of butyrylcholinesterase, together with its location at the mouth of the active-site gorge, supports the hypothesis that D70 is a component of the peripheral anionic site of butyrylcholinesterase. Mutants containing aromatic amino acids at the mouth of the gorge had increased binding affinity for propidium and fasciculin, but unaltered function, suggesting that aromatic amino acids are not important to the function of the peripheral anionic site of butyrylcholinesterase.
We have applied the technique of PCR-SSCP (polymerase chain reaction-single stranded conformation polymorphism) to characterise the molecular basis of cholinesterase deficiency and variants in a Jordanian family. PCR-SSCP proved to be a quick and sensitive method of screening cholinesterase variants in a clinical setting. An AG insertion at position 351 was found to cause a silent allele, for which the parents were heterozygous and three children homozygous. In addition, the father and two sons were heterozygous for an A to G transition at position 209, known to cause the dibucaine resistant variant. No linkage to the K variant was found, which has been reported previously in white populations. These findings suggest considerable homogeneity in the molecular basis of CHE variants between different ethnic groups.
Title: Peripheral Anionic Site of Wild-Type and Mutant Human Butyrylcholinesterase Masson P, Froment MT, Bartels CF, Lockridge O Ref: In Enzyme of the Cholinesterase Family - Proceedings of Fifth International Meeting on Cholinesterases, (Quinn, D.M., Balasubramanian, A.S., Doctor, B.P., Taylor, P., Eds) Plenum Publishing Corp.:230, 1995 : PubMed
Variant alleles of the butyrylcholinesterase gene, BCHE, have often been used to trace the genetic histories of populations. The D70G substitution in BCHE causes prolonged postanesthesia apnea ("atypical" phenotype); H322N substitution in the closely related acetylcholinesterase gene, ACHE, is the basis of the mutually incompatible Yt blood groups. In both genes, additional point mutations were reported to be linked to these phenotypically evident ones. To examine whether the intragenic linkage reported for the ACHE and BCHE mutations in Americans is universal, we studied frequencies of these mutations in trans-Caucasian Georgian Jews, a population that has remained relatively isolated for 1500 years. To this end we employed PCR amplification followed by DNA sequencing and enzymatic restriction and compared the frequencies we found to corresponding reported phenotype data. Georgian Jews' N322 ACHE was a rather low 7.0% and was totally linked to a P446 mutation, in agreement with a recent report. In BCHE, however, G70 was a relatively high 5.8%, and the V497 and T539 mutations were not found, either in Georgian or in Ashkenazi Jews, in contrast to reported findings in Americans. Our findings reveal distinct displays of ACHE and BCHE haplotypes in Georgian Jews and suggest different founder effects, genetic drifts, and/or selection pressures in the evolution of each of these genes.
Title: Intramolecular relationships in cholinesterases revealed by oocyte expression of site-directed and natural variants of human BCHE Neville LF, Gnatt A, Loewenstein Y, Seidman S, Ehrlich G, Soreq H Ref: EMBO Journal, 11:1641, 1992 : PubMed
Structure-function relationships of cholinesterases (CHEs) were studied by expressing site-directed and naturally occurring mutants of human butyrylcholinesterase (BCHE) in microinjected Xenopus oocytes. Site-directed mutagenesis of the conserved electronegative Glu441,Ile442,Glu443 domain to Gly441,Ile442,Gln443 drastically reduced the rate of butyrylthiocholine (BTCh) hydrolysis and caused pronounced resistance to dibucaine binding. These findings implicate the charged Glu441,Ile442,Glu443 domain as necessary for a functional CHE catalytic triad as well as for binding quinoline derivatives. Asp70 to Gly substitution characteristic of 'atypical' BCHE, failed to alter its Km towards BTCh or dibucaine binding but reduced hydrolytic activity to 25% of control. Normal hydrolytic activity was restored to Gly70 BCHE by additional His114 or Tyr561 mutations, both of which co-appear with Gly70 in natural BCHE variants, which implies a likely selection advantage for these double BCHE mutants over the single Gly70 BCHE variant. Gly70 BCHE variants also displayed lower binding as compared with Asp70 BCHE to cholinergic drugs, certain choline esters and solanidine. These effects were ameliorated in part by additional mutations or in binding solanidine complexed with sugar residues. These observations indicate that structural interactions exist between N' and C' terminal domains in CHEs which contribute to substrate and inhibitor binding and suggest a crucial involvement of both electrostatic and hydrophobic domains in the build-up of the CHE active center.
Title: Aspartate-70 to glycine substitution confers resistance to naturally occurring and synthetic anionic-site ligands on in-ovo produced human butyrylcholinesterase Neville LF, Gnatt A, Loewenstein Y, Soreq H Ref: Journal of Neuroscience Research, 27:452, 1990 : PubMed
The "atypical" allelic variant of human butyrylcholinesterase (BCHE) can be characterized by its failure to bind the local anesthetic dibucaine, the muscle relaxant succinylcholine, and the naturally occurring steroidal alkaloid solanidine, all assumed to bind to the charged anionic site component within the normal BCHE enzyme. A single nucleotide substitution conferring a change of aspartate-70 into glycine was recently reported in the CHE gene encoding BCHE from several individuals having the "atypical" BCHE phenotype, whereas in two other DNA samples, this mutation appeared together with a second alteration conferring a change of serine-425 into proline. To separately assess the contribution of each of these mutations toward anionic site interactions in BCHE, three transcription constructs were engineered with each of these substitutions alone or both of them together. Xenopus oocyte microinjection of normal or mutated synthetic BCHEmRNA transcripts was employed in conjunction with biochemical analyzes of the resultant recombinant BCHE variants. The presence of the Gly-70 mutation alone was found to render the enzyme resistant to 100 microM solanidine and 5 mM succinylcholine; concentrations sufficient to inhibit the "normal," Asp-70 containing BCHE by over 50%. Furthermore, when completely inhibited by the organophosphorous poison diisopropylfluorophosphate (DFP), Gly-70 BCHE failed to be reactivated by 10 mM of the cholinesterase-specific oxime pyridine 2-aldoxime methiodide (2-PAM); a concentration restoring about 50% of activity in the "normal" Asp-70 recombinant enzyme. The Pro-425 mutation alone had no apparent influence on BCHE interactions with any of these ligands. However, it conferred synergistic effects on some of the anionic site changes induced by the Gly-70 mutation.
A point mutation in the gene for human serum cholinesterase was identified that changes Asp-70 to Gly in the atypical form of serum cholinesterase. The mutation in nucleotide 209, which changes codon 70 from GAT to GGT, was found by sequencing a genomic clone and sequencing selected regions of DNA amplified by the polymerase chain reaction. The entire coding sequences for usual and atypical cholinesterases were compared, and no other consistent base differences were found. A polymorphic site near the C terminus of the coded region was detected, but neither allele at this locus segregated consistently with the atypical trait. The nucleotide-209 mutation was detected in all five atypical cholinesterase families examined. There was complete concordance between this mutation and serum cholinesterase phenotypes for all 14 heterozygous and 6 homozygous atypical subjects tested. The mutation causes the loss of a Sau3A1 restriction site; the resulting DNA fragment length polymorphism was verified by electrophoresis of 32P-labeled DNA restriction fragments from usual and atypical subjects. Dot-blot hybridization analysis with a 19-mer allele-specific probe to the DNA amplified by the polymerase chain reaction distinguished between the usual and atypical genotypes. We conclude that the Asp-70----Gly mutation (acidic to neutral amino acid substitution) accounts for reduced affinity of atypical cholinesterase for choline esters and that Asp-70 must be an important component of the anionic site. Heterogeneity in atypical alleles may exist, but the Asp-70 point mutation may represent an appreciable portion of the atypical gene pool.
Frequencies of the CHE1*A allele were estimated on the basis of a sample of 999 Caucasians (1.5%) and 1,015 Negroids (0.84%) from Curitiba, Brazil. The frequency found in the Negroid subsample allows an estimate of 50 +/- 15% of Caucasoid admixture and an average gene flow in the white-black direction of the order of 5.6% per generation.
Title: Some statistical data on atypical cholinesterase of human serum Kalow W, Gunn DR Ref: Annals of Human Genetics, 23:239, 1959 : PubMed
Title: A method for the detection of atypical forms of human serum cholinesterases. Determination of dibucaine numbers Kalow W, Genest K Ref: Canadian Journal of Biochemistry, 35:339, 1957 : PubMed
Title: On distribution and inheritance of atypical forms of human serum cholinesterase, as indicated by dibucaine numbers Kalow W, Staron N Ref: Canadian Journal of Biochemistry, 35:1305, 1957 : PubMed
