Epitope tagging of endogenous proteins for genome-wide ChIP-chip studies

We developed a strategy to introduce epitope tag-encoding DNA into endogenous loci by homologous recombination-mediated 'knock-in'. The tagging method is straightforward, can be applied to many loci and several human somatic cell lines, and can facilitate many functional analyses including western blot, immunoprecipitation, immunofluorescence and chromatin immunoprecipitation-microarray (ChIP-chip). The knock-in approach provides a general solution for the study of proteins to which antibodies are substandard or not available.

Heterozygous mutations in natriuretic peptide receptor-B (NPR2) are associated with short stature

CONTEXT

C-type natriuretic peptide (CNP) is an important regulator of skeletal growth. Loss-of-function mutations affecting the CNP receptor natriuretic peptide receptor-B (gene NPR2) cause the autosomal recessive skeletal dysplasia, acromesomelic dysplasia, Maroteaux type (AMDM). The phenotype of heterozygous carriers of NPR2 mutations is less clear.

OBJECTIVE

The objective of the study was to determine the phenotypic features of heterozygous carriers of NPR2 mutations.

DESIGN AND SETTING

This was a case-control study from the general community.

SUBJECTS

Thirty-nine members of a family in which one member has AMDM were studied.

INTERVENTION

This was an observational study.

MAIN OUTCOME MEASURE

The primary measure was stature, with the hypothesis that carriers have reduced stature compared with noncarriers.

RESULTS

Sixteen family members were NPR2 mutation carriers. Height z-scores of these carriers were -1.8 +/- 1.1 (mean +/- sd), which was significantly less than the 23 noncarrier family members (-0.4 +/- 0.8, P < 0.0005) and the general population (P < 0.0005). However, there was no difference in body proportion between carriers and noncarriers. The proband with AMDM had low IGF-I levels and evidence of GH resistance, as well as very high plasma levels of CNP and its amino-terminal propeptide. Levels of these peptides were normal in the heterozygous carriers.

CONCLUSIONS

We have shown that heterozygous mutations in NPR2 are associated with short stature. Assuming one in 700 people unknowingly carry an NPR2 mutation, our data suggest that approximately one in 30 individuals with idiopathic short stature are carriers of NPR2 mutations.

Clinical and molecular findings in osteoporosis-pseudoglioma syndrome

Mutations in the low-density lipoprotein receptor-related protein 5 gene (LRP5) cause autosomal recessive osteoporosis-pseudoglioma syndrome (OPPG). We sequenced the coding exons of LRP5 in 37 probands suspected of having OPPG on the basis of the co-occurrence of severe congenital or childhood-onset visual impairment with bone fragility or osteoporosis recognized by young adulthood. We found two putative mutant alleles in 26 probands, only one mutant allele in 4 probands, and no mutant alleles in 7 probands. Looking for digenic inheritance, we sequenced the genes encoding the functionally related receptor LRP6, an LRP5 coreceptor FZD4, and an LRP5 ligand, NDP, in the four probands with one mutant allele, and, looking for locus heterogeneity, we sequenced FZD4 and NDP in the seven probands with no mutations, but we found no additional mutations. When we compared clinical features between probands with and without LRP5 mutations, we found no difference in the severity of skeletal disease, prevalence of cognitive impairment, or family history of consanguinity. However, four of the seven probands without detectable mutations had eye pathology that differed from pathology previously described for OPPG. Since many LRP5 mutations are missense changes, to differentiate between a disease-causing mutation and a benign variant, we measured the ability of wild-type and mutant LRP5 to transduce Wnt and Norrin signal ex vivo. Each of the seven OPPG mutations tested, had reduced signal transduction compared with wild-type mutations. These results indicate that early bilateral vitreoretinal eye pathology coupled with skeletal fragility is a strong predictor of LRP5 mutation and that mutations in LRP5 cause OPPG by impairing Wnt and Norrin signal transduction.

Consequences of Disease-causing Mutations on Lubricin Protein Synthesis, Secretion, and Post-translational Processing

Lubricin, a protein product of the gene PRG4, is a secreted mucin-like proteoglycan that is a major lubricant in articulating joints. Mutations in PRG4 cause the autosomal recessive, human disorder camptodactyly-arthropathy-coxa vara-pericarditis syndrome. We developed rabbit polyclonal antibodies against human lubricin to determine the consequence of disease-causing mutations at the protein level and to study the protein's normal post-translational processing. Antiserum generated against an epitope in the amino-terminal portion of lubricin detected protein in wild-type synovial fluid and in conditioned media from wild-type cultured synoviocytes. However, the antiserum did not detect lubricin in synovial fluid or cultured synoviocytes from several patients with frameshift or nonsense mutations in PRG4. Antiserum generated against an epitope in the protein's carboxyl-terminal, hemopexin-like domain identified a post-translational cleavage event in wild-type lubricin, mediated by a subtilisin-like proprotein convertase (SPC). Interestingly, in contrast to wild-type lubricin, one disease-causing mutation that removes the last 8 amino acids of the protein, including a conserved cysteine residue, was not cleaved within the hemopexin-like domain when expressed in COS-7 cells. This suggests that formation of an intrachain disulfide bond is required for SPC-mediated cleavage and that SPC-mediated cleavage is essential to protein function.

Reaction kinetics of biotinylated organophosphorus toxicant, FP-biotin, with human acetylcholinesterase and human butyrylcholinesterase

A biotinylated organophosphate could be useful for identifying proteins that react with organophosphorus toxicants (OP). FP-biotin, 10-(fluoroethoxyphosphinyl)-N-(biotinamidopentyl)decanamide, was synthesized and found to be stable in methanol and chloroform but less stable in water. Because acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BChE, EC 3.1.1.8) are known to be sensitive targets of OP, their reactivity with FP-biotin was tested. The rate constant for reaction with human AChE was 1.8 x 10(7) M(-1) min(-1), and for human BChE, it was 1.6 x 10(8) M(-1) min(-1). A phosphorus stereoisomer, constituting about 50% of the FP-biotin preparation, appeared to be the reactive species. The binding affinity was estimated to be >85 nM for AChE and >5.8 nM for BChE. It was concluded that FP-biotin is a potent OP, well-suited for searching for new biomarkers of OP exposure.

Mutations in the transmembrane natriuretic peptide receptor NPR-B impair skeletal growth and cause acromesomelic dysplasia, type Maroteaux

The homodimeric transmembrane receptor natriuretic peptide receptor B (NPR-B [also known as guanylate cyclase B, GC-B, and GUC2B]; gene name NPR2) produces cytoplasmic cyclic GMP from GTP on binding its extracellular ligand, C-type natriuretic peptide (CNP). CNP has previously been implicated in the regulation of skeletal growth in transgenic and knockout mice. The autosomal recessive skeletal dysplasia known as "acromesomelic dysplasia, type Maroteaux" (AMDM) maps to an interval that contains NPR2. We sequenced DNA from 21 families affected by AMDM and found 4 nonsense mutations, 4 frameshift mutations, 2 splice-site mutations, and 11 missense mutations. Molecular modeling was used to examine the putative protein change brought about by each missense mutation. Three missense mutations were tested in a functional assay and were found to have markedly deficient guanylyl cyclase activity. We also found that obligate carriers of NPR2 mutations have heights that are below the mean for matched controls. We conclude that, although NPR-B is expressed in a number of tissues, its major role is in the regulation of skeletal growth.

High activity of human butyrylcholinesterase at low pH in the presence of excess butyrylthiocholine

Butyrylcholinesterase is a serine esterase, closely related to acetylcholinesterase. Both enzymes employ a catalytic triad mechanism for catalysis, similar to that used by serine proteases such as alpha-chymotrypsin. Enzymes of this type are generally considered to be inactive at pH values below 5, because the histidine member of the catalytic triad becomes protonated. We have found that butyrylcholinesterase retains activity at pH <or= 5, under conditions of excess substrate activation. This low-pH activity appears with wild-type butyrylcholinesterase as well as with all mutants we examined: A328G, A328I, A328F, A328Y, A328W, E197Q, L286W, V288W and Y332A (residue A328 is at the bottom of the active-site gorge, near the pi-cation-binding site; E197 is next to the active-site serine S198; L286 and V288 form the acyl-binding pocket; and Y332 is a component of the peripheral anionic site). For example, the kcat value at pH 5.0 for activity in the presence of excess substrate was 32900 +/- 4400 min(-1) for wild-type, 55200 +/- 1600 min(-1) for A328F, and 28 700 +/- 700 min(-1) for A328W. This activity is titratable, with pKa values of 6.0-6.6, suggesting that the catalytic histidine is protonated at pH 5. The existence of activity when the catalytic histidine is protonated indicates that the catalytic-triad mechanism of butyrylcholinesterase does not operate for catalysis at low pH. The mechanism explaining the catalytic behaviour of butyrylcholinesterase at low pH in the presence of excess substrate remains to be elucidated.

The mutational spectrum of brachydactyly type C

Growth/differentiation factor-5 (GDF5), also known as cartilage-derived morphogenetic protein-1 (CDMP-1), is a secreted signaling molecule that participates in skeletal morphogenesis. Heterozygous mutations in GDF5, which maps to human chromosome 20, occur in individuals with autosomal dominant brachydactyly type C (BDC). Here we show that BDC is locus homogeneous by reporting a GDF5 frameshift mutation segregating with the phenotype in a family whose trait was initially thought to map to human chromosome 12. We also describe heterozygous mutations in nine additional probands/families with BDC and show nonpenetrance in a mutation carrier. Finally, we show that mutant GDF5 polypeptides containing missense mutations in their active domains do not efficiently form disulfide-linked dimers when expressed in vitro. These data support the hypothesis that BDC results from functional haploinsufficiency for GDF5.

Wild-type and A328W mutant human butyrylcholinesterase tetramers expressed in Chinese hamster ovary cells have a 16-hour half-life in the circulation and protect mice from cocaine toxicity

Human butyrylcholinesterase (BChE) hydrolyzes cocaine to inactive metabolites. A mutant of human BChE, A328W, hydrolyzed cocaine 15-fold faster compared with wild-type BChE. Although the catalytic properties of human BChE secreted by Chinese hamster ovary (CHO) cells are identical to those of native BChE, a major difference became evident when the recombinant BChE was injected into rats and mice. Recombinant BChE disappeared from the circulation within minutes, whereas native BChE stayed in the blood for a week. Nondenaturing gel electrophoresis showed that the recombinant BChE consisted mainly of monomers and dimers. In contrast, native BChE is a tetramer. The problem of the short residence time was solved by finding a method to assemble the recombinant BChE into tetramers. Coexpression in CHO cells of BChE and 45 residues from the N terminus of the COLQ protein yielded 70% tetrameric BChE. The resulting purified recombinant BChE tetramers had a half-life of 16 h in the circulation of rats and mice. The 16-h half-life was achieved without modifying the carbohydrate content of recombinant BChE. The protective effect of recombinant wild-type and A328W mutant BChE against cocaine toxicity was tested by measuring locomotor activity in mice. Pretreatment with wild-type BChE or A328W tetramers at a dose of 2.8 units/g i.p. reduced cocaine-induced locomotor activity by 50 and 80%. These results indicate that recombinant human BChE could be useful for treating cocaine toxicity in humans.

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.

The active site of human paraoxonase (PON1)

Ideally we would like to treat people exposed to nerve agents with an enzyme that rapidly destroys nerve agents. The enzymes considered for such a role include human butyrylcholinesterase (BChE), acetylcholinesterase (AChE), carboxylesterase and paraoxonase (PON1). Success has been achieved in endowing BChE with the ability to hydrolyze organophosphates. The G117H mutant of BCHE hydrolyzes sarin and VX, whereas the double mutant G117H/E197Q hydrolyzes soman (Millard et al. Biochemistry 1995; 34: 15925-15933; 1998; 37: 237-247). However, the rates of organophosphate hydrolysis are slow and a faster organophosphate hydrolase is being sought. Native PON1 hydrolyzes paraoxon with a catalytic efficiency, of 2.4 x 10(6) M(-1) x min(-1), and our goal is to improve the organophosphate hydrolase activity of PON1. To achieve this we need to identify the amino acids in the active site of PON1. Using site-directed mutagenesis and expression in human 293T cells, we have identified the following eight amino acids as being essential to PON1 activity: W280, H114, H133, H154, H242, H284, E52 and D53. Fluorescence of PON1 complexed to terbium ion shows that at least one tryptophan is close to the calcium binding site.

Determination of the DNA sequences of acetylcholinesterase and butyrylcholinesterase from cat and demonstration of the existence of both in cat plasma

Cat serum contains 0.5 mg/L of butyrylcholinesterase (BChE, EC 3.1.1. 8) and 0.3 mg/L of acetylcholinesterase (AChE, EC 3.1.1.7); this can be compared with 5 mg/mL and < 0.01 mg/L, respectively, in human serum. Cat BChE differed from human BChE in the steady-state turnover of butyrylthiocholine, having a 3-fold higher k(cat) and 2-fold higher K(m) and K(ss) values. Sequencing of the cat BCHE cDNA revealed 70 amino acid differences between cat and human BChE, three of which could account for these kinetic differences. These amino acids, which were located in the region of the active site, were Phe398Ile, Pro285Leu, and Ala277Leu (where the first amino acid was found in human and the second in cat). Sequencing genomic DNA for cat and human ACHE demonstrated that there were 33 amino acid differences between the cat and human AChE enzymes, but that there were no differences in the active site region. In addition, a polymorphism in intron 3 of the human ACHE gene was detected, as well as a silent polymorphism at Y116 of the cat ACHE gene.

The butyrylcholinesterase K-variant shows similar cellular protein turnover and quaternary interaction to the wild-type enzyme

A recent study has linked the butyrylcholinesterase (BChE) K-variant and the apolipoprotein epsilon4 isoform to late-onset Alzheimer's disease. These findings have been controversial and have led us to examine the differences between wild-type and K-variant BChE in enzyme activity, protein stability, and quaternary structure. J-variant BChE (E497V/A539T) was also studied because it is associated with the K-variant mutation. The K-variant mutation (A539T) is located in the C-terminal tetramerization domain. Wild-type, K-variant, and J-variant BChE were expressed in Chinese hamster ovary cells and purified. The purified enzymes had similar binding affinity (Km) values and catalytic rates for butyrylthiocholine and benzoylcholine. In pulse-chase studies the K-variant, J-variant, and wildtype BChE were degraded rapidly within the cell, with a half-time of approximately 1.5 h. Less than 5% of the intracellular BChE was exported. The C-terminal peptide containing the K-variant mutation interacted with itself as strongly as did the wild-type peptide in the yeast two-hybrid system. Both K-variant and wild-type BChE assembled into tetramers in the presence of poly-L-proline or the proline-rich attachment domain of the collagen tail. The native K-variant BChE in serum showed the same proportion of tetramers as the native serum wild-type BChE. We conclude that the K-variant BChE is similar to wild-type BChE in enzyme activity, protein turnover, and tetramer formation.

A naturally occurring genetic variant in the human chorionic gonadotropin-beta gene 5 is assembly inefficient

The hCGbeta gene family is composed of six homologous genes linked in tandem repeat on chromosome 19; the order of the genes is 7, 8, 5, 1, 2, and 3. Previous studies have shown that hCGbeta gene 5 is highly expressed during the first trimester of pregnancy. The purpose of our study was to identify naturally occurring polymorphisms in hCGbeta gene 5 and determine whether these alterations affected hCG function. The data presented here show that hCGbeta gene 5 was highly conserved in the 334 asymptomatic individuals and 41 infertile patients examined for polymorphisms using PCR followed by single stranded conformational polymorphism analysis. Most of the polymorphisms detected were either silent or located in intron regions. However, one genetic variant identified in beta gene 5 exon 3 was a G to A transition that changed the naturally occurring valine residue to methionine in codon 79 (V79M) in 4.2% of the random population studied. The V79M polymorphism was always linked to a silent C to T transition in codon 82 (tyrosine). To determine whether betaV79M hCG had biological properties that differed from those of wild-type hCG, a beta-subunit containing the V79M substitution was created by site-directed mutagenesis and was coexpressed with the glycoprotein hormone alpha-subunit in Chinese hamster ovary cells and 293T cells. When we examined betaV79M hCG biosynthesis, we detected atypical betaV79M hCG folding intermediates, including a betaV79M conformational variant that resulted in a beta-subunit with impaired ability to assemble with the alpha-subunit. The inefficient assembly of betaV79M hCG appeared to be independent of beta-subunit glycosylation or of the cell type studied, but, rather, was due to the inability of the betaV79M subunit to fold correctly. The majority of the V79M beta-subunit synthesized was secreted as unassembled free beta. Although the amount of alphabeta hCG heterodimer formed and secreted by betaV79M-producing cells was less than that by wild-type beta-producing cells, the hCG that was secreted as alphabeta V79M heterodimer exhibited biological activity indistinguishable from that of wild-type hCG.

An improved cocaine hydrolase: the A328Y mutant of human butyrylcholinesterase is 4-fold more efficient

Butyrylcholinesterase (BChE) has a major role in cocaine detoxication. The rate at which human BChE hydrolyzes cocaine is slow, with a kcat of 3.9 min(-1) and Km of 14 microM. Our goal was to improve cocaine hydrolase activity by mutating residues near the active site. The mutant A328Y had a kcat of 10.2 min(-1) and Km of 9 microM for a 4-fold improvement in catalytic efficiency (kcat/Km). Since benzoylcholine (kcat 15,000 min(-1)) and cocaine form the same acyl-enzyme intermediate but are hydrolyzed at 4000-fold different rates, it was concluded that a step leading to formation of the acyl-enzyme intermediate was rate-limiting. BChE purified from plasma of cat, horse, and chicken was tested for cocaine hydrolase activity. Compared with human BChE, horse BChE had a 2-fold higher kcat but a lower binding affinity, cat BChE was similar to human, and chicken BChE had only 10% of the catalytic efficiency. Naturally occurring genetic variants of human BChE were tested for cocaine hydrolase activity. The J and K variants (E497V and A539T) had k(cat) and Km values similar to wild-type, but because these variants are reduced to 66 and 33% of normal levels in human blood, respectively, people with these variants may be at risk for cocaine toxicity. The atypical variant (D70G) had a 10-fold lower binding affinity for cocaine, suggesting that persons with the atypical variant of BChE may experience severe or fatal cocaine intoxication when administered a dose of cocaine that is not harmful to others.

Butyrylcholinesterase-catalysed hydrolysis of aspirin, a negatively charged ester, and aspirin-related neutral esters

Although aspirin (acetylsalicylic acid) is negatively charged, it is hydrolysed by butyrylcholinesterase (BuChE). 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 BuChE 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' BuChE 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-BuChE 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 BuChE indicated perpendicular interactions between the aromatic rings of W82 and aspirin. Kinetic study of BuChE-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.

Aging of di-isopropyl-phosphorylated human butyrylcholinesterase

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.

Importance of aspartate-70 in organophosphate inhibition, oxime re-activation and aging of human butyrylcholinesterase

Asp-70 is the defining amino acid in the peripheral anionic site of human butyrylcholinesterase (BuChE), 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 BuChE. 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 BuChE 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 BuChE by 2-PAM, but this will be offset by the lower rate of irreversible aging of inhibited BuChE, allowing some regeneration by spontaneous hydrolysis.

Role of aspartate 70 and tryptophan 82 in binding of succinyldithiocholine to human butyrylcholinesterase

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.

Asp7O in the peripheral anionic site of human butyrylcholinesterase

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.

Characterization of 12 silent alleles of the human butyrylcholinesterase (BCHE) gene

The silent phenotype of human butyrylcholinesterase (BChE), present in most human populations in frequencies of approximately 1/100,000, is characterized by the complete absence of BChE activity or by activity <10% of the average levels of the usual phenotype. Heterogeneity in this phenotype has been well established at the phenotypic level, but only a few silent BCHE alleles have been characterized at the DNA level. Twelve silent alleles of the human butyrylcholinesterase gene (BCHE) have been identified in 17 apparently unrelated patients who were selected by their increased sensitivity to the muscle relaxant succinylcholine. All of these alleles are characterized by single nucleotide substitutions or deletions leading to distinct changes in the structure of the BChE enzyme molecule. Nine of the nucleotide substitutions result in the replacement of single amino acid residues. Three of these variants, BCHE*33C, BCHE*198G, and BCHE*201T, produce normal amounts of immunoreactive but enzymatically inactive BChE protein in the plasma. The other six amino acid substitutions, encoded by BCHE*37S, BCHE*125F, BCHE*170E, BCHE*471R, and BCHE*518L, seem to cause reduced expression of BChE protein, and their role in determining the silent phenotype was confirmed by expression in cell culture. The other four silent alleles, BCHE*271STOP, BCHE*500STOP, BCHE*FS6, and BCHE*I2E3-8G, encode BChES truncated at their C-terminus because of premature stop codons caused by nucleotide substitutions, a frame shift, or altered splicing. The large number of different silent BCHE alleles found within a relatively small number of patients shows that the heterogeneity of the silent BChE phenotype is high. The characterization of silent BChE variants will be useful in the study of the structure/function relationship for this and other closely related enzymes.

Tissue distribution of human acetylcholinesterase and butyrylcholinesterase messenger RNA

Cholinesterase inhibitors occur naturally in the calabar bean (eserine), green potatoes (solanine), insect-resistant crab apples, the coca plant (cocaine) and snake venom (fasciculin). There are also synthetic cholinesterase inhibitors, for example man-made insecticides. These inhibitors inactivate acetylcholinesterase and butyrylcholinesterase as well as other targets. From a study of the tissue distribution of acetylcholinesterase and butyrylcholinesterase mRNA by Northern blot analysis, we have found the highest levels of butyrylcholinesterase mRNA in the liver and lungs, tissues known as the principal detoxication sites of the human body. These results indicate that butyrylcholinesterase may be a first line of defense against poisons that are eaten or inhaled.

Endogenous butyrylcholinesterase in SV40 transformed cell lines: COS-1, COS-7, MRC-5 SV40, and WI-38 VA13

Comparison of proteins expressed by SV40 transformed cell lines and untransformed cell lines is of interest because SV40 transformed cells are immortal, whereas untransformed cells senesce after about 50 doublings. In MRC-5 SV40 cells, only seven proteins have previously been reported to shift from undetectable to detectable after transformation by SV40 virus. We report that butyrylcholinesterase is an 8th protein in this category. Butyrylcholinesterase activity in transformed MRC-5 SV40 cells increased at least 150-fold over its undetectable level in MRC-5 parental cells. Other SV40 transformed cell lines, including COS-1, COS-7, and WI-38 VA13, also expressed endogenous butyrylcholinesterase, whereas the parental, untransformed cell lines, CV-1 and WI-38, had no detectable butyrylcholinesterase activity or mRNA. Infection of CV-1 cells by SV40 virus did not result in expression of butyrylcholinesterase, showing that the butyrylcholinesterase promoter was not activated by the large T antigen of SV40. We conclude that butyrylcholinesterase expression resulted from events related to cell immortalization and did not result from activation by the large T antigen.