Purification of Acetylcholinesterase by Affinity Chromatography and Determination of Active Site Stoichiometry

Biogenesis, assembly and trafficking of acetylcholinesterase

Acetylcholinesterase (AChE) is expressed as several homomeric and heterooligomeric forms in a wide variety of tissues such as neurons in the central and peripheral nervous systems and their targets including skeletal muscle, endocrine and exocrine glands. In addition, glycolipid-anchored forms are expressed in erythropoietic and lymphopoietic cells. While transcriptional and post-transcriptional regulation is important for determining which AChE oligomeric forms are expressed in a given tissue, translational and post-translational regulatory mechanisms at the level of protein folding, assembly and sorting play equally important roles in assuring that the AChE molecules reach their intended sites on the cell surface in the appropriate numbers. This brief review will focus on the latter events in the cell with the goal of providing novel therapeutic interventional strategies for the treatment of organophosphate and carbamate pesticide and nerve agent exposure. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Molybdenum-containing nitrite reductases: Spectroscopic characterization and redox mechanism

OBJECTIVES

This review summarizes the spectroscopic results, which will provide useful suggestions for future research. In addition, the fields that urgently need more information are also advised.

BACKGROUND

Nitrite-NO-cGMP has been considered as an important signaling pathway of NO in human cells. To date, all the four known human molybdenum-containing enzymes, xanthine oxidase, aldehyde oxidase, sulfite oxidase, and mitochondrial amidoxime-reducing component, have been shown to function as nitrite reductases under hypoxia by biochemical, cellular, or animal studies. Various spectroscopic techniques have been applied to investigate the structure and catalytic mechanism of these enzymes for more than 20 years.

METHODS

We summarize the published data on the applications of UV-vis and EPR spectroscopies, and X-ray crystallography in studying nitrite reductase activity of the four human molybdenum-containing enzymes.

RESULTS

UV-vis has provided useful information on the redox active centers of these enzymes. The utilization of EPR spectroscopy has been critical in determining the coordination and redox status of the Mo center during catalysis. Despite the lack of substrate-bound crystal structures of these nitrite reductases, valuable structural information has been obtained by X-ray crystallography.

CONCLUSIONS

To fully understand the catalytic mechanisms of these physiologically/pathologically important nitrite reductases, structural studies on substrate-redox center interaction are needed.

Purification of soluble acetylcholinesterase from Japanese quail brain by affinity chromatography

The purification of a soluble acetylcholinesterase from Japanese quail brain using affinity chromatography on concanavalin A-Sepharose and edrophonium-Sepharose is described. The affinity matrix was synthesized by coupling an inhibitor edrophonium to epoxy-activated Sepharose. Acetylcholinesterase was purified 10,416-fold with a specific activity of 2500 U/mg protein. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and mercaptoethanol gave only one band with a molecular weight of 62.5 kDa. The molecular weight of the purified acetylcholinesterase was estimated to be 245.5 kDa by gel chromatography on Sephacryl S-200 under nondenaturing conditions. Based on the molecular weight obtained by both SDS-PAGE and gel filtration the purified acetylcholinesterase was assumed to be a tetrameric form.

Active-site peptides of acetylcholinesterase of Electrophorus electricus: labelling of His-440 by 1-bromo-[2-14C]pinacolone and Ser-200 by tritiated diisopropyl fluorophosphate

To characterize the structure of the active site of acetylcholinesterase (AChE) from the electric organ of E. electricus, we identified sites of incorporation of two active-site affinity labels, [3H]diisopropyl fluorophosphate ([3H]DFP), and 1-bromo-2-[14C]pinacolone ([14C]BrPin). AChE was isolated, purified, inactivated and digested with trypsin, and peptides containing 3H or 14C were purified by reverse-phase HPLC and characterized by N-terminal sequence analysis. [3H]DFP, labelling Ser-200, was found in a single peptide, QVTIFGESAGAASVGMHLLSPDSR, 83% identical with the sequence from Thr-193 to Arg-216 deduced for AChE of T. californica, with Gln, Ala, Leu, and Asp in place of Thr-193, Gly-203, Ile-210 and Gly-214, respectively, and 87% identical with that from bovine and human brain AChEs. Inactivation by [14C]BrPin led to two radioactive peptides. One, ASNLVWPEWMGVIHGYEIEFVFGLPLEK, was 96% identical with that extending from Ala-427 to Lys-454 of T. californica. Release of 14C in cycle 14 established reaction of [14C]BrPin with active-site His-440, protected by 5-trimethylammonio-2-pentanone (TAP). The other peptide, LLXVTENIDDAER, 77% homologous with that of T. californica extending from Leu-531 to Arg-543, had label associated with the third cycle, not protected by TAP, corresponding to Asn-533. The slow inactivation of eel AChE by reaction of [14C]BrPin at His-440 contrasts with that of AChE from T. nobiliana, where it reacts rapidly with a free cysteine, Cys-231, not present in eel AChE. For both AChEs, inactivation by BrPin prevents subsequent reaction with [3H]DFP, and prior inactivation by DFP does not prevent reactions with [14C]BrPin.

Reactions of 1-bromo-2-[14C]pinacolone with acetylcholinesterase from Torpedo nobiliana. Effects of 5-trimethylammonio-2-pentanone and diisopropyl fluorophosphate

1-Bromo-2-[14C]pinacolone, (CH3)3C14COCH2Br [( 14C]BrPin), was prepared from [1-14C]acetyl chloride and tert-butylmagnesium chloride with cuprous chloride catalyst, followed by bromination. It was examined as an active-site directed label for acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) (AcChE). AcChE, isolated from Torpedo nobiliana, has k(cat) = (4.00 +/- 0.04).10(3) s-1, Km = 0.055 +/- 0.008 mM in hydrolysis of acetylthiocholine, and k(cat) = (5.6 +/- 0.2).10(3) s-1, Km = 0.051 +/- 0.003 mM in hydrolysis of acetylcholine. BrPin, binding in the trimethyl cavity, acts initially as a reversible competitive inhibitor, Ki = 0.20 +/- 0.09 mM, and, with time, as an irreversible covalently bound inactivator. Introduction of 14C from [14C]BrPin into Torpedo AcChE at pH 7.0 was followed by SDS-PAGE, autoradiography and scintillation counting, in the absence and presence of 5-trimethylammonio-2-pentanone (TAP), a competitive inhibitor (Ki = 0.075 +/- 0.001 mM) isosteric with acetylcholine; 1.8-1.9 14C was incorporated per inactivated enzyme unit at 50% inactivation. TAP retarded inactivation by [14C]BrPin, and prevented introduction of 0.9-1.1 14C per unit of enzyme protected. Prior inactivation of AcChE by BrPin prevents reaction with [3H]diisopropyl fluorophosphate [( 3H]DFP). Prior inactivation by DFP or [3H]DFP does not prevent reaction with [14C]BrPin, and this subsequent reaction with BrPin does not displace the [3H] moiety. [14C]BrPin alkylates a nucleophile in the active site, and this reaction does not alkylate or utilize the serine-hydroxyl.

Physiochemical and immunological properties of acetylcholine receptors from human muscle

The acetylcholine receptor protein from human muscle was extracted with the non-ionic detergent Triton X-100 and purified by affinity chromatography on alpha-Naja toxin sepharose 4B. Further purification on Dicap-MP sepharose 4B, a choline analog compound, led to ACHR preparations with specific activities of 2-7 nmol/mg protein. The isolated receptor, labeled with 125I-alpha-bungarotoxin was characterized by different methods and compared to ACHRs from Torpedo californica electroplax and rat-denervated skeletal muscle. Gel filtration on Ultrogel AcA 34 resulted in a stokes radius of 70 A for the receptor monomer and 99 A for the dimeric form. Sucrose density gradient centrifugation showed sedimentation coefficients of 9.1 S and 13.5 S. From these data the molecular weight of the ACHR monomer was estimated as 254 000 D and 540 000 D for the receptor dimer. The isoelectric point of the 125I-alpha-bgt-ACHR complex was determined by thin-layer isoelectric focussing to be pH 5. Purified ACHRs were used for immunization of rats and mice which developed an EAMG as verified by clinical observation and electrophysical measurements. Sera from the immunized animals as well as from myasthenia gravis patients were subsequently used to compare the cross-reactivity of ACHR preparations from different sources. While antibodies of rats immunized with Torpedo ACHRs cross-reacted with ACHR preparations from rat and human skeletal muscle, antibodies from mice immunized with rat ACHR only reacted with preparations from rats and mice. Antibodies from mice immunized with ACHR of human origin exhibited a broad cross-reactivity, as did antibodies from MG patients.

Affinity chromatography of acetylcholinesterase from Electrophorus electricus electroplax. Investigations on 9-aminoalkylacridine affinity ligands

The neural enzyme 11-S acetylcholinesterase (E.C. 3.1.1.7) was purified by affinity chromatography from a trypsin digest of Electrophorus electricus electric organ. Unquaternized affinity ligands were reported which were comparable in efficacy to the routinely employed "quaternized acridine MAC ligand". A study was made of the quaternization reactions of various 9-aminoalkylacridines and 9-aminoacridine along with their relative binding affinities to acetylcholinesterase. Ease of synthesis in conjunction with the column performance of these unquaternized 9-aminoalkylacridine compounds made them the preferred affinity ligand in acetylcholinesterase chromatography. A new carbodiimide synthetic route for these unquaternized ligands was described.

Comparative affinity chromatography of acetylcholinesterases from five vertebrate species

The efficacy of N-methylacridinium affinity chromatography in the purification of acetylcholinesterases from chicken, rat, calf and human brain and from the electric organ of the electric fish Torpedo marmorata has been investigated. Retention of the enzymes on the N-methylacridinium columns exceeded 90% in all instances except for the chicken enzyme, where 40-80% retention was observed depending on the acridinium concentration. Sucrose density gradient centrifugation profiles revealed no difference between the distribution of molecular forms in the crude extracts and in the partially purified fractions eluted from the columns by decamethonium iodide.

Altered acetylcholinesterase isozyme patterns in mice with hereditary muscular dystrophy

Normal and dystrophic mouse muscles were separated into a predominantly white muscle fraction (gastrocnemius, extensor digitorum longus) and a predominantly red muscle fraction (diaphragm). Acetylcholinesterase (AChE) was extracted from each muscle fraction using a Triton X-100/NaCl buffer. Six forms of AChE were separated from each muscle homogenate by velocity sedimentation on linear sucrose gradients. Their apparent sedimentation coefficients in each case were 19.7S, 16.0S, 13.3S, 10.4S, 7.6S, and 3.9S. Gel electrophoresis of crude muscle homogenates under nondenaturing conditions (native gels) and of ech separate isozyme fraction gave one band of AChE activity with a consistent Rf (relative mobility) value. Reelectrophoresis of native gel bands on SDS/acrylamide slab gels revealed a similar monomeric subunit protein from either crude muscle homogenates or isozyme fractions with an apparent molecular weight of approximately 69,000 daltons. Our results indicate that the AChE distribution and activity are severely affected in dystrophic "white" muscles (anaerobic) but much less so in "red" muscles (aerobic). Dystrophic predominantly white muscles weigh less, contain less protein, and have a decreased total AChE activity in comparison with their normal counterparts. Furthermore, the relative proportions of AChE activity in each isozyme fraction is altered between normal white and dystrophic white muscle fractions: i.e., dystrophic white muscle contains a decreased proportion of a low molecular weight form (7.6S) and increased proportions of higher molecular weight forms (16.0S, 19.7S). In contrast, no significant differences occur in AChE activity or distribution between normal and dystrophic predominantly red muscle. The changes in white muscle AChE are toward a pattern common to red muscle. This suggests that the effect of muscular dystrophy and its related stress on mouse white muscle is at least in part a shift from a predominantly anaerobic, fatigable metabolism to an aerobic, fatigue-resistant metabolism.

Acetylcholinesterase kinetics

Three mechanisms have been suggested to describe the inhibition of acetylcholinesterase (EC. 3.1.1.7) by an excess of acetylcholine. (i) Substrate inhibition occurs through the reaction of acetylcholine with acetylated enzyme. The deacetylation of this ternary complex is supposed to be completely inhibited. (ii) A ternary complex is formed as in (i). However, the deacetylation is not completely inhibited. (iii) A two-site-mechanism is discussed. Acetylcholine binds either to the active site or to the modifier site. Binding to the latter changes the activity of the active site. Steady state treatment was applied to (i)-(iii). A least squares fit led to catalytic parameters. It is demonstrated that mechanism (ii) is the most simple one which can describe satisfactorily the experimental data. Limits for a set rate constants are derived from the catalytic parameters. A numerical integration shows that the steady state approximation may be used even when the mechanisms are rather complex.

Major component of acetylcholinesterase in Torpedo electroplax is not basal lamina associated

Electroplax tissue from Torpedo californica contains two major structural forms of the enzyme acetylcholinesterase. One form, composed of tetrameric protomers which are further aggregated by interactions among associated collagenous "tail fibers", has been well characterized previously. This form is associated in situ with the basal lamina. The other form is described and characterized herein. This latter form accounts for at least 50% of the acetylcholinesterase activity of the tissue. This enzyme associated with the tissue phospholipids. It aggregates in aqueous solution but readily dissociates to dimers in 1% sodium cholate solution, a solvent in which it is both soluble and catalytically fully active. The same dimer is obtained in sodium dodecyl sulfate solution where the enzyme is denatured. Denaturation in the presence of the reductant dithiothreitol results in the formation of a single 80000-dalton subunit. The purified enzyme contains no collagenous component. It is not derivable from the collagenous "tailed-enzyme" form in the tissue homogenate. However, the two enzymes have similar molecular weight catalytic subunits and the same substrate-dependent turnover numbers (per active site) for a variety of choline esters which are generally utilized to distinguish specific esterase function. In the tissue homogenate each form of the enzyme is associated with a characteristic structural component (phospholipid or collagen). By implication, acetylcholinesterase function is localized in situ in the phospholipid membrane as well as at the basal lamina.

On the homogeneity of 11-S acetylcholinesterase

11-S acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) purified by affinity chromatography of trypsin-digested homogenates was shown to be contaminated with three other active forms of enzyme. The initial purification used an affinity column of the inhibitor, N-methylacridinium ion. Chromatography of the "affinity-pure" sample on hydroxyapatite resulted in two peaks of acetylcholinesterase activity. One peak contained only a form sedimenting at 11-S (approx. 85% of the recovered activity). The other peak consisted of a 9.5-S form, in addition to 14-S and 18-S forms. The 9.5-S form (approx. 7% of the activity) co-electrophoresed with 11-S in 6% polyacrylamide gels and co-sedimented with the same form in sucrose density gradients containing 0.1 M NaCl. The purified 11-S enzyme was shown to be homogeneous by sucrose density gradient centrifugation and electrophoresis. These results indicate that 11-S acetylcholinesterase may be unsuitable for some characterization studies due to undetected contamination by the 9.5-S form.

Purification of acetylcholinesterase from house fly brain by affinity chromatography

Acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) from the heads of house flies (Musca domestica L.) was purified by affinity chromatography. The enzyme was adsorbed from the crude extracts on an affinity column containing trimethyl(p-aminophenyl) ammonium chloride hydrochloride (Ki approximately 1.7 - 10(-4) M), covalently linked to Sepharose 4B, then eluted with a solution of a selective reversible inhibitor, 1,5-bis (4-allyl dimethyl ammoniumphenyl)-pentan-3-one dibromide (BW 284C51; Ki approximately 1 - 10(-7) M). The enzyme was purified 1223 times in one step and had a specific activity of 752 units/mg protein. Disc gel electrophoresis in polyacrylamide gel revealed five protein bands, four corresponding to the enzyme activity bands and one devoid of enzyme activity. On the basis of periodic acid-Schiff stain intensity, the slower moving isozyme I and the contaminating band appear to be rich in carbohydrate. The purity of the enzyme estimated by disc gel electrophoresis was 94%. Density gradient centrifugation in sucrose showed two major species each of which ran as a single band on disc gel electrophoresis. The average molecular weights were 306000 (+/- 11 150) for heavy (s20,w = 11.5 S) form and 143000 (+/- 4700) for light (s20,w = 6.9 S) form.

Properties of tyrosine-inhibitable 3-deoxy-d-arabinoheptulosonic acid-7-phosphate synthase from Salmonella

Tyrosine-inhibitable 3-deoxy-D-arabinoheptulosonic acid-7-phosphate (DAHP) synthase was purified to homogeneity without significant loss of sensitivity to inhibition by tyrosine from an operator-constitutive strain (tyrOc) of Salmonella. The enzyme had an apparent molecular weight of 76,000 by gel filtration and a subunit molecular weight of 40,000 by sodium dodecyl sulfate-gel electrophoresis and by reaction with dimethyl suberimidate. It had an isoelectric point of 4.68. Inhibition by L-tyrosine showed a Hill coefficient of 1.8 at pH 7.0, suggesting cooperative interaction between tyrosine-binding sites, and was competitive with phosphoenol pyruvate and noncompetitive with erythrose-4-phosphate.

Affinity chromatography of acetylcholinesterase. The importance of hydrophobic interactions

An easily prepared affinity column for acetylcholinesterase is described, which may be operated at ionic strength high enough to prevent aggregation of the asymmetric forms of the enzyme. Specific elution by tetraethylammonium or decamethonium was quantitative. The performance of this column is comparable to that of the column described by Dudai and Silman. It is shown that the hexyl 'spacer arm' strongly participates in the enzyme binding and that its replacement with the more hydrophilic spermine chain lowers the affinity. The hexyl chain itself is shown to bind acetylcholinesterase, although with lower affinity and capacity than the complete column. This binding is also partly reversed by inhibitors. Such hydrophobic columns bind the native asymmetric forms of the enzyme more strongly than the lytic globular ones. The aromatic quaternary ligang inhibits Electrophorus but not Torpedo acetylcholinesterase; therefore the column does not retain the Torpedo enzyme. Differences in Km between acetylcholinesterases of the two species also point to differences in their active sites.

Molecular forms of Electrophorus acetylcholinesterase. Molecular weight and composition

Molecular weights for the series of six Electrophorus acetylcholinesterase forms have been determined either by the sedimentation-diffusion equilibrium method or, particularly in the case of the very scarce G' and G inches forms, from their Stokes radius and sedimentation coefficient values. Both methods are in excellent agreement. The results provide good evidence for the model previously proposed, G inches, G' and G containing one, two and four subunits, whereas A, C and D possess, in addition to respectively one, two and three tetrameric sets of such subunits, a structural element, the tail. Although the amino acid composition of 'tailed' and globular forms did not reveal any significant feature of this element, its mass, about 100 000 daltons, could be deduced from a comparison of molecular weights for the two classes of acetylcholinesterase forms. This value is in close agreement with electron microscopic data. The tail is thought to consist of three 30 000-dalton strands.

Structure-activity relationships in acetylcholinesterase reactions. Hydrolysis of non-ionic acetic esters

The Michaelis-Menten parameters kcat, Ks(app) and the second-order rate constants kII = k2/Ks of acetylcholinesterase-catalyzed hydrolysis of 25 acetic esters with non-ionic leaving groups have been determined at 25 degree C and pH 7.5 in 0.15 M KCL. A linear relationship between the substrate noncovalent binding capacity and the leaving group hydrophobicity, and a multiparameter correlation of the acetylation reaction rate constant logarithm with the leaving group inductive effect, hydrophobicity, and steric effect, have been established. The acetyl-enzyme deacetylation rate constant has been calculated. Taken together, a fairly complete understanding of acetylcholinesterase specificity is possible. The data are consistent with a model of the acetylcholinesterase active site, in which the catalytically active groups are located at the bottom of a jaws-like slit with a limited range of hydrophobic walls that provide the sorption of the substrate leaving groups not longer than that in n-butyl acetate.

Differences in subunit activities in acetylcholinesterase as possible cause for apparent deviation from normal Michaelis-Menten kinetics

1. Form Gp of acetylcholinesterase (EC 3.1.1.7) from electric eel gave curved Lineweaver-Burk plots with acetylcholine. The Hill coefficients were 0.50-0.55 at low ionic strength and increased to 0.93 with increasing ionic strength. In presence of atropine or hexamethonium normal Michaelis-Menten kinetics were observed. 2. Inhibition of the enzyme by iPr2P-F was biphasic. One half of the total enzyme activity decreased at a faster rate than the other half. With [3H]iPr2P-F, the extent of labelling was determined for the light and heavy subunits. At low [3H]iPr2P-F concentration only the light subunit was phosphorylated. Higher [3H]iPr2P-F concentrations and prolonged treatment increased the amount of label in the heavy subunit. 3. From these data it is concluded that the two subunits are labelled at different rates indicating different reactivity towards iPr2P-F as well as towards acetylcholine. These data might account for the apparent non-Michaelis-Menten type kinetics obtained at low ionic strength.