The role of sulfhydryl groups in the activity of type A botulinum toxin

Purification and Characterization of Botulinum Neurotoxin FA from a Genetically Modified Clostridium botulinum Strain

Botulinum neurotoxins (BoNTs), produced by neurotoxigenic clostridial species, are the cause of the severe disease botulism in humans and animals. Early research on BoNTs has led to their classification into seven serotypes (serotypes A to G) based upon the selective neutralization of their toxicity in mice by homologous antibodies. Recently, a report of a potential eighth serotype of BoNT, designated "type H," has been controversial. This novel BoNT was produced together with BoNT/B2 in a dual-toxin-producing Clostridium botulinum strain. The data used to designate this novel toxin as a new serotype were derived from culture supernatant containing both BoNT/B2 and novel toxin and from sequence information, although data from two independent laboratories indicated neutralization by antibodies raised against BoNT/A1, and classification as BoNT/FA was proposed. The sequence data indicate a chimeric structure consisting of a BoNT/A1 receptor binding domain, a BoNT/F5 light-chain domain, and a novel translocation domain most closely related to BoNT/F1. Here, we describe characterization of this toxin purified from the native strain in which expression of the second BoNT (BoNT/B) has been eliminated. Mass spectrometry analysis indicated that the toxin preparation contained only BoNT/FA and confirmed catalytic activity analogous to that of BoNT/F5. The in vivo mouse bioassay indicated a specific activity of this toxin of 3.8 × 10(7) mouse 50% lethal dose (mLD50) units/mg, whereas activity in cultured human neurons was very high (50% effective concentration [EC50] = 0.02 mLD50/well). Neutralization assays in cells and mice both indicated full neutralization by various antibodies raised against BoNT/A1, although at 16- to 20-fold-lower efficiency than for BoNT/A1. IMPORTANCE Botulinum neurotoxins (BoNTs), produced by anaerobic bacteria, are the cause of the potentially deadly, neuroparalytic disease botulism. BoNTs have been classified into seven serotypes, serotypes A to G, based upon their selective neutralization by homologous antiserum, which is relevant for clinical and diagnostic purposes. Even though supportive care dramatically reduces the death rate of botulism, the only pharmaceutical intervention to reduce symptom severity and recovery time is early administration of antitoxin (antiserum raised against BoNTs). A recent report of a novel BoNT serotype, serotype H, raised concern of a "treatment-resistant" and highly potent toxin. However, the toxin's chimeric structure and characteristics indicate a chimeric BoNT/FA. Here we describe the first characterization of this novel toxin in purified form. BoNT/FA was neutralized by available antitoxins, supporting classification as BoNT/FA. BoNT/FA required proteolytic activation to achieve full toxicity and had relatively low potency in mice compared to BoNT/A1 but surprisingly high activity in cultured neurons.

An improved method for development of toxoid vaccines and antitoxins

Botulinum neurotoxins are the most potent toxins known and causative agents of human botulism. Treatment comprises of administering purified polyclonal antitoxin or the prophylactic use of a vaccine containing formaldehyde inactivated toxoid. Whilst formaldehyde inhibits toxin activity, it induces so many structural changes in the molecule that immunisation often results in low levels of neutralising antibodies. We describe here for the first time a simple, less time consuming, novel method for producing a non-toxic toxoid that is structurally and antigenically more similar to the native toxin. Toxin is chemically inactivated by alkylation with iodoacetamide in the presence of reversibly denaturing conditions. This reduces neurotoxic activity by at least 7-orders of magnitude to undetectable levels. Following immunisation, in vivo neutralising antibody levels were 600-times higher than those produced with formaldehyde toxoid, despite generating equivalent ELISA antitoxin binding titres. These studies demonstrate that the new toxoid retains more of the native toxins structure and critical epitopes responsible for inducing life-saving neutralising antibody. Toxoid produced by the new method should substantially improve both antitoxin and vaccine production and be applicable to other toxins and immunogens.

Antibody mapping to domains of botulinum neurotoxin serotype A in the complexed and uncomplexed forms

The domain organization of the botulinum neurotoxin serotype A was studied by using antibody mapping of 44 monoclonal single-chain variable fragments. The analysis was carried out on (i) the individual domains of botulinum neurotoxin holotoxin (binding, translocation, and catalytic), (ii) botulinum neurotoxin holotoxin, (iii) the botulinum neurotoxin holotoxin in complex with the nontoxic portion, and (iv) botulinum neurotoxin holotoxin and nontoxic portion of the complex recombined in vitro. All 44 antibodies mapped to individual domains of botulinum neurotoxin. Forty of the 44 single-chain variable fragments bound the botulinum neurotoxin holotoxin relative to the isolated domains, suggesting that 4 epitopes are covered when the individual domains are in the holotoxin form. Only 20 of the antibodies showed a positive reaction to the toxin while in complex with the nontoxic portion. All of the covered epitopes were mapped to the binding domain of botulinum neurotoxin, which suggested that the binding domain is in direct contact with the nontoxic portion in the complex. Based on the antibody mapping to the different domains of the botulinum neurotoxin holotoxin and the entire complex, a model of the botulinum neurotoxin complex is proposed.

Crystallization and preliminary X-ray analysis of botulinum neurotoxin type A

Botulinum neurotoxin serotype A was isolated from liquid culture of Clostridium botulinum. The pure Mr approximately 150,000 neurotoxin, composed of Mr approximately 50,000 light and Mr approximately 100,000 heavy chains, has been crystallized in three different crystal morphologies; all three have the same crystal form. The most suitable crystal form for X-ray analysis are bipyrimidal and crystallize in the hexagonal space group P3(1)21 (or P3(2)21) with one dimer per asymmetric unit. The unit cell dimensions are a = b = 170.5 A, c = 161.7 A. The crystals diffract to 3 A resolution.

Clostridium botulinum types A, B, C1, and E produce proteins with or without hemagglutinating activity: do they share common amino acid sequences and genes?

Clostridium botulinum produce the antigenically distinct 150 kD neurotoxin serotypes (e.g., A, B, C1, and E) and simultaneously proteins, A Hn+, B Hn+, C Hn+, and E Hn-, that have high, low, and no hemagglutinating activity. A Hn+ and B Hn+ are serologically cross-reactive. A Hn+, B Hn+, and C Hn+ found as large aggregates (900-220 kD) can be dissociated on SDS-PAGE into multiple subunits, the smallest for A Hn+, B Hn+ is 17 kD and 27 kD for C Hn+. The 116 kD E Hn- does not aggregate. We determined the sequences of 10-33 amino terminal residues of the 17, 21.5, 35, and 57 kD subunits of A Hn+ and B Hn+. Each of these subunits have unique sequences, indicating that the larger units studies are not homomers or heteromers of smaller units. The subunits of A Hn+ and B Hn+ of comparable size have striking sequence identity (e.g., 21.5 kD subunits from the two are identical and 57 kD subunits have 80% identity). In vitro proteolysis of 116 kD E Hn- with different proteases did not impart hemagglutinating activity to the fragments. The 116 kD E Hn- and one of its proteolytic fragments (87 kD) were partially sequenced. Sixty-two base pairs downstream from the termination codon of the cloned 33 kD subunit of C Hn+, there is an initiation codon followed by an open reading frame for at least 34 amino acid residues (Tsuzuki et al., 1990). The derived amino acid sequence of this open reading frame, we found, has 73-84% sequence identity with those of the 17 kD subunits of A Hn+ and B Hn+ and significant identity with the N-terminal of E Hn-. These highly conserved sequences show existence of genetic linkage among the Hn+ and Hn- proteins.

Changes in the molecular topography of the light and heavy chains of type A botulinum neurotoxin following their separation

Botulinum neurotoxin serotype A, an approx. 150 kDa protein, is composed of two subunits, the light and heavy chains (approximately 50 and approximately 100 kDa, respectively). The neurotoxin's mode of action is believed to depend on coordinated but independent actions of the two subunit chains. The molecular environments of the aromatic amino acid residues of the dichain neurotoxin and the two isolated subunit chains were analyzed using near-ultraviolet circular dichroism (CD) (between 250 and 320 nm) and second-derivative ultraviolet absorption spectroscopy (between 240 and 320 nm) to investigate the conformational variations of the subunit chains in separated and conjugated forms. The mean residue weight ellipticities showed virtually no change (i.e., 1.7%) in the vicinities of Phe (268 nm), and only a small change (11%) around Tyr (279 nm) residues following dissociation of the subunit chains. However, significant changes (23-26%) at 286 nm as well as at 292 nm were noted, suggesting considerable alteration in the conformation of the subunits. Second-derivative ultraviolet absorption spectra indicated the degree of Tyr exposure in the dichain neurotoxin, isolated heavy and light chains at 70.7, 81.5 and 46.4%, respectively. A weighted mean of the degree of exposed Tyr residues in the separated heavy and light chains was 69.6%, virtually same as the 70.7% exposed Tyr residues observed in the intact dichain neurotoxin, indicating no difference in their Tyr exposure upon separation of the two chains. This was corroborated by the CD data which revealed only small changes in the CD signals of Tyr residues, and no alteration in those of the Phe residues following separation of the subunit chains. However, a change in the CD signal at 292 nm suggested that the conformations of Trp-containing segments of the two chains were significantly influenced upon their separation. The heavy and light chains of the neurotoxin therefore appear to exist as two semi-independent domains, in spite of being linked by disulfide and noncovalent bonds, and at least part of their conformations depends on interactions between them.

Effect of tetranitromethane on the biological activities of botulinum neurotoxin types A, B and E

Botulinum neurotoxin serotypes A, B and E were modified at pH 7.9 with tetranitromethane, a reagent highly specific for tyrosine residues. The type B and E neurotoxins were completely detoxified without significant damage to their serological activities. Under similar modification conditions, the type A neurotoxin was incompletely detoxified with some alteration in its serological reactivity. Modification of only tyrosine residues to nitrotyrosine was evident from amino acid analysis of the acid hydrolysates of the modified proteins. The completely detoxified type B and E neurotoxins, used as toxoid, elicited antibodies in rabbits. The antisera precipitated and neutralized the homologous neurotoxin. The two toxoids, type B and E, were prepared with greater than 99% pure neurotoxins as tested by sodium dodecyl sulfate-polyacrylamide gel electrophoresis whereas the traditional toxoids produced with formaldehyde are very crude preparations of the neurotoxin (approximately 90% impure). Chemical modification using tetranitromethane is more specific than products that form during approximately 7 days of reaction between a protein and formaldehyde. The toxoids produced with tetranitromethane may be considered second-generation toxoids, compared with the first-generation toxoids (crude preparation of neurotoxins detoxified with formaldehyde).

Modification of carboxyl groups in botulinum neurotoxin types A and E

Effects of chemical modification of carboxyl groups of botulinum neurotoxin serotypes A and E were studied by using a water soluble carbodiimide-nucleophile reaction that is highly specific for modifying carboxyl groups of proteins. In both types A and E, increasing levels of the reagents, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and norleucine methyl ester or glycine methyl ester, at pH 4.8 caused increased loss of toxicity. More glycine could be incorporated than norleucine. Amino acid analysis did not reveal modification of any amino acid residue other than carboxyl groups (possible reaction of sulfhydryl groups was not studied). Loss of one carboxyl group did not severely affect toxicity, but modification of three carboxyl groups caused greater than 95% detoxification in both types. Complete detoxification could not be achieved with any amount of the reagents. Modification of three to five carboxyl groups did not affect serological activity.

Botulinum neurotoxin type A: cleavage of the heavy chain into two halves and their partial sequences

The 145-kDa type A botulinum neurotoxin (NT) is produced by the bacteria Clostridium botulinum (strain, Hall). The heavy (H) and light (L) chains (97- and 53-kDa, respectively) of this protein are linked by at least one disulfide bond. The N- and C-terminal halves of the H chain appear to have different functions in the mechanism of action of the NT [1987) FEBS Lett. 226, 115-120). Well-characterized and highly purified preparations of the two halves of the H chain are needed for such studies. Two different approaches were taken to cut the H chain with trypsin and isolate the fragments. In one method the cleavage products were: (i) 94-kDa fragment made of the L chain linked to the N-terminal half of the H chain (49 kDa) by a disulfide bond(s), and (ii) the C-terminal 44-kDa fragment. The N-terminal half of H chain was separated from the L chain by reducing the disulfide bond(s) linking them and then purified by ion-exchange chromatography. The 1-27 residues of 49-kDa N-terminal half of the H chain were Ala-Leu-Asn-Asp-Leu-Cys-Ile-Lys-Val-Asn-Asn-Trp-Asp-Leu-Phe-Phe-Ser-Pro- Ser-Glu - Asp-Asn-Phe-Thr-Asn-Asp-Leu-. The sequence of the other half of the H chain (44 kDa) was X-Ile-Ile-Asn-Leu-X-Ile-Leu-Asn-Leu-Arg-Tyr-Glu-X-Asn-His-Leu-Ile-Asp-Le u-Lys- X-Tyr-Ala-Ser-. In the second method, the H chain was first separated from the L chain, purified, and then cleaved. One product of cleavage, the 44-kDa fragment, was partially sequenced; the first 25 residues were identical to the sequence of the 44-kDa fragment generated by the first method. The present work also demonstrated that (i) The cysteine residue(s) located on the N-terminal half of the H chain form the -S-S- link(s) with the L chain. (ii) The other half of the H chain (44-kDa fragment, apparently the C-terminal half) is not linked via -S-S- to the L-chain or to the N-terminal half (49-kDa fragment) of the H chain.(ABSTRACT TRUNCATED AT 400 WORDS)

Circular dichroic and fluorescence spectroscopic study of the conformation of botulinum neurotoxin types A and E

Botulinum neurotoxin (NT) is synthesized by Clostridium botulinum in any of seven antigenically distinct forms, called types A through G. Protease(s) endogenous to the bacteria, or trypsin, nicks the single chain protein to a dichain molecule which generally is more toxic. The conformation of dichain type A (nicked by endogenous protease), single chain type E, and dichain type E NT (nicked by trypsin) have been determined using circular dichroism (CD) and fluorescence spectroscopy. The high degree of ordered secondary structure (alpha helix 28%, beta sheet 42%, total 70%) found in type A NT at pH 6.0 was similar to that found at pH 9.0 (alpha 22%, beta 47%, total 69%). The secondary structure of the single chain type E NT at pH 6.0 (alpha 18%, beta 37%, total 55%) differed somewhat from these values at pH 9.0 (alpha 22%, beta 43%, total 65%). The dichain type E NT at pH 6.0 assumed a secondary structure (alpha 20%, beta 47%, total 67%) more similar to that of dichain type A than the single chain type E NT. Examination with the fluorogenic probe toluidine napthalene sulfonate revealed that the hydrophobicity of the type A and E NTs were higher at pH 9.0 than at pH 6.0. Also, the hydrophobicity of the dichain type E NT was higher than its precursor the single chain protein and appeared similar to that of the dichain type A NT. The CD and fluorescence studies indicate that conversion of the single chain type E NT to the dichain form (i.e. nicking by trypsin) induced changes in conformation.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that subunits of type A botulinum toxin need not be linked by disulfide

Type A neurotoxin of Clostridium botulinum strain 62A was purified by a modification of the procedure of TSE et al. (1982). Electrophoresis in sodium dodecyl sulfate - polyacrylamide gels (SDS - PAGE) indicated the mol. wt of the intact dichain molecule is 140,000 and that of its L subunit is 52,000, both expected from published values. However the mol. wt of 83,000 for the H subunit was lower than the mol. wt of 97,000 in the literature. The purified toxin separated in SDS-PAGE into H and L subunits when pretreated with 2-mercaptoethanol but it unexpectedly behaved similarly without the pretreatment. Specific toxicity (approximately 3 x 10(8) mouse LD50/mg protein) was not affected by the spontaneous molecular change that made dissociation into subunits possible. The subunits of dichain botulinum toxins are believed to be covalently joined by intersubunit disulfide(s) since they have been demonstrated only when samples are treated with 2-mercaptoethanol or dithiothreitol. Since it is not always needed, the pretreatment is apparently not reducing a disulfide that connects the subunits. The strong chelating activity also possessed by the pretreating agents suggest that the subunits may be joined by a metallic divalent cation.

Effect of diethylpyrocarbonate on the biological activities of botulinum neurotoxin types A and E

The single chain (unnicked) type-E and the dichain (nicked) type-A botulinum neurotoxins were modified with diethylpyrocarbonate (ethoxyformic anhydride), a reagent highly specific for histidine residues. The type-E neurotoxin could be completely detoxified without causing detectable damage to its serological reactivity. Under identical modification reaction conditions, the type A was incompletely detoxified with some alteration in its serological reactivity. Modification of histidine residues was evident from the increase in absorbance at 240 nm, and reactivation of the detoxified proteins by reversing the modification with hydroxylamine. The completely detoxified type-E neurotoxin, used as toxoid, elicited antibodies in rabbits. The antiserum precipitated and neutralized the neurotoxin. This toxoid is homogeneous as tested by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, whereas the traditional toxoid produced with formaldehyde is heterogeneous.

Purification and amino acid composition of type A botulinum neurotoxin

A method to purify type A botulinum neurotoxin from a 64 liter bacterial culture is reported. The procedure includes cation exchange chromatography at pH 7.0. The final product, essentially homogeneous (according to polyacrylamide gel-sodium dodecylsulfate electrophoresis), is a mixture of two forms of the neurotoxin (mol. wt 145,000); the dichain or nicked form (over 95%) and its precursor the single chain or unnicked form. Two batches of the neurotoxin purified by the method described here and one batch purified according to the method of Sugii and Sakaguchi were similar in purity and amino acid composition. The best estimate of the number of amino acid residues per neurotoxin molecule (mol. wt 145,000) is: Asp200Thr75Ser79Glu114Pro44Gly64Ala53Val70CyS10Met22Ile111Leu104Tyr71 Phe68Lys100His14Arg43Trp17.

Nature of intracellular type A botulinum neurotoxin

The neurotoxin in cells of young Clostridium botulinum type A culture was extracted with lysozyme. Highly purified neurotoxin preparation, obtained by processing the extract in two chromatographic steps had only unnicked (single-chain) molecules of molecular weight comparable to that of the dichains isolated from type A crystals. Trypsinization converted the unnicked molecules into dichains whose component subunits were of sizes indistinguishable from those of the neurotoxin from crystals. The enzymatic treatment increased toxicity of crude extract 30-fold but did not activate the purified intracellular neurotoxin preparation. The results indicated that intracellular type A botulinum neurotoxin is unnicked, is not fully activated, and is activated in the time between its extraction and purification. Since trypsinization nicked all of the single chains without increasing toxicity, nicking was not causally related to toxicity activation.

Affinity chromatography purification of type A botulinum neurotoxin from crystalline toxic complex

Type A botulinum neurotoxin was purified from toxic crystals by adsorption to p-aminophenyl-beta-D-thiogalactopyranoside coupled to CH-Sepharose 4B. At pH 6.3, the toxic complex was held by the binding between the ligand and the hemagglutinin of the complex; the toxin is eluted selectively by dissociating the complex with buffer-saline of pH 7.9. The single-step affinity chromatography recovered 50 to 60% of applied toxicity as preparations of greater than 99% purity.

Purification and properties of Clostridium botulinum type F toxin

Clostridium botulinum type F toxin of proteolytic Langeland strain was purified. Toxin in whole cultures was precipitated with (NH4)2SO4. Extract of the precipitate was successively chromatographed on diethylaminoethyl-cellulose at pH 6,0, O-(carboxymethyl) cellulose at pH 4.9, and finally diethylaminoethyl-cellulose at pH 8.1. The procedure recovered 14 percent of the toxin assayed in the starting culture. The toxin was homogeneous by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, double gel diffusion serology, and isoelectric focusing. Purified toxin had a molecular weight of 150,000 by gel filtration and 155,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Specific toxicity was 9.6 X 10-6 mean lethal doses per absorbancy (278 nm) unit. Sub-units of 105,000 and 56,000 molecular weight are found when purified toxin is treated with a disulfide reducing agent and electrophoresed on sodium dodecyl sulfate-polyacrylamide gels. Reciprocal cross neutralizations were demonstrated when purified type F and E toxins were reacted with antitoxins which were obtained with immunizing toxoids prepared with purified toxins.

Evaluation of type A botulinal toxin assays that use antitoxin to crystalline toxin

The type A botulinal toxin assay by the reverse passive hemagglutination procedure which uses antitoxin to crystalline toxin was examined for specificity. The analysis was based on the fact that crystalline type A toxin is a complex of neurotoxic protein (Aalpha) and a nontoxic protein (Abeta). By using these components, obtained in essentially pure forms, it was shown that the antitoxin to crystalline toxin has a significantly higher titer to Abeta than to Aalpha. When Formalin-treated red blood cells were sensitized with this antitoxin, the antibodies coupled to the cells were, for practical results, only anti-Abeta. When the suspension is reacted with dilutions of type A toxic solutions, the limiting dilutions are determined by Abeta and not by the neurotoxin, which should be the determinant if the assay is to measure toxicity. These observations may be pertinent to the development of serological assays for other botulinal toxin types.