Primary structure of alpha-bungarotoxin. Six amino acid residues differ from the previously reported sequence

Alpha neurotoxins

α-Neurotoxins have been isolated from hydrophid, elapid and, more recently, colubrid snake venoms. Also referred to as postsynaptic neurotoxins or 'curare mimetic' neurotoxins, they play an important role in the capture and/or killing of prey by binding to the nicotinic acetylcholine receptor on the skeletal muscle disrupting neurotransmission. They are also thought to cause respiratory paralysis in envenomed humans. This review will discuss the historical background into the discovery, isolation, structure and mechanism of action of the α-neurotoxins, including targets and cellular outcomes, and then will examine the potential uses of α-neurotoxins as pharmacological tools and/or as drug leads.

Identification of alpha-bungarotoxin (A31) as the major postsynaptic neurotoxin, and complete nucleotide identity of a genomic DNA of Bungarus candidus from Java with exons of the Bungarus multicinctus alpha-bungarotoxin (A31) gene

The Malayan krait (Bungarus candidus) is one of the most medically significant snake species in Southeast Asia. No specific antivenom exists to treat envenoming by this species. Death within 30 min after its bite has been reported from Java, suggesting the presence of highly lethal postsynaptic neurotoxins in the venom of these snakes. We purified and identified the major postsynaptic toxin in the venom of B. candidus from Java. The toxin was indistinguishable from alpha-bungarotoxin (A31), a toxin originally isolated from Bungarus multicinctus, in its mass (7983.75 Da), LD50 (0.23 microg/g in mice i.p.), affinity to nicotinic acetylcholine receptors, and by its 40 N-terminal amino acid residues as determined by Edman degradation. Identity with alpha-bungarotoxin was confirmed by cloning and sequencing a genomic DNA from B. candidus which encodes the 74 amino acid sequence of alpha-bungarotoxin (A31) and part of its signal peptide, revealing complete identity to the alpha-bungarotoxin (A31) gene in exon and 98.9% identity in intron sequences. The entire mitochondrial cytochrome b gene of the krait species B. candidus from Java and B. multicinctus from Taiwan was sequenced for comparison, suggesting that these snakes are phylogenetically closely related. alpha-Bungarotoxin appears to be widely present and conserved in Southeast and East Asian black-and-white kraits across populations and taxa.

Primary structure of gamma-bungarotoxin, a new postsynaptic neurotoxin from venom of Bungarus multicinctus

The primary structure of gamma-bungarotoxin, a new toxin from Bungarus multicinctus venom, was determined using mass spectrometry and Edman degradation. The toxin has a mass of 7524.7 D and consists of 68 residues having the following sequence: MQCKTCSFYT CPNSETCPDG KNICVKRSWT AVRGDGPKRE IRRECAATCP PSKLGLTVFC CTTDNCNH. Gamma-bungarotoxin is structurally similar to both kappa-bungarotoxin and elapid long postsynaptic neurotoxins. Its C-terminal nine residues are identical to those of the kappa-toxins. Its disulfide bond locations appear identical to those of several elapid toxins of unknown pharmacology and its hydrophobicity profile is also strikingly similar. However, with an LD50 of 0.15 microg/g i.v. in mice, gamma-bungarotoxin is 30-150-fold more toxic than other members of this latter class. Its toxicity is comparable to those of alpha-nicotinic acetylcholine receptor antagonists.

Disulfide isomers of alpha-neurotoxins from King cobra (Ophiophagus hannah) venom

Two novel alpha-neurotoxins, Oh-6A and Oh-6B, isolated from the king cobra (Ophiophagus hannah) venom, consist of 70 amino acid residues with 10 cysteine residues and share the same amino acid sequences as determined by Edman degradation on the peptide fragments generated from the proteolytic hydrolysates. Their sequences share 46-53% homology with Oh-4, Oh-5, Toxin a, and Toxin b from the same venom. The finding that Oh-6A and Oh-6B had different retention times in the reversed-phase column suggested that the two toxin molecules should not have the same conformation. Selective reduction on the disulfide bond, Cys26--Cys30, at the tip of their loop II structures resulted in the production of the partially reduced derivatives eluted at the same position. Under redox conditions, the partially reduced Oh-6A and 6B exclusively converted into native Oh-6A as evidenced by HPLC analyses. This suggests that Oh-6A and Oh-6B are disulfide isomers which probably arise from cis-trans isomerization of the Cys26--Cys30 disulfide bond. Alternatively, the two toxins exhibited binding activity toward nAChR and lethal toxicity equally. It reflects that the diversity around the extra loop at the loop II structure does not exert a significant effect on the manifestation of the neurotoxicity of Oh-6A and Oh-6B.

Studies on the status of amino groups in alpha-bungarotoxin

The positive charges of amino groups in alpha-bungarotoxin (alpha-BuTX) were neutralized by reaction with trinitrobenzene sulfonate (TNBS) and were converted into negative charges with 4-chloro-3,5-dinitrobenzoate (CDNB). Three derivatives monotrinitrophenylated (TNP-) at Lys-38, 64, or 70; three di-TNP at Lys-38 and 64, Lys-38 and 70, and Lys-64 and 70; one tri-TNP at Lys-38, 64 and 70; and one penta-TNP at Lys-38, 51, 52, 64 and 70 as well as one mono-carboxydinitrophenylated (CDNP-) at Lys-38; di-CDNP at Lys-38 and 70, and tri-CDNP at Lys-38, 64 and 70 were isolated, respectively. The epsilon-amino groups at Lys-38, 64 and 70 are the most accessible to trinitrophenylation, Lys-51 and 52 are less reactive, while the N-terminus and Lys-26 are the least reactive. Each mono-TNP and CDNP derivative showed approximately 50% residual binding activity to nicotinic acetylcholine receptor (AChR)-rich membranes isolated from Torpedo californica and 50% of the lethality of alpha-BuTX. However, the activities were progressively lost as the accumulative modifications proceeded, and led finally to the formation of almost inactive penta-TNP derivative. The antigenicity of alpha-BuTX was still retained essentially intact after one or two amino groups of lysine residues were modified, whereas tri-TNP and CDNP-derivatives modified at Lys-38, 64 and 70 lost 46 and 70% of their antigenicity, respectively. Pronounced alteration in antigenicity was observed after five amino groups were trinitrophenylated. The present study indicates that the amino groups in alpha-BuTX may participate in the multipoint contact between the toxin and AChR, but none of the individual amino groups is definitely essential for the binding.

The role of an invariant tryptophan residue in alpha-bungarotoxin and cobrotoxin. Investigation of active derivatives with the invariant tryptophan replaced by kynurenine

Ozone oxidation converted the single, invariant, tryptophan residue to N2-formylkynurenine in alpha-bungarotoxin and cobrotoxin. Upon this modification, the lethal toxicity was significantly reduced in cobrotoxin but mostly retained in alpha-bungarotoxin. Each neurotoxin containing kynurenine instead of tryptophan retained the same antigenicity as the native toxin. Fluorescence and CD spectroscopy revealed that, although the environment and state of the kynurenine residue were similar, [Kyn29]cobrotoxin was much more sensitive to pH change than alpha-[Kyn28]bungarotoxin. In terms of lethal toxicity and conformational stability, the invariant tryptophan residue appears to play a more important role in cobrotoxin, imparting a higher lethal toxicity than that in alpha-bungarotoxin, which has a disulfide bond at Cys29-Cys33.

Neutralizing monoclonal antibody specific for alpha-bungarotoxin: preparation and characterization of the antibody, and localization of antigenic region of alpha-bungarotoxin

We prepared an alpha-bungarotoxin-specific monoclonal antibody that neutralizes the biological activity of the toxin in vivo. The antigenic determinant combining specifically with this antibody was determined on the basis of cross-reaction experiments using three other long neurotoxins and peptide fragments of alpha-bungarotoxin. The antigenic determinant was located on the peptide fragment containing S34-S35-R36-G37-K38, which forms a part of the expected site that binds to the acetylcholine receptor proteins.

Preparation and characterization of monoclonal antibody specific for alpha-bungarotoxin and localization of the epitope

We prepared a monoclonal antibody (mAb) specific for alpha-bungarotoxin (alpha-BuTX) which can neutralize the lethal toxicity of the toxin and inhibit the binding of [3H]-alpha-BuTX to the nicotinic acetylcholine receptor. The radiolabelled toxin has a high affinity for the mAb. alpha-BuTX was digested with acid protease A and the resulting peptide fragments were isolated by reverse-phase HPLC. The epitope recognized by the mAb has been localized on the basis of competition radioimmunoassay between [3H]-alpha-BuTX and the peptide fragments of alpha-BuTX towards the antibody. The epitope specific for the mAb may be located in the second loop of alpha-BuTX, probably involving residues 34-41.

Isolation of antigenically reactive peptide fragments and localization of antigenic regions of alpha-bungarotoxin

alpha-Bungarotoxin was digested with acid protease A and the resulting peptide fragments were isolated by reverse-phase HPLC. The antigenic activity of the peptide fragments were examined by competitive solid phase radioimmunoassay and the probably sequences were assigned. The two antigenic regions are likely located around the residues 34-41 and 68-74 of alpha-bungarotoxin, and the other two may be within the residues 3-26 and 45-67, respectively. Radioimmunoassay revealed a maximum number of four antigenic determinants of alpha-bungarotoxin that can simultaneously be occupied by antibody.