Identification of a locality in snake venom alpha-neurotoxins with a significant compositional similarity to marine snail alpha-conotoxins: implications for evolution and structure/activity

Biochemistry of envenomation

Venoms and toxins are of significant interest due to their ability to cause a wide range of pathophysiological conditions that can potentially result in death. Despite their wide distribution among plants and animals, the biochemical pathways associated with these pathogenic agents remain largely unexplored. Impoverished and underdeveloped regions appear especially susceptible to increased incidence and severity due to poor socioeconomic conditions and lack of appropriate medical treatment infrastructure. To facilitate better management and treatment of envenomation victims, it is essential that the biochemical mechanisms of their action be elucidated. This review aims to characterize downstream envenomation mechanisms by addressing the major neuro-, cardio-, and hemotoxins as well as ion-channel toxins. Because of their use in folk and traditional medicine, the biochemistry behind venom therapy and possible implications on conventional medicine will also be addressed.

Neuromuscular effects of candoxin, a novel toxin from the venom of the Malayan krait (Bungarus candidus)

1 Candoxin (MW 7334.6), a novel toxin isolated from the venom of the Malayan krait Bungarus candidus, belongs to the poorly characterized subfamily of nonconventional three-finger toxins present in Elapid venoms. The current study details the pharmacological effects of candoxin at the neuromuscular junction. 2 Candoxin produces a novel pattern of neuromuscular blockade in isolated nerve-muscle preparations and the tibialis anterior muscle of anaesthetized rats. In contrast to the virtually irreversible postsynaptic neuromuscular blockade produced by curaremimetic alpha-neurotoxins, the neuromuscular blockade produced by candoxin was rapidly and completely reversed by washing or by the addition of the anticholinesterase neostigmine. 3 Candoxin also produced significant train-of-four fade during the onset of and recovery from neuromuscular blockade, both, in vitro and in vivo. The fade phenomenon has been attributed to a blockade of putative presynaptic nicotinic acetylcholine receptors (nAChRs) that mediate a positive feedback mechanism and maintain adequate transmitter release during rapid repetitive stimulation. In this respect, candoxin closely resembles the neuromuscular blocking effects of d-tubocurarine, and differs markedly from curaremimetic alpha-neurotoxins that produce little or no fade. 4 Electrophysiological experiments confirmed that candoxin produced a readily reversible blockade (IC(50) approximately 10 nM) of oocyte-expressed muscle (alphabetagammadelta) nAChRs. Like alpha-conotoxin MI, well known for its preferential binding to the alpha/delta interface of the muscle (alphabetagammadelta) nAChR, candoxin also demonstrated a biphasic concentration-response inhibition curve with a high- (IC(50) approximately 2.2 nM) and a low- (IC(50) approximately 98 nM) affinity component, suggesting that it may exhibit differential affinities for the two binding sites on the muscle (alphabetagammadelta) receptor. In contrast, curaremimetic alpha-neurotoxins have been reported to antagonize both binding sites with equal affinity.

Snake venom alpha-neurotoxins and other 'three-finger' proteins

The review is mainly devoted to snake venom alpha-neurotoxins which target different muscle-type and neuronal nicotinic acetylcholine receptors. The primary and spatial structures of other snake venom proteins as well as mammalian proteins of the Ly-6 family, which structurally resemble the 'three-finger' snake proteins, are also briefly discussed. The main emphasis is placed on recent data characterizing the alpha-neurotoxin interactions with nicotinic acetylcholine receptors.

Mass spectrometric investigations on Conus peptides

Molecular masses and primary structure determination of Conus peptides, such as alpha-, mu- and omega-conotoxins, conantokins and conopressins, were accurately measured by state-of-the-art mass spectrometric techniques using only 1-2 pmole quantities. Soft ionization of Conus peptides under electrospray, matrix-assisted laser desorption and continuous flow frit-FAB conditions produced their corresponding singly and multiply charged molecular ions which can be detected by mass spectrometric analysis. The molecular masses of Conus peptides were obtained by the deconvolution of the multiply charged pseudo-molecular ions. Mixture analysis without chromatographic separation can be accomplished by this approach. The ions formed during collision-induced dissociation of either singly or multiply charged ions of any reduced and derivatized peptide provided the corresponding sequences of the amino acids. Preliminary investigations indicate that the developed techniques and procedures could be applied in order to characterize the peptides present in unknown Conus venoms from the Bay of Bengal region.

Acetylcholine receptor binding characteristics of snake and cone snail venom postsynaptic neurotoxins: further studies with a non-radioactive assay

The binding of postsynaptic neurotoxins from snake and marine cone snail (Conus sp.) venoms to nicotinic acetylcholine receptor (AchR) was investigated with an ELISA-based, non-radioactive assay. Three snake postsynaptic toxins from the long-chain group (Naja naja kaouthia cobratoxin, Naja oxiana neurotoxin I, Bungarus multicinctus alpha-bungarotoxin) and short-chain group (Naja naja atra cobrotoxin, Naja oxiana neurotoxin II, and Laticauda semifasciata erabutoxin b) were studied. Both types of snake postsynaptic toxins showed a dose-response with constant AchR (50 micrograms/ml) and varying toxin concentrations (50-0.035 micrograms/ml). The minimum detection limits of the assay for snake toxins ranged from 310 to 1240 ng/ml (40-160 pmole/ml), depending on the toxin. Unlike any of the short-chain toxins, long-chain toxins consistently bound less receptor and reached maximum absorbance levels with toxin concentrations of 10-50 micrograms/ml. Competition for AchR binding between cone snail postsynaptic neurotoxins (conotoxins GI, MI, SI) and alpha-bungarotoxin or cobrotoxin resulted in a dose-response. The postsynaptic conotoxins were uniformly better competitors for AchR binding with alpha-bungarotoxin than with cobrotoxin. Heat stability studies with neurotoxin I, erabutoxin b, or cobrotoxin revealed a loss in AchR binding activity with increasing temperature. alpha-Bungarotoxin heated at 90 degrees C had increased AchR binding activity by 105%, relative to 25 degrees C samples, but lost the majority of its binding activity after 100 degrees C. The enhanced binding of heated alpha-bungarotoxin to AchR was specific, as evidenced by a competitive dose-response with unheated alpha-bungarotoxin, but heated toxin lacked any biological activity in the mouse lethal assay. When conotoxins GI or MI were heated at 100 degrees C, there was no detectable loss in AchR binding activity, and only a slight decrease in mouse lethality.

Mambin, a potent glycoprotein IIb-IIIa antagonist and platelet aggregation inhibitor structurally related to the short neurotoxins

The purification, complete amino acid sequence, functional activity, and structural modeling are described for mambin, a platelet glycoprotein GP IIb-IIIa antagonist and potent inhibitor of platelet aggregation from the venom of the Elapidae snake Dendroaspis jamesonii (Jameson's mamba). Mambin is 59 residues in length and contains four disulfide linkages and an RGD amino acid sequence found in protein ligands that bind to GP IIb-IIIa. Mambin inhibits ADP-induced platelet aggregation (IC50 = 172 +/- 22 nM) and inhibits the binding of purified platelet fibrinogen receptor GP IIb-IIIa to immobilized fibrinogen (IC50 = 3.1 +/- 0.8 nM). Mambin has very little sequence similarity to the Viperidae family of platelet aggregation inhibitors, except for the RGD-containing region in the protein. However, mambin does have ca. 47% similarity to the short-chain postsynaptic neurotoxins found in other Elapidae venoms, which do not contain the RGD sequence and do not act as GP IIb-IIIa antagonists. On the basis of its circular dichroism spectrum, mambin has a beta-sheet structure characteristic of the neurotoxins. Molecular modeling of the mambin sequence onto the erabutoxin b structure predicts a very similar structure within the entire protein except for the loop containing the RGD sequence. Mambin may therefore represent a genetic hybrid of neurotoxic and hemotoxic proteins found in snake venoms.

Immunoreactivity, epitope mapping and protection studies with anti-conotoxin GI sera and various conotoxins

Goat or rabbit anti-conotoxin GI sera recognized native or dithiothreitol-reduced alpha-conotoxins GI, MI and SI in an enzyme-linked immunosorbent assay (ELISA). Native mu-conotoxin GIIIA or omega-conotoxin GVIA did not react with either anti-conotoxin GI serum in the assay. The goat anti-conotoxin GI serum neutralized 2.5 LD50S of alpha-conotoxins GI or MI in mouse lethal assays, while the rabbit antiserum had little protective capabilities. Epitope mapping of synthesized conotoxin peptide fragments revealed that both anti-conotoxin GI sera recognized linear sequences from five different alpha-conotoxins: GI, GIA, GII, MI and SI. The CCNPAC sequence was optimally recognized by both antisera.