A-Conotoxins

Acetylcholine nicotinic (LGIC) Na+, K+ and Ca2 + conductance Nicotine, suxamethonium Tubocurarine, a-conotoxins, a-bungarotoxin... 

In the venom of C. geographus and other fish-hunting species, the conotoxins isolated so far can be divided into three major classes (1-4) o -conotoxms which block neuronal calcium channels at the presynaptic terminus of the neuromuscular junction, a-conotoxins which inhibit the acetylcholine receptor at the postsynaptic terminus, and x-conotoxins which block Na channels on the muscle membrane. 

Between 6 and 10 homologous peptides have been extensively characterized for each toxin class. Although uj- and a-conotoxins have been isolated from several fish-hunting Conus species, x-conotoxins have so far been isolated only from C. geographus venom. 

Peptides in the a-conotoxin family are inhibitors of nicotinic acetylcholine receptors. They were first isolated from C. geographus venom as components which cause paralysis in mice and fish when injected intraperitoneally (27). Early physiological experiments (28) indicated that a-conotoxins GI, GII, and GIA (see Table III) all act at the muscle end plate region. Mini end-plate potentials and end plate potentials evoked in response to nerve stimulation are inhibited in the presence of a-conotoxins in the nM to pM range. a-Conotoxin GI was subsequently shown to compete with rf-tubocurarine and a-bungarotoxin for the acetylcholine receptor (29). 

Like the other paralytic toxins from Conus venom, a-conotoxins are small and very tightly folded, structural features which may be advantageous for rapid paralysis of prey (1). a-Conotoxins are typically 13 to 15 amino acids long with two disulfide bridges (see Table III). In addition to the five a-conotoxins shown, two new a-conotoxins (SIA and SIB) from C. striatus have recently been isolated, sequenced, and chemically synthesized. SIA is very unusual because it is 19 amino acid residues long and it contains 6 cysteine residues, three of which are contiguous near the amino terminus (C. Ramilo et al., unpublished results). 

Radioiodinated derivatives have been prepared to define more closely the target site of a-conotoxins on the acetylcholine receptor (R. Myers, unpublished data). In membrane preparations from Torpedo electroplax, photoactivatable azidosalicylate derivatives of a-conotoxin GIA preferentially label the p and 7 subunits of the acetylcholine receptor. However, when the photoactivatable derivative is cross-linked to detergent solubilized acetylcholine receptor (AChR), only the 7 subunit is labeled. Since snake a-neurotoxins mainly bind to the a subunits of AChR and a-conotoxins compete directly with a-bungarotoxin, the cross-linking results above are both intriguing and problematic. 

The a -, /z-, and a-conotoxins are the best characterized of the peptides isolated from Conus venoms so far. However, a large number of other peptides are found in these venoms. These comprise both paralytic toxins to immobilize the prey of the cone snail, and other biologically active peptides which are not themselves directly paralytic. Only the briefest overview of these peptide components will be presented here. 

The cysteine motifs in this polydnavirus gene family are structurally analogous to the cysteine motifs of the >-conotoxins. The a>-conotoxins are high-affinity ligands with different receptor specificity for voltage-sensitive ion channels (64, 65). The mature w-conotoxins have three... 

Fig. 3 Potential subunit compositions of nAChRs expressed in dopaminergic nerve terminals. A combination of ligand binding ([ H]-epibatidine and [ I]-a-conotoxin Mil), immunoprecipita-tion, and dopamine release data have led to the conclusion that rodent brain expresses a minimum of five different nAChR subtypes. Three of these (the two forms of a4p2 and a4aSP2) do not bind a-conotoxin Mil with high affinity (a-conotoxin Mll-resistant). The three a6-containing subtypes bind a-conotoxin Mil with high affinity (conotoxin Mil-sensitive). In general, the conotoxin-sensitive nAChR subtypes are activated by lower concentrations of agonist than are required to activate the a-conotoxin Mll-resistant subtypes (Salminen et al. 2007)...

Salminen O, Whiteaker P, Grady SR, CoUins AC, McIntosh JM, Marks MJ (2005) The subunit composition and pharmacology of a-conotoxin Mll-binding nicotinic acetylcholine receptors studied by a novel membrane-binding assay. Neuropharmacology 48 696-705... 

Salminen O, Drapeau J, McIntosh JM, CoUins AC, Marks MJ, Grady SR (2007) Pharmacology of a-conotoxin Mil-sensitive subtypes of nicotinic acetylcholine receptors isolated by breeding of null mutant mice. Mol Pharmacol 71 1563-1571... 

Xen-2174 (220) (isolated from the snail Conus marmoreus) 13-Amino-acid peptide (a conotoxin) Xen-2174 (220) Neurology (Pain) Inhibits norepinephrine transporter (NET) Phase II (against acute postoperative pain and chronic pain in cancer patient) Xenome 970-973... 

An interesting approach based entirely on the solid-phase route has been reported for the synthesis of a-conotoxin SI (Scheme 14) where a combination of acid-labile and acid-stable thiol protecting groups are employed for the required orthogonality.[107]... 

Of this family of peptides containing two intramolecular disulfide bonds the most studied in terms of oxidative refolding are a-conotoxins with two adjacent cysteine residues, i.e. with m = 0, the bee venom toxins, for example apamin and mast cell degranulating peptide, and snake venom toxins, exemplified by sarafotoxins, and endothelins, mammalian peptide hormones with the characteristic Cys-(Xaa)rCys/Cys-(Xaa)3-Cys motif (Scheme 2). With m = 0 or 1 all these peptides are expected to show a weak tendency to form the isomer 3 with a disulfide bond between two proximal cysteine residues. This was fully confirmed by oxidative refolding experiments. 

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