Sea Anemone Toxins

Jiri Patocka a and Anna Strunecka b

a Department of Toxicology, Military Medical Academy, 500 01 Hradec Kralove;
b Department of Animal Physiology and Developmental Biology, Faculty of Sciences, Charles University, 128 00 Prague, Czech Republic. E-mail: patocka@pmfhk.cz, strun@prfdec.natur.cuni.cz.

Introduction

Toxins are non-replicating chemical agents of biological origin. The toxicity of many toxins is very high, often comparable with chemical warfare agents. Some toxins are among the most toxic substances known [1] and may be misused for military or terrorist purposes [2]. Roughly speaking, toxins can be divided according to their pharmacological effects: neurotoxins, myotoxins, hepatotoxins, nephrotoxins, hematological toxins and locally acting toxins. Toxins also can be divided according to their sources, such as snakes, scorpions, mussels, spiders, marine creatures, sea anemones, etc. This article describes the structure and function of a common, easily available toxin.

Sea anemone toxins

Sea anemones (Actiniaria) are solitary, ocean dwelling member of the phylum Cnidaria and the class Anthozoa. These carnivorous animals are common along all of the sea shores of the world. Most sea anemones adhere by their bases to hard substrates, such as rocks, corals, other animals or ship bottoms. They have tentacles that surround a central mouth opening and these are used to catch and transfer food (mollusks, crustaceans, small fish) to their mouth. "Burning cells", on the edges of tentacles, loose specific toxins [3].

Toxins of sea anemones are peptides that contain 46-49 amino acid residues in a single polypeptide chain that is cross-linked by three disulfide bridges [4]. Shorter and longer polypeptide chains were also found in the venom of some species [5, 6]. At this time, more than forty toxic peptides have been isolated from different sea anemone species and their amino acid sequences and positions of disulfide bridges estimated. The chemical structure of a typical polypeptide toxin, anthopleurin A, isolated from the sea anemone Anthopleura xanthogrammica, is given in Fig. 1 [7].

Sea anemone toxins can also be characterized as neurotoxins [4]. The isolated toxins enhance the normal release of neurotransmitters because they can selectively block some neuronal sodium and potassium ion channels. The ion channels of excitable cells can be described as water-filled pores with a membrane controlled by electrosensitive gates. Normally, when the membrane potential is negative enough, most of the channels are closed. During depolarization they open within fractions of millisecond (activation) and then become non-conducting again (inactivation). These ion channels help control the excitability of nerve cells and aid in the regulation of neurotransmitter release. Ligand-gated ion channels are neurotransmitter receptors for both excitatory (acetylcholine, serotonin, glutamate) and inhibitory (GABA and glycine) transmitters. These receptors contain an intrinsic ion channel, which is opened by the binding of the neurotransmitter. Several subunits, encoded by individual genes, have been cloned for most of these. The distribution of receptor subunits varies in different brain regions and different neuronal cell types. The different subunit compositions of the receptors has provided the impetus to search for the structural determinants of various toxins’ action by comparing the sensitivities of different receptor subtypes with the toxins.

Sodium and potassium channels are the primary targets for a number of neurotoxins, each of which causes specific alterations in channel functions. Blocking of ion channels by sea anemone toxins leads to paralysis of neuronal transmission in skeletal muscles [8]. This produces among other things, heart arrhythmia and respiratory failure ending in cardiac failure. No specific therapy is known.

The toxicity of sea anemone toxins is high, not only for prey animals, but also for others, including many vertebrates. The lethal dose for a mouse, expressed by a LD50 value, ranges from 1 to 100 g/kg [9]. This is comparable with the most toxic organophosphate chemical warfare agents [10]. It is clear that these toxins have potential as warfare agents. Sea anemone toxins are more available than many other animal toxins, for example snake, scorpion or spider toxins. Their molecules are relative simple and may be prepared by solid phase peptide synthesis or more sophisticated biotechnologies. However, these toxins can also be used as guides leading to new, more effective therapies. Because the target of their action, neuronal sodium and potassium channels, is known very well [10, 11], it is possible to use structure-based drug design [12] and find those compounds that have good geometrical and chemical complementarity for the target structure.

Conclusions

Sea anemone toxins represent a group of polypeptides, containing 46-49 amino acid residues in a single polypeptide chain that is cross-linked by three disulfide bridges. The toxicity of sea anemone toxins is comparable with the most toxic organophosphate chemical warfare agents.

References

1. Franke, S. Lehrbuch der Militaerchemie. 2nd Ed., Militaerverlag er DDR, 1977.

2. Geissler, E. Biological and Toxin Weapons Today. SIPRI, Oxford University Press, Oxford 1986.

3. Patton. W.K. Sea Anemones. Academic American Encyclopedia, 1995 ed. Manoleras, N, 4. Norton R.S. Three-dimensional structure in solutions of neurotoxin III from the sea anemone Anemonia sulcata. Biochemistry 1994; 33: 11051-11061.

5. Schweitz, H., Bidard, J.N., Frelin, C., Pauron, D., Vijverberg, H.P., Mhasneh, D.M., Lazdunski , M., Vilbois, F., Tsugita, A. "Purification, sequence, and pharmacological properties of sea anemone toxins from Radianthus paumotensis. A new class of sea anemone toxins acting on the sodium channel." Biochemistry 1985; 24: 3554-3561.

6. Schweitz, H., Vincent, J.P., Barhanin, J., Frelin, C., Linden, G., Hugues, M., Lazdunski, M. "Purification and pharmacological properties of eight sea anemone toxins from Anemonia sulcata, Anthopleura xanthogrammica, Stoichactis giganteus, and Actinodendron plumosum." Biochemistry 1981; 20: 5245-5252.

7. Norton, T.R. "Cardiotonic polypeptides from Anthopleura xanthogrammica (Brandt) and elegantissima (Brandt)." Fed.Proc. 1981; 40: 21-25.

8. Rogers, J.C., Qu, Y., Tanada, T.N., Scheuer, T, Catterrall, W.A. Molecular determinants of high affinity binding of alpha-scorpion toxin and sea anemone toxin in the S3-S4 extracellular loop in domain IV of the Na+ channel alpha subunit. J. Biol. Chem. 1996; 271:15950-15962.

9. Norton, R.S. "Structure and structure-function relationships of sea anemone proteins that interact with the sodium channel." Toxicon 1991; 29: 1051-1084.

10. Munro N.B., Ambrose K.R., Watson A.P. "Toxicity of the organophosphate chemical warfare agents GA, GB, and VX: Implications for public protection. Environ. Health Perspect. 1994; 102: 18-38.

11. Catterall W.A. Molecular properties of voltage-sensitive sodium channels. Annu. Rev. Biochem. 1986; 55: 953-985.

12. Kuntz I.D., Roe D.C. What is structure based drug design? Pharmaceutical News 1995; 2: 13-15.

Editor’s Note: There are many aspects in developing a toxin weapon, not just toxicity. This article is worthwhile because it alerts us to a toxin that can be readily obtained and also is very useful in exploring the neuronal ion channels.

Figure 1

Fig. 1.

Chemical structure of Anthopleurin A, neurotoxin from Anthopleura xanthogrammica, polypeptide composed from 49 amino acid residues with three disulfide bridges between Cys 4 - Cys 46, Cys 6- Cys 36, and Cys 29 - Cys 47, respectively.

One letter code for description of amino acid residues was used in this figure: A = Alanine, R = Arginine, N = Asparagine, D = Aspartic acid, C = Cysteine, G = Glycine, H = Histidine, I = Isoleucine, L = Leucine, K = Lysine, P = Proline, S = Serine, T = Threonine, W = Tryptophan, Y = Tyrosine and V = Valine.

ASA 99-1, issue no. 70


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