Anatoxins

The basic mechanism of muscle contraction consists of the neurotransmitter acetylcholine propagating from the axon of a nerve cell to the ion channels in the wall of a muscle cell. The acetylcholine binds to the ion channel, opening it and allowing the migration of ions across the muscle cell wall, thus propagating the electrical signal for the cell to contract. It is then the job of the enzyme cholinesterase to deactivate the cholinesterase and allow the ion channel to re-close, relaxing the contraction.

Anatoxin-a is an alkaloid that acts as an acetylcholine mimic, and is thus able to bind to and open the ion channel. It cannot, however, be deactivated acetylcholinesterase, and so the ion channel stays open. The muscle cell continues to contract until it fails from exhaustion.

Anatoxin-a(s), the second type of anatoxin, only occurs in the species Anabaena flos-aquae, one of the most toxic strains of cyanobacteria. It is an acetylcholinesterase inhibitor which binds to the enzyme and renders it unable to deactivate the acetylcholine. Since the acetylcholine is not deactivated, the ion channel is left open, once again destroying muscle function through exhaustion. Anatoxin-a(s) is an organic phosphate, similar in its action to synthetic organophosphate pesticides such as parathion and malathion. Anatoxin-a(s) is the only natural organophosphate known. Since it is more water soluble than synthetic organophosphates, with consequently less tendency to bioaccumulate and stay in fat cells and cell membranes, it may yield a less nefarious pesticide than its synthetic brethren.

Anatoxins

The anatoxins are a group of low molecular weight neurotoxic alkaloids first described in the fresh-water cyanobacteria Anabaena flos-aquae from Canada (Gorham et al., 1964). Anatoxins have not yet been identified in Australian cyanobacteria. three common anatoxins have been described: anatoxin-a and homoanatoxin-a are secondary amines and anatoxin-a(S) is a phosphate ester of a cyclic N-hydroxyguanine structure (Figure 1).

Figure 1: Anatoxin

Further Reading

Toxic Cyanobacteria in Water: a guide to their public health consequences, monitoring and management. Chorus, E. & Bartram, J. (Eds.) World Health Organisation 1999, E&FN Spoon London & New York.

Reference

Gorham PJ, McLachlan J, Hammer UT & Kim, W.K. (1964) Isolation and toxic strains of Anabaena flos-aquae (lyngb.) de Breb. Verh. Internat. Verein. Limnol. XV, 796-804

Anatoxin production

The anatoxins have been identified in freshwater cyanobacteria from Europe, North America and Japan (see Table 1) but have not been identified in Australian cyanobacteria. Anatoxins are produced by various species of cyanobacteria, in isolation or in combination with hepatotoxins (Fig. 1). Not all strains of the causative species are toxic and there are no taxonomical or behavioural clues to the presence or absence of anatoxin in a particular sample.

Table 1. Sources of anatoxins worldwide*

*Adapted from Toxic Cyanobacteria in Water: a guide to their public health consequences, monitoring and management. Chorus, E. & Bartram, J. (Eds.) World Health Organisation 1999, E&FN Spoon London & New York.

Anatoxin poisoning

Anatoxin-a and homoanatoxin-a are postsynaptic depolarising neuromuscular blocking agents (Carmichael et al. 1979) that bind strongly to the nicotinic acetylcholine receptor (Spivak et al., 1980). These compounds are potent neurotoxins which cause rapid death in mammals by respiratory arrest (mouse LD50 approx 250 m g/kg IP, Devlin et al., 1977).

Anatoxin-a(S) is more potent towards mice (LD50 20-50 m g/kg) and is a cholinesterase inhibitor (Mahmood & Carmichael, 1986;1987). Unlike anatoxin-a, anatoxin-a(S) induces hypersalivation in mammals as well as other symptoms more typical of neurotoxicity such as diarrhoea, shaking and nasal mucus discharge (Cook et al., 1991).

Detailed reading

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Page last updated - December 18, 2008

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