Anatoxin-a (ANTX) is a neurotoxic cyanobacterial toxin. Already in the 1930´s cyanobacterial blooms were observed causing very fast deaths in animals. The causing agents were therefore called “very fast death factor (VFDF)”. The toxin was later identified and named anatoxin-a. It was the first cyanobacterial toxin which was chemically and functionally defined in 1972.
Anatoxin-a is a bicyclic secondary amine with the general structure 2,9-Diacetyl-9-azabicyclo[4,2,1]non-2,3-ene. Besides ANTX, homoanatoxin-a a methylated anatoxin-a analogue , and 4-hydroxyanatoxin-a an oxygenated homoanatoxin-a analogue, have been isolated and described.
Anatoxin–a was first isolated and identified from the cyanobacterium Anabaena flos-aquae in 1972. Anatoxin-a was later found in various members of the cyanobacterial genera Anabaena, Aphanizomenon, Cylindrospermum, Oscillatoria, Planktothrix and Raphidiopsis. Homoanatoxin-a was first detected in Oscillatoria sp. in 1992 and later found to be also produced by members of the cyanobacterial genera Anabaena, Raphidiopsis and Phormidium. 4-hydroxyanatoxin-a was isolated from a Rhaphidiopsis strain in 2003.
Light microscopy photo of Anabaena flos-aquae
Anatoxin-a is a potent agonist of the muscular and neuronal nicotinic acetylcholine receptor. Anatoxin-a is a structural analogue to the neurotransmitter acetylcholine and binds to the nicotinic acetylcholine receptor with a higher affinity than acetylcholine. It acts as a postsynaptic depolarizing neuromuscular blocking agent and cannot be degraded by acetylcholinesterase thus causing muscle overstimulation. Signs of poisoning are staggering, muscle fasciculations leading to fatigue and paralysis, gasping, convulsions, and death by respiratory arrest in vertebrates. Homoantoxin-a is a potent analogue of anatoxin-a. Anatoxin-a and homoanatoxin-a are highly toxic with a LD50=200-250 µg kg-1 (mouse, intraperitoneal). 4-Hydroxyanatoxin-a does not show apparent toxicity to mice.
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