Bioactive Toxins

Fatal intoxications of fish, water birds and domestic animals in aquatic habitats are frequently observed with blooms of cyanobacteria. The latter produces two types of toxins : cytotoxins & biotoxins and a few antibiotics (allelopaths) such as acutiphycins, indolcarbazoles, tantazoles, tolytoxins etc. Cytotoxins exhibit a wide spectrum of activity against bacteria, fungi, algae and mammalian cell lines whereas biotoxins commonly produce intoxications in animals and man. Two different kinds of biotoxins produced by cyanobacteria have been investigated: neurotoxins and hepatotoxins. It was observed that blooms of algae secreting hepatotoxins are almost twice as common as neurotoxic ones. Statistical associations were frequent between hepatotoxicity and occurrence of Microcystis aeruginosa, M. viridis, M. wesenbergii, Anabaena flos-aquae and A. spiroides. Hepatotoxins mainly target liver in mice where they form marked congestion and necrosis in the parenchymal cells around the central vein. Their LD50 values were found between 140 to 160 µg/kg bw. Neurotoxicity was, by and large, linked with prevalence of certain species of the genera Anabaena, Aphanizomenon, Oscillatoria and Trichodesmium. Of these neurotoxins, saxitoxin (STX) and neosaxitoxin, also termed as 'aphantoxins' are produced by Aphanizomenon flos-aquae. Anatoxin-a (ANTX-a) is an alkaloid neurotoxin produced by Anabaena flos-aquae, A. spiroides and Oscillatoria. The LD50 in mice is about 200 µg/kg body weight as against 10 µg/kg dosages of aphantoxins. Mice injected intraperitoneally with lethal doses of ANTX-a die from respiratory arrest within some minutes showing symptoms like muscle fasciculation, heavy abdominal breath, cyanosis, convulsions and collapse. Another neurotoxin - Anatoxin-a(s) {ANTX-a(s)} has similar properties to an organophosphate insecticide, being a potent inhibitor of cholinesterase. On treatment with 20 µg/kg bw of dosage (LD50) animals show marked salivation, mucoid nasal discharge, ataxia, diarrhea, dyspnea and cyanosis. Anatoxin-b (ANTX-b), a neurotoxin with acetocholinesterase activity; is more toxic than anatoxin-a and a(s). However, its exact nature and pharmacological action have yet to be worked out