Anatoxin-a(s), a Naturally Occurring Organophosphate, is an Irreversible Active Site-directed Inhibitor of Acetylcholinesterase (EC 3.1.1.7)

Anatoxin-a(s) is a guanidine methyl phosphate ester (unprotonated molecular ion equals 252 daltons) isolated from the freshwater cyanobacterium (blue-green alga) Anabaena flos-aquae strain NRC 525-17. Previous work has shown anatoxin-a(s) to be a potent irreversible inhibitor of electric eel acetylcholinesterase (EC 3.1.1.7, AChE). In the present study the interaction of anatoxin-a(s) with AChE was investigated by protection studies and since similarities have been noted between anatoxin-a(s) and the synthetic organophosphate anticholinesterases, the ability of reactivators to reactivate the inhibited enzyme was investigated. Treatments directed toward eliminating poisoning symptoms and in vivo protection from anatoxin-a(s) poisonings were investigated using oxime reactivators and atropine or pretreatment with a carbamate and atropine. Anatoxin-a(s) was shown to be an active site-directed inhibitor of acetylcholinesterase which is resistant to oxime reactivation due to the structure of its enzyme adduct. In vivo pretreatment with physostigmine and high concentrations of 2-PAM were the only effective antagonists against a lethal dose of anatoxin-a(s).