The relationship of esterases to organophosphorus insecticide tolerance in mosquitofish

The levels of peripheral acetylcholinesterase and carboxylesterases and their organophosphate sensitivities were studied in two populations of mosquitofish (Gambusia affinis). One population, highly resistant to organochlorine insecticides, demonstrated a low tolerance to parathion and methyl parathion. The organochlorine resistant fish possessed higher levels of both peripheral acetylcholinesterase and carboxylesterases. The sensitivities of these esterases to organophosphate inhibition were the same in both populations. The esterases were more sensitive to paraoxon than to methyl paraoxon. Carboxylesterases were far more sensitive to organophosphate inhibition than was acetylcholinesterase. Carboxylesterases, by their higher affinity for the organophosphates, may serve to protect acetylcholinesterase from inhibition.