Brain Acetylcholinesterase Inhibition and Hepatic Activation of Acephate and Fenitrothion in Rainbow Trout (Salmo gairdneri)

Two factors, potency of brain acetylcholinesterase (AChE) inhibition and biotransformation by liver homogenate, were investigated to understand the temperature-dependent toxicity of fenitrothion (FTN) in rainbow trout (Salmo gairdneri) and the 600- to 1000-fold differences in concentration between FTN and another organophosphate (OP) insecticide, acephate (ATE), required to produce death in these trout. Concentrations required to produce 50% inhibition of brain AChE were similar for ATE and FTN, ~125 mmol/L and 80 mmol/L, respectively, whereas fenitrooxon (FTO), the oxidative desulfuration metabolite of FTN, was approximately five orders of magnitude more potent. Incubation with liver homogenate, however, demonstrated that a more potent brain AChE inhibitor was produced from ATE, but not from FTN. It is concluded that hepatic biotransformation of FTN to FTO does not explain previous observations of FTN temperature dependency and differences in concentrations producing lethality.Key words: acephate, acetylcholinesterase, biotransformation, brain, fenitrothion, fenitrooxon, liver