Acute toxicity of the pesticide trichlorfon and inhibition of acetylcholinesterase in Colossoma macropomum (Characiformes: Serrasalmidae)

In Brazilian fish farms, trichlorfon has been widely used to control acanthocephalan infections in Colossoma macropomum. Toxicity tests were conducted to estimate the median lethal concentration (LC50-96 h) and evaluate the effects of trichlorfon on acetylcholinesterase (AChE) and glutathione S-transferase (GST) from different tissues of C. macropomum. The LC50-96 h of trichlorfon was estimated to be 0.87 mg L-1. In the sublethal toxicity tests, concentrations of 0.26 mg L-1 (30% of LC50-96 h) and 0.43 mg L-1 (50% of LC50-96 h) were used. AChE and GST activities were measured in the brain, muscle, intestine, and liver. In vitro studies were conducted to estimate the kinetic properties and half-maximal inhibitory concentration (IC50) values of AChE in the brain and muscle for trichlorfon. In the sublethal toxicity experiments, inhibition of more than 90% of AChE in the brain, muscle, and intestine was observed. However, the activity of GST did not vary in any of the tissues studied. This finding suggests that trichlorfon is not metabolised by this enzyme. The in vitro assay results suggest that trichlorfon tends to be a classic uncompetitive inhibitor of AChE in both the brain and muscle, since K-m and V-max values decrease, while the slope remains unchanged. The IC50 values of muscle AChE are lower than those of the brain. All these results show that C. macropomum has low tolerance to this pesticide and suggest that brain AChE can be used as a biochemical biomarker, while muscle AChE may be used as an indicator of mortality in toxicological studies.