Different sensitivities to paraoxon of brain and serum cholinesterases from Pacu, an indigenous Brazilian fish

In pacu, Piaractus mesopotamicus, which is an economically important fish in Brazil, the response of serum and brain cholinesterase activity to paraoxon was investigated. Paraoxon results from parathion bioactivation. Results showed that paraoxon concentrations five times lower than those used to produce inhibition of brain cholinesterase activities were able to produce nearly total inhibition of serum activities. The I sub(50) values for brain acetylcholinesterase and serum butylcholinesterase were calculated at 254 and 1.8 nM, respectively. In vitro inhibition curves showed that either of the butylcholinesterase determinations, using brain or serum, accurately detected paraoxon concentrations under the detection limit of gas chromatography.