Cunha Bastos_1999_Pestic.Biochem.Physiol_64_149

A constant influx of parathion into livers of benthic fishes could overflow their ability to both activate (into paraoxon) and detoxify (through hydrolysis) molecules and metabolites of this phosphorothioate. This could allow parathion to be activated in brain cells with consequent nervous intoxication. Assay of acetylcholinesterase (AChE) enzyme activities allowed the nanomolar quantification of paraoxon produced from parathion in vitro by liver and brain homogenates of cascudo, a Brazilian suckermouth benthic fish. Using comparable kinetic conditions, we have determined that the initial velocities at which parathion desulfuration occurred in homogenates of liver and brain of cascudo responded to increasing micromolar parathion concentrations as a sigmoidal function. Our data also suggest that cascudo brain tissues may produce paraoxon from parathion at a rate approximately two-thirds that of the liver tissues, when parathion concentrations are lower than 30 uM. Moreover, cascudo brain would be faster than liver to promote oxidative desulfuration of parathion between 35 and 55 uM. As no parathion cleavage was measured using brain or liver homogenates of cascudo it became clear that direct hydrolysis of parathion would not work as a detoxification process in these tissues. These results may reveal typical processes of organophosphate metabolism present in benthic fish which could produce sublethal nervous intoxication.