Inhibition of acetylcholinesterase and aliesterases of fingerling channel catfish by chlorpyrifos, parathion, and S, S, S-tributyl phosphorotrithioate (DEF)

Serine esterases can be inhibited by organophosphorus compounds. The in vitro potency of the organophosphorus pesticide DEF and oxons of the phosphorothionate insecticides chlorpyrifos (chlorpyrifos-oxon, Cpxn) and parathion (paraoxon, Pxn) were determined for brain, gill, liver, muscle, and plasma for acetylcholinesterase (AChE) and gill, liver, and plasma aliesterases (ALiE). AChE activity was inhibited less than 15% in all tissues in the presence of 1 mM DEF ®. AChE I 50 values for Cpxn were 28–33 nM, and for Pxn were 446–578 nM. ALiE I 50 values for Cpxn were 0.1–0.2 nM, for DEF were 24–163 nM, and for Pxn were 6–46 nM. Fish were exposed to chlorpyrifos (Cp), parathion (Pth), DEF, and combinations of the phosphorothionates with DEF for 4 h followed by a 384-h recovery period. AChE inhibition following Cp and Pth exposures was rapid. Cp led to more persistent inhibition than Pth. DEF treatments yielded low levels of AChE inhibition in brain and muscle, and high levels of inhibition in gill, liver, and plasma. In vitro and in vivo results suggest that DEF's disposition and/or mode of action are different than those of Cp or Pth. Exposure to DEF resulted in persistent, high level inhibition of ALiE activity. Greater AChE inhibition in DEF plus Cp or Pth treatments was not evident, suggesting that ALiEs do not serve to appreciably protect AChE in channel catfish, even though ALiEs are inherently more sensitive to inhibition.