Inhibition of Acetylcholinesterase in the Anaxyrus cognatus Liver by Chlorpyrifos Oxon

Abstract

This study investigated the effects of a commonly used organophosphate pesticide, chlorpyrifos, on the liver of the Great Plains toad, Anaxyrus cognatus.  The activity of acetylcholinesterase in the liver of the toad was evaluated by the Ellman method using a plate reader, and its in vitro sensitivity to a range of concentrations of chlorpyrifos oxon, the active metabolite of chlorpyrifos.  At chlorpyrifos oxon concentrations from 100 nM to 100 µM, concentration-dependent inhibition was noted (IC50 [concentration that inhibited 50% of the enzyme activity] = 1.8 µM).  This concentration is about 100-fold higher than published for mammalian brain acetylcholinesterase inhibition; thus, the data suggest the toad may be resistant to acute toxicity of this pesticide.  The findings suggest that organophosphate pesticides may have lesser acute toxicity potential in the Great Plains toad compared to mammals.