Methyl-paraoxon comparative inhibition kinetics for acetylcholinesterases from brain of neotropical fishes

Acetylcholinesterase (AChE) sensitivity to the organophosphorus (OP) pesticide methyl-paraoxon was measured in fourteen species of Neotropical marine and freshwater fish found in the waters of Brazil. The rate constant for phosphorylation, k p, the dissociation constant, k d, the second order rate constant, k i , and the IC 50 value were measured at 28 °C in pH 7.5 buffer for AChE extracted from brain. In addition, the substrate affinity constant, k m, was measured with acetylthiocholine. The IC 50 for 30 min of inhibition ranged from 123 nM ( Prochilodus lineatus) to 3340 nM ( Percophis brasiliensis), which corresponded to k i values of 187–6.9 mM −1 min −1. A 10-fold range in k p values from 0.21 min −1 ( Paralonchurus brasiliensis) to 2.1 min −1 ( Dules auriga) was associated with a 37-fold range in k d values from 4 to 150 μM. These large differences in reactivity with methyl-paraoxon were not reflected in the binding affinity for acetylthiocholine; k m values were approximately 0.1–0.3 mM for all species. These results predict that the amino acid sequence involved in AChE sensitivity differs in these fishes, and that consequently some fish species may be resistant to the toxicity of methyl-paraoxon.