Binary combinations of organophosphorus pesticides exhibit differential toxicity under oxidised and un-oxidised conditions.

Acetylcholinesterase (AChE) inhibition has been demonstrated to be useful as a biomarker for exposure to organophosphorus (OP) insecticides in many environments. The objective of this study was to investigate the response of housefly (Musca domestica) head AChE (HF-AChE) exposed to five OPs as individual compounds and their binary mixtures under in vitro conditions. To examine the effects of oxidation on OP potency in the HF-AChE system, bromine water was used as an oxidisng agent. With oxidation, the sensitivity of HF-AChE to chlorpyrifos (CPF), malathion (MLT) and triazophos (TRZ) increased significantly. Monocrotophos (MCP) and profenofos (PRF) did not exhibit any significant differences in toxicity under oxidised and un-oxidised conditions. The toxicological interaction of five organophosphorus pesticides was evaluated using the concentration addition model, the combination index-isobologram equation and the toxic unit approach. All three models provided similar predictions for the 10 binary combinations of OPs under oxidised and un-oxidised conditions. In the present study, the antagonistic effects of the binary combination of OPs (CPF+PRF, CPF+MLT, MCP+MLT, PRF+MLT, MLT+TRZ and PRF+TRZ) were observed under oxidised conditions. This may be due to dispositional and/or receptor antagonism. Most of the binary combinations assayed under un-oxidised conditions exhibited synergistic responses. Triazophos showed very strong synergism in binary combinations with CPF, MCP and PRF un-oxidised conditions. In contrast, under oxidised conditions, only CPF+TRZ exhibited synergism. The results obtained indicate differential toxicity of binary combinations of OPs under oxidised and un-oxidised conditions. This information could be a valuable tool in understanding the mechanisms of OPs interactions and the interpretation of future in vivo studies with mixtures of OP insecticides.