Binary combinations of organophosphorus pesticides exhibit differ- ential toxicity under oxidised and un-oxidised conditions Sumitra Arora a,b , Anu Kumar b,n a ICAR-National Centre for Integrated Pest Management, PUSA Campus, New Delhi 12, India b Land and Water, CSIRO, Waite Campus, Urrbrae, South Australia 5064, Australia article info Article history: Received 15 July 2014 Received in revised form 4 January 2015 Accepted 7 January 2015 Keywords: Housefly acetylcholinesterase Concentration addition model Toxic unit Combination index–isobologram Antagonism Synergism abstract 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 in- vestigate the response of housefly(Musca domestica) head AChE (HF-AChE) exposed to five OPs as in- dividual compounds and their binary mixtures under in vitro conditions. To examine the effects of oxi- dation 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) in- creased significantly. Monocrotophos (MCP) and profenofos (PRF) did not exhibit any significant differ- ences in toxicity under oxidised and un-oxidised conditions. The toxicological interaction of five orga- nophosphorus 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 þPRF, CPF þMLT, MCP þMLT, PRF þMLT, MLT þTRZ and PRF þTRZ) 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 þTRZ ex- hibited 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. & 2015 Elsevier Inc. All rights reserved. 1. Introduction Organophosphate insecticides (OPs) are a highly diverse family of organic chemicals generally used as stomach and contact poi- sons, fumigants, and systemic insecticides for nearly every type of insect control (Matsumura, 1985). Among the predominant classes of pesticides used in India, insecticides account for 61% of total usage, followed by fungicides (19%) and herbicides (17%; Indira, 2010). Within insecticide group of pesticides, organophosphates constitute 50% of the total insecticide usage in India (Chennama- neni, 2006). Organophosphate pesticides have been used ex- tensively in agriculture as replacement chemicals for organo- chlorine pesticides as they are more rapidly degraded than the more persistent chlorinated compounds. In the United States, there are at present 27 OPs active ingredients registered for use in hundreds of different pesticide products (Laetz et al., 2009, 2013). Pesticides have potentially adverse effects on human health and contaminates vegetables, fruits, animal resources (Dewan and Saiyed, 1998; Zaidi et al., 2000; Gupta, 2004; Mandal and Singh, 2010). Agricultural runoff water and effluent discharges contain complex mixtures of pesticides. Traditionally, the evaluation of the quality of the aquatic environment has been undertaken at the chemical level, identifying and quantifying chemical residues in the different compartments of the ecosystem. However, to make an accurate assessment of the environmental risk, the quantifica- tion of chemical residues should be complemented with the study of biomarkers that serve as early warning signals (Domingues et al., 2010; Cacciatore et al., 2012; Laetz et al., 2013). Acet- ylcholinesterase (AChE) is an enzyme that has been widely used as a sensitive biomarker of exposure to OPs and carbamate pesticides (Timbrell, 2000; Walker et al., 2001; Kumar et al., 2010; Laetz et al., 2013). The enzyme hydrolyses the neurotransmitter Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ecoenv Ecotoxicology and Environmental Safety http://dx.doi.org/10.1016/j.ecoenv.2015.01.003 0147-6513/& 2015 Elsevier Inc. All rights reserved. n Corresponding author. E-mail addresses: sumitraarora@hotmail.com (S. Arora), anupama.kumar@csiro.au (A. Kumar). Ecotoxicology and Environmental Safety 115 (2015) 93–100