Comparing the in vitro effects of Diazinon and Chlorpyrifos on acetylcholinesterase activity of marine medaka (Oryzias javanicus) as a test species

Abstract

Organophosphorus pesticides (OPs) like chlorpyrifos and diazinon are one of the extensively used pesticides in the agricultural industry. The toxicity of these pesticides results from the inhibition of the enzyme acetylcholinesterase (AChE) which catalyze hydrolysis of the neurotransmitter acetylcholine (ACh). Because of the potential for significant acute toxicity, there is a need to evaluate the effects of OPs exposure in the aquatic environment. Recently, the activity of acetylcholinesterase has been widely used as a biomarker for monitoring of exposure to OPs in the environment. Although numerous studies have been done in the effects of OPs on freshwater fish, its effect on marine fish is not well known. In this study, in vitro effects of chlorpyrifos and diazinon, on AChE inhibition in head post-mitochondrial supernatant (PMS) of marine medaka (Oryzias javanicus) were investigated. Marine medaka was selected as a model marine test fish because of its short life cycle and easiness to rear in a laboratory. The PMS was exposed to two pesticides in the concentration range of 0.02 ~ 3mM. Chlorpyrifos and diazinon inhibited the AChE activity in a dose-dependent manner, and the rate of inhibition was similar to each other. In case of the diazinon exposure both the maximal velocity (Vmax) and the Michaelis-Menten constant (Km) were decreased by 27% and 20% to the control, respectively, while only Vmax is decreased by 32% in the presence of chlorpyrifos. With a Lineweaver-burk plot, we analyzed that diazinon and chlorpyrifos inhibite AchE in the un-competitive and non-competitive manner, respectively.