Brain Acetylcholinesterase Activity as a Potential Biomarker for the Rapid Assessment of Chlorpyrifos Toxicity in a Euryhaline Fish,Oreochromis mossambicus

Acute studies of chlorpyrifos, O,O-diethyl-O-(3,5,6-trichloro-2-pyridyl) phosphorothioate insecticide on fish, Oreochromis (Tilapia) mossambicus, were carried out to assess the toxicity in relation to the marker enzyme, acetylcholinesterase (AChE, EC 3.1.1.7) activity in a semi-static system. Chlorpyrifos can be rated as highly toxic to O. mossambicus, with median lethal concentrations (LC50) of 43.52, 35.89, 30.6 and, 25.78 μg L−1 for 24, 48, 72, and 96 h respectively. The time required for 50% inhibition (IT50) of acetylcholinesterase (AChE) activities were measured in the brain of O. mossambicus, exposed to different median lethal concentrations indicated a decrease in time as the concentration increased. After exposure, survival fish were transferred to clean (chemical free) water and studied. AChE recovery were recorded in regular intervals of 3, 7, 14 and 21 days as a biomarker of exposure to this compound. The fish exposed to the LC50 for 24 h (43.52 μg L−1) showed a significant inhibition of AChE activity at 6 h (>40%) and reached to a maximum of 90% at 24 h. However, the recovery of AChE in the recuperation period was much slower and required >100d to reach normal activity. A similar pattern of AChE recovery was observed in fish exposed for 48, 72 and 96 h (with respective LC50s'), which showed comparatively quicker recovery than the fish exposed to LC50 of 24 h. A graphical comparison of respective IT50s' of brain AChE activity with LC50 concentrations suggested a negative correlation, whereas 50 and 100% recovery tenures (RT50 & RT100) with individual LC50 concentrations revealed a positive correlation. Based on the derived linear regressions, one can easily predict the required concentration, in relation to selected time for 50% inhibition or vice versa. To assess the validity, assumed time required for 50% inhibition was calculated for 40 μg L−1 and compared with actual experiments that revealed the same value. The present correlation offers the advantage of accurately predicting, without performing any experiments, the percent inhibition or recovery in brain AChE activity of O. mossambicus, based on any any sub-lethal concentration of chlorpyrifos. This prediction method can save a lot of time.