Characterization, effect of metal ions and organophosphates on the brain acetylcholinesterase of Nile Tilapia (Oreochromis niloticus) in vitro.

Acetylcholinesterase (AChE; EC 3.1.1.7) inhibition in vitro is an effective method for monitoring pesticide contamination, particularly organophosphates, offering advantages over in vivo approaches. Environmental factors like temperature, age, and stress can alter enzyme activity in vivo, making controlled in vitro studies necessary. This research focused on the brain AChE of Nile tilapia (Oreochromis niloticus), a key species in global aquaculture, to detect organophosphate contamination. The study examined pesticides such as chlorpyrifos, fenitrothion, malathion, and temephos. Optimal pH (8.0) and temperature (55°C) conditions were determined for AChE activity. The enzyme also showed sensitivity to several heavy metals, including Co²⁺, Cd²⁺, Cu²⁺, Hg²⁺, Mn²⁺, and Zn²⁺, with mercury causing 100% inhibition. AChE exhibited high substrate affinity, with a Vmax of 0.651 ± 0.26 mU/mg and a Michaelis-Menten constant (Km) of 399 mU/mg. Pesticide inhibition varied, with chlorpyrifos and temephos showing the highest efficacy at lower concentrations. IC20, IC50, and Ki values ranked pesticide potency as follows: malathion, fenitrothion, temephos, and chlorpyrifos. These results support the use of Nile tilapia AChE as a reliable environmental biomarker for detecting pesticides, offering valuable insights for environmental monitoring and filling gaps in the existing literature.