Characterisation of cholinesterase and histopathological features of brain of Clarias gariepinus following exposure to cadmium

Aim : Cholinesterase (ChE) inhibition and histopathological features of brain tissues of Clarias gariepinus were investigated with a purpose to develop biomarkers for monitoring heavy metals. Methodology : The effects of cadmium toxicity on C. gariepinus were assessed by ChE inhibition and characterisation of ChE by affinity chromatography and histological variations in brain tissues were studied by H&E staining under light microscope. Results : The purified enzyme gave 10.58 fold purification with the recovery of 12% with specific activity of 159.729 U mg'1. The Michaelis-Menten constant (Km) and Vmax value obtained was 1.985 mM and 0.4479 |jmol min-1 mg-1, respectively. The enzyme showed the ability to hydrolyse acetylthiocholine iodide (ATC) at a faster rate compared to other two synthetic substrates, propionylcholine iodide (PTC) and butyrylthiocholine iodide (BTC). ChE showed highest activity at 35°C in Tris-HCl pH 7.5. The metal inhibition study also found the maximal inhibitory concentration (IC50) value for cadmium to be 0.6517 mg l-1. Histological study revealed decrease in the number of cells, degeneration of Purkinje cells, increased aggregation area and cells necrosis. Interpretation : Inhibition of cholinesterase activity and degeneration of Purkinje cells observed in the present study can be used as a tool for further developing a biomarker for detecting heavy metals in the aquatic ecosystems.