Objectives: Human exposure to heavy metals and pesticides is a worldwide major health problem. These environmental pollutants have been considered as the most neurotoxic agents and responsible to causing neurological toxicity. Plant-based therapeutic supplement may be used in the event of toxicity. Citrus limon contains several useful bioactive ingredients including flavonoids, dietary fiber, carotenoids, vitamins, pectin, minerals, and essential oils, which are responsible for its therapeutic potential. In the present investigation, we have studied the toxicity of heavy metals such as lead (Pb) and a carbamate pesticide such as cartap (Cp) on rat brain acetylcholinesterase (AChE).
Methods: The chemical characterization of C. limon involved determination of total antioxidants and 2,2-diphenyl-1-picryl-hydrazyl-hydrate free radical scavenging activity using known methods. The AChE activity and its kinetic characterization were performed by assaying the enzyme activity at varying substrate concentrations, pH, temperature, and time of reaction. Its different kinetic parameters such as K i , V max, K m , K cat, and K cat/K m were determined by using standard procedures. The amelioration potential of the extract was evaluated on the neurotransmission system of rat brain AChE treated with Pb, Cp, and their combination (Pb-Cp), considering their 50% inhibitory concentration (IC50) values.
Results: The optimal activity of rat brain AChE was recorded at 25 µg of protein, pH of 7.4, substrate concentration [S] of 0.5 mM, and temperature of 37.4-40°C. The enzyme was stable for 10 min when incubated at 37.4 °C in vitro. The enzyme displayed 70% of its activity remaining even after 160 min of incubation in this condition. It may be stable up to 1 month when stored at -20°C. The IC50 values for Pb, Cp, and Pb-Cp were found to be 75, 2.9, and 5 mM, respectively. Pb, Cp, and Pb-Cp inhibited the activity of rat brain AChE in the noncompetitive, mixed, and uncompetitive manners, respectively, with their respective K i values to be 675, 2.37, and 22.72 mM.
Conclusions: The results indicated that the Pb and Cp were able to cause significant alterations in the level and properties of AChE. However, the introduction of lemon juice on Pb- and Cp-treated AChE indicated protection of its activity from their adverse effects. The results may be useful in prospective therapeutic applications of lemon juice or as a food supplement to protect mammalian systems from adverse effects of these toxicants.
Keywords: Citrus limon; acetylcholinesterase; cartap; kinetics; lead; toxicity.
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