Background and objective: Peroxidases catalyze protein oxidation and lipid peroxidation. The activity of these enzymes in nerve cells is involved in causing disorders such as Alzheimer's and Parkinson's disease. This study investigated the effect of Citrus aurantium, Foeniculum vulgare and Rosmarinus officinalis essential oils on activity of peroxidase enzyme. Methods: All three medicinal plants were dried at room temperature. Their essential oil was extracted by steam distillation using a Clevenger apparatus. Optimal reaction conditions were determined in the presence of hydrogen peroxide and guaiacol as substrate and hydrogen donor, respectively. Enzyme kinetics of zucchini peroxidase were evaluated by increasing the amount of essential oils in optimal reaction conditions. Enzyme reaction rate for each of the essential oils and the Km and Vmax values were determined. Results: The results indicated concentration-dependent effect of the extracted essential oils on enzyme kinetics at optimum temperature of 50 °C and optimal pH of 6.5. The essential oil of Citrus aurantium had noncompetitive inhibitory effects on the enzyme with Km of 6.25 mM, while the enzyme’s Vmax significantly reduced by increasing the concentration. Foeniculum vulgare showed mixed inhibition effect with Km of 7.14 mM per 20 μl of the essential oil, but had a decreasing effect on the Vmax in smaller amounts. Finally, Rosmarinus officinalis showed activating effects by reducing the Km to 4-5.88 mM. Conclusion: The essential oils of Citrus aurantium and Foeniculum vulgare are inhibitors of the peroxidase enzyme and can be further studied as natural herbal medicines.
