Alteration of Activation Energy of a Reaction in the Catalase of Human Erythrocyte by Cimetidine, an In vitro Thermokinetic Study

Background: Hydrogen peroxide is normally formed during the metabolic pathway of the body. It is a toxic compound for vital cells, which can oxidize many macromolecules and cause damage in cells. Catalase can degrade H2O2 in cells and prevent cell injury. Cimetidine is a histamine H2 receptor blocker which decreases the release of stomach acid and is used for gastrointestinal diseases. Cimetidine inhibited catalase by mixed inhibition. Objective: In this study, the effect of temperature on the binding of cimetidine to human erythrocyte catalase was investigated and kinetic factors of the binding were determined. Results: Dixon plot confirmed the mixed type of inhibition and determined the Ki of the drug. The maximum activity of the enzyme was observed at 30°C. Arrhenius plot demonstrated that the activation energy of the enzyme reaction in the absence and presence of cimetidine was about 4.7 and 8.13 kJ/mol, respectively. The temperature coefficient (Q3040) was determined as about 1.11 and 1.09 in the absence and presence of cimetidine. Conclusion: Cimetidine was able to increase the activation energy of the reaction of catalase, which confirmed the inhibition of the enzyme based on the kinetic results.