MPO and cancer: Myeloperoxidase (MPO) catalyzes the reaction of hydrogen peroxide with chloride ions to form HOCl in biological systems.6 It is essential for preserving the intracellular redox equilibrium, reducing inflammation, and fending off infections.7,8 However, too much HOCl can result in the production of dangerous substances in water (such as trihalomethanes) and cause a number of oxidative stress-related illnesses in people, such as cancer, arthritis, cardiovascular disease, and neurodegeneration.