Peroxidases are a large family of oxidoreductase enzymes that accelerate specific chemical reactions. They facilitate the transfer of electrons between molecules. Their primary function involves the breakdown of peroxides, such as hydrogen peroxide. Peroxidases are widely distributed across nature, synthesized by plants, animals, and microorganisms.
The Core Biological Function
Peroxidases function largely as a cellular defense mechanism against the harmful byproducts of normal metabolism. Cellular respiration and other metabolic processes constantly generate Reactive Oxygen Species (ROS), including hydrogen peroxide, which can damage cell components. If not quickly neutralized, these compounds can initiate destructive chain reactions leading to oxidative stress and cellular dysfunction.
The primary role of peroxidase is to catalyze the detoxification of hydrogen peroxide. This reaction converts the highly reactive hydrogen peroxide into two stable molecules: water and oxygen. The enzyme achieves this by using a second molecule, often an organic compound, as an electron donor.
This catalytic action allows organisms to manage the toxic effects of oxygen chemistry, which is fundamental to life. In plants, for instance, peroxidases are involved in building cell walls and defending against pathogens. Managing oxidative stress is a universal requirement for survival.
The detoxification process is carried out through a specific oxidation-reduction reaction sequence. The peroxidase molecule first binds with hydrogen peroxide, which results in an intermediate compound with a high oxidation state. This intermediate then oxidizes the electron-donating molecule, returning the enzyme to its original state and converting the peroxide to water.
Peroxidases in Human Health and Immunity
In the human body, peroxidases are highly specialized, playing direct roles in defense and cellular maintenance. One prominent example is Myeloperoxidase (MPO), an enzyme stored in immune cells like neutrophils and monocytes. MPO is released during an immune response, where it catalyzes the reaction between hydrogen peroxide and chloride ions.
This reaction produces hypochlorous acid, a powerful oxidant highly effective at destroying invading bacteria and other pathogens. This process is often called the “oxidative burst,” representing a coordinated chemical attack by the immune system. The release of MPO must be tightly regulated due to the destructive potential of hypochlorous acid on the body’s own tissues.
Another group is the Glutathione Peroxidases (GPx), which protect the cell’s interior from oxidative damage. These enzymes utilize glutathione to reduce hydrogen peroxide and lipid hydroperoxides, safeguarding cellular membranes and genetic material.
Persistent over-release of MPO is strongly linked to chronic inflammation and tissue damage, particularly in the cardiovascular system. Elevated MPO levels in the bloodstream serve as a biomarker for increased risk of cardiovascular events, including coronary artery disease and heart attack. The enzyme contributes to atherosclerosis by promoting the modification of low-density lipoprotein (LDL) into an oxidized form that triggers plaque formation.
Practical Uses in Industry and Diagnostics
The robust and specific chemical activity of peroxidases makes them valuable tools outside of biological systems, particularly in laboratory diagnostics and industrial processing. Horseradish Peroxidase (HRP), isolated from the horseradish plant, is one of the most widely used enzymes in biotechnology due to its stability and ability to accept a wide range of electron donors.
In diagnostics, HRP is routinely employed as an enzyme label in tests like the Enzyme-Linked Immunosorbent Assay (ELISA) and pregnancy tests. The enzyme is chemically attached to an antibody or antigen. When its target is present, HRP catalyzes a reaction with a chromogenic substrate. This reaction produces an intense, measurable color change, allowing scientists and clinicians to detect minute quantities of specific biological molecules.
Peroxidase activity is also a measure of success in the food industry, specifically during blanching, a heat-treatment process. Vegetables are briefly heated before freezing or canning to inactivate enzymes that cause off-flavors, discoloration, and texture changes. Peroxidase is known for its high thermal stability, meaning it is one of the last enzymes destroyed by heat.
Food scientists use the inactivation of peroxidase as an indicator enzyme for the entire blanching process. If the heat treatment destroys peroxidase activity, it is assumed that all other less heat-resistant spoilage enzymes have also been inactivated, ensuring product quality. Beyond food and medicine, microbial peroxidases are used in environmental applications, such as breaking down industrial pollutants like phenols and synthetic dyes in wastewater treatment.