Lactoperoxidase (LPO, EC 1.11.1.7) is a peroxidase enzyme secreted from mammary, salivary, tears and other mucosal glands including the lungs, bronchii and nose that function as a natural, first line of defense against bacteria and viral agents. Lactoperoxidase is a member of the heme peroxidase family of enzymes. In humans, lactoperoxidase is encoded by the LPO gene.
Lactoperoxidase catalyzes the oxidation of several inorganic and many organic substrates by hydrogen peroxide. Lactoperoxidase rapidly oxidizes iodide and slowly oxidizes bromide and is designated a haloperoxidase. Another important substrate is the pseudo-halide thiocyanate. The oxidized products display potent, non-specific bactericidal and antiviral activities, including destruction of the influenza virus. Lactoperoxidase together with its inorganic ion substrates, hydrogen peroxide, DUOX1 and DUOX2 and products are termed the lactoperoxidase system. Hence LPO is considered a very important defense against invasive bacteria and viral agents such as influenza and the SARS-CoV-2 virus when sufficient iodine is provided.
The LPO system plays an important role in the innate immune system by destroying bacteria in milk and mucosal (linings of mostly endodermal origin, covered in epithelium involved in absorption and secretion) secretions. Hence augmentation of the lactoperoxidase system may have therapeutic applications and applications for controlling bacteria in food and consumer health care products. This system does not vigorously attack DNA and is not mutagenic. However, under certain conditions, the LPO system may contribute to oxidative stress though recent evidence indicates LPO to be protective. LPO may contribute to the initiation of breast cancer, through its ability to oxidize estrogenic hormones producing free radical intermediates.
