Bactericidal Activity of the Oxidants Derived from Mammalian Heme Peroxidases

Abstract

Myeloperoxidase and lactoperoxidase play important roles in the innate immune system. Myeloperoxidase is highly expressed in phagocytic white blood cells called neutrophils. It converts hydrogen peroxide and chloride to hypochlorous acid, which has strong antimicrobial properties. Lactoperoxidase is present in epithelial lining fluids, particularly the respiratory tract. It converts hydrogen peroxide and thiocyanate to hypothiocyanous acid, which is a weaker oxidant than hypochlorous acid but can also inhibit bacterial growth. This chapter focuses on the susceptibility of pathogenic bacteria to hypochlorous and hypothiocyanous acids. We also discuss what is known about the bacterial systems targeted by these oxidants. It is extremely difficult for microbes to survive high doses of hypohalous acids; however, some pathogenic bacteria are more resistant, helping them to evade immune defenses. Further understanding of the reaction of hypochlorous acid and hypothiocyanous acid with bacteria, and how bacteria attempt to mitigate the damage, may reveal new therapeutic opportunities for assisting the innate immune system to contain pathogens.