Bioorganic Chemistry of Hypothiocyanite

Myeloperoxidase Modulates Hydrogen Peroxide Mediated Cellular Damage in Murine Macrophages

Myeloperoxidase (MPO) is involved in the development of many chronic inflammatory diseases, in addition to its key role in innate immune defenses. This is attributed to the excessive production of hypochlorous acid (HOCl) by MPO at inflammatory sites, which causes tissue damage. This has sparked wide interest in the development of therapeutic approaches to prevent HOCl-induced cellular damage including supplementation with thiocyanate (SCN^-) as an alternative substrate for MPO. In this study, we used an enzymatic system composed of glucose oxidase (GO), glucose, and MPO in the absence and presence of SCN^-, to investigate the effects of generating a continuous flux of oxidants on macrophage cell function. Our studies show the generation of hydrogen peroxide (H₂O₂) by glucose and GO results in a dose- and time-dependent decrease in metabolic activity and cell viability, and the activation of stress-related signaling pathways. Interestingly, these damaging effects were attenuated by the addition of MPO to form HOCl. Supplementation with SCN^-, which favors the formation of hypothiocyanous acid, could reverse this effect. Addition of MPO also resulted in upregulation of the antioxidant gene, NAD(P)H:quinone acceptor oxidoreductase 1. This study provides new insights into the role of MPO in the modulation of macrophage function, which may be relevant to inflammatory pathologies.

The role of hypothiocyanous acid (HOSCN) in biological systems

Hypohalous acids (HOX), produced by peroxidase-catalysed reactions of halide and pseudohalide ions with H(2)O(2), play an important role in the human immune system. However, there is compelling evidence that these oxidants also mediate host tissue damage and contribute to the progression of a number of inflammatory diseases. Although it is well established that significant amounts of hypothiocyanous acid (HOSCN) are formed under physiological conditions, the reactions of this oxidant with host biological systems are relatively poorly characterized. It is generally accepted that HOSCN is a mild oxidant that reacts selectively with thiols. However, it is becoming increasingly recognized that this selectivity can result in the induction of significant cellular damage, which may contribute to disease. This review will outline the formation and reactivity of HOSCN and the role of this oxidant in biological systems.