Antioxidant binding of caeruloplasmin to myeloperoxidase: Myeloperoxidase is inhibited, but oxidase, peroxidase and immunoreactive properties of caeruloplasmin remain intact

The neutrophil enzyme myeloperoxidase (MPO) purposefully makes hypochlorous acid (HOCl) as part of the cells defence against microbial infections. Dur ing cell lysis, however, MPO will be released into the extracellular enviro nment where production of HOCl, a powerful oxidant, will lead to molecular damage. Extracellular MPO binds to the copper-containing protein caerulopla smin (Cp) and prevents MPO making HOCl. Cp has several important antioxidan t functions in extracellular fluids associated with its ability to catalyse oxidation of ferrous ions and to remove peroxides. The binding of MPO to C p did not inhibit these important extracellular antioxidant activities of C p, but in so doing it provided additional antioxidant protection against fo rmation of HOCl.