Myeloperoxidase (MPO) is the enzyme of azurophilic granules of neutrophils, which catalyzes two electron oxidation of either chloride or bromide in the so-called "halogenating cycle". Interaction of hydrogen peroxide with MPO in the presence of chloride ions leads to formation of hypochlorous acid (HOCl). Ceruloplasmin (CP) is known to be an effective physiological inhibitor of the MPO activity. However, despite the large excess of CP in blood plasma, MPO-dependently modified biomolecules were found in variety of inflammation loci, including vessel walls. This study shows that CP, which is supposed to inhibit MPO, can provide its action in physiological conditions due to hydrogen peroxide formation during oxidation of free cysteine. The key role of labile copper ions in said process is also demonstrated. (C) 2018 Elsevier Inc. All rights reserved.