Description_x000D_ Biochem/physiol Actions_x000D_ Ceruloplasmin (Cp), the principal carrier of copper in plasma, is thought to be involved in oxidative modification of lipoproteins and participate in the acute phase reaction to stress. Cp is an effective antioxidant for a variety of radicals and has a potent peroxidase activity to decompose hydrogen peroxide in the presence of reduced glutathione. It inhibits the peroxidase activity of myeloperoxidase in a concentration-dependent manner and shows selective binding to myeloperoxidase in vitro. This suggests that ceruloplasmin may participate in the clearance and inactivation of myeloperoxidase in vivo. At physiological (micromolar) concentrations it was found to inhibit relaxation of rabbit aorta induced by endothelium-dependent agonists like acetylcholine or ADP. However, it was found ineffective toward vasodilation due to direct stimulation of smooth muscle cells by nitroglycerin. Cp plays an essential role in iron homeostasis, regulating the efflux of iron from macrophages and the oxidation of Fe2+ to Fe3+. Cp deficiency leads to neurodegenerative disorders, possibly by starving neurons of iron._x000D_ Unit Definition_x000D_ One unit will cause a δA550 of 0.01 per min using N,N-dimethyl-p-phenylenediamine as substrate at pH 5.5 and 37 °C, in a 7 mL reaction volume._x000D_ Physical form_x000D_ Pale blue powder containing sodium chloride and sodium acetate_x000D_ Analysis Note_x000D_ Protein determined by biuret