Iron loading into ferritin by an intracellular ferroxidase.

An intracellular, membrane-bound enzyme exhibiting both p-phenylenediamine oxidase activity and ferrous iron oxidase activity was isolated with the plasma membrane fraction of horse heart and studied for its ability to load iron into ferritin. The ferroxidase activity of the tissue oxidase was stimulated approximately twofold by horse spleen apoferritin, and the iron was loaded into ferritin. The loading of iron into ferritin by the tissue oxidase was inhibited by anti-horse serum ceruloplasmin antibody. The stoichiometry of iron oxidation and oxygen consumption during iron loading into ferritin by the tissue-derived oxidase and serum ceruloplasmin were 3.6 +/- 0.2 and 3.9 +/- 0.2, respectively. These data provide evidence that an enzyme analogous to ceruloplasmin is present on the plasma membrane of horse heart and that this ferroxidase is capable of catalyzing the loading of iron into ferritin. The implications of these data on the present models for the uptake and storage of iron by cells are discussed.