Isolation and characterization of the serum ferroxidase inhibitor

strong evidence suggests that the serum ferroxidases (ceruloplasmin andferroxidase II) promote the formation of Fe-III transferrin and thereby stimulate the turnover of iron from tissue stores. Ceruloplasmin is the major ferro.xidase in human serum; whereas, ferroxidase II accotmts for an increased proportion of the activity in less highly developed animals. A large fold increase in total ferroxidase II activity was observed when both human and rabbit sera were subjected to gel-filtration on sephadex G-200. This indicated that whole serum might contain a potent inhibitor of ferroxidase II. Such an inhibitor has been isolated and purified to homogeneity by a combination of gel-filtration and ion exchange chromatography. It has a molecular weight of 64,0<YJ-67,0CJO. The molecular weight, chromatographic behavior, electrophoretic mobility, electrofocusing pH, carbohydrate content, and reactivity with anti-human albumin in an imnnmodiffusion system indicate that the ferroxidase inhibitor could be serum albumin. Furthermore, commercial human serum albumin exhibits an inhibitory activity with both serum ferroxidases that is equivalent to the ferroxidase inhibitor purified from whole rabbit and human serum. Serum albumin can be fragmented into several smaller peptide:;;, one of which contains a specific binding site for copper. it was this fragment that had inhibitory activity towards the serum ferroxidases. "In vivo" studies demonstrated that the content of the ferroxidase inhibitor in serum decreased when iron mobilization was accelerated from tissue stores by either dietary manipulation or repetitive bleeding. Furthermore, an inverse relationship was observed between the total serum content of the inhibitor and the total serum ferroxidase activity. By modulating the activity of the serum ferroxidases, serum album:in could participate in the regulation of the efflux of iron from tissue stores.