Characteristics of a ferritin-binding protein present in human serum

The ferritin present in human serum differs from the ferritins found in tissues and other body fluids in having negligible proportions of H subunits. This has been related to the possible presence of binding factors which would form complexes with H-subunit containing ferritins and thereby determine their rapid clearance and/or interference with immunoassays ('serum inhibition'). In this work we have tried to identify and characterize these binding factors. Dotting and blotting experiments demonstrated an interaction between tissue ferritins and human serum. This was stronger with human heart and recombinant H-type ferritin obtained from E. coli than with human liver ferritin. The serum binder appeared to be a glycoprotein migrating in the beta-2 region and with a molecular weight of about 200,000 and pI between 4 and 5. Two different approaches to purification of the ferritin-binding protein yielded enriched fractions containing also the complement proteins C3 and C4, the plasma protease inhibitor alpha-2-macroglobulin, and immunoglobulins. These in vitro findings may have physiological relevance.