Role of the four c-Helix bundle channel of rat liver ferritin for iron i.oading using ceruloplasmin

We reported that the heavy chain was required for loading iron into ferritin using cerutoplasmin as a Ferroxidase [J.-H. Guo, M. Abedi, and S.D. Aust (t996) Arch. Biochem. Biol?h3,.v. 335, 197-204]. We have now investigated the iron loading channel of rat liver fcrritin. Site-directed mutagenesis (K58E and G61H) on rat liver light chain Ferritin was done to construct an iron loading channel similar to that in the four c-helix bundle of the heavy chain of ferritin, and on the ferritin heavy chain (E62K and H65G) to close the channel, similar to what is thought to exist in the ferritin light chain. We Found that both variants expressed in the baculovirus expression system were soluble and able to form multi-subunit homopolymers. The heavy chain mutant homopolymer could not be loaded, whereas light chain mutant homopolymer was loaded with iron by ceruloplasmin However, the initial rate of loading iron into light chain mutant homopolymer by ceruloplasmin was 50% of that for loading iron into Ferritin heavy chain homopolymer. Furthermore, the ferroxidase activity of ceruloplasmin was enhanced in the presence of ferritin heavy chain and its mutant, but not in the presence of ferritin light chain or its mutant. These results suggest the importance oF association between ceruloplasmin and t)l-ritin heavy chain and the four a-helix bundle channel for loading iron into Ferritin by cemloplasmin (Supported by Nit-! Grant ES05056).