Vanadyl(IV) binding to mammalian ferritins. An EPR study aided by site-directed mutagenesis

During its metabolism, vanadium is known to become associated with the iron storage protein, ferritin. To elucidate probable vanadium binding sites on the protein, VO²⁺ binding to mammalian ferritins was studied using site-directed mutagenesis and EPR spectroscopy. VO²⁺-apoferritin EPR spectra of human H-chain (100% H), L-chain (100% L), horse spleen (84% L, 16% H) and sheep spleen (45% L, 55% H) ferritins revealed the presence of α and β VO²⁺ species in all the proteins, implying that the ligands for these species are conserved between the H- and L-chains. The α species is less stable than the β species and decreases with increasing pH, demonstrating that the two species are not pH-related, a result contrary to earlier proposals. EPR spectra of site-directed HuHF variants of several residues conserved in H- and L-chain ferritins (Asp-131, Glu-134, His-118 and His-128) suggest that His-118 near the outer opening of the three-fold channel is probably a ligand for VO²⁺ and is responsible for the β signals in the EPR spectrum. The data indicate that VO²⁺ does not bind to the Asp-131 and Glu-134 residues within the three-fold channels nor does it bind at the ferroxidase site residues Glu-62 or His-65 or at the putative nucleation site residues Glu-61,64,67. While the ferroxidase site is not a site for VO²⁺ binding, mutation of residues Glu-62 and His-65 of this site to Ala affects VO²⁺ binding at His-118, located some 17 Å away. Thus, VO²⁺ spin probe studies provide a window on structural changes in ferritin not seen in most previous work and indicate that long-range effects caused by point mutations must be carefully considered when drawing conclusions from mutagenesis studies of the protein. (C) 2000 Elsevier Science Inc.