The ferroxidase center is essential for ferritin iron loading in the presence of phosphate and minimizes side reactions that form Fe(III)-phosphate colloids.
Ferritin iron loading was studied in the presence of physiological serum phosphate concentrations (1 mM), elevated serum concentrations (2-5 mM), and intracellular phosphate concentrations (10 mM). Experiments compared iron loading into homopolymers of H and L ferritin with horse spleen ferritin. Prior to studying the reactions with ferritin, a series of control reactions were performed to study the solution chemistry of Fe(2+) and phosphate. In the absence of ferritin, phosphate catalyzed Fe(2+) oxidation and formed soluble polymeric Fe(III)-phosphate complexes. The Fe(III)-phosphate complexes were characterized by electron microscopy and atomic force microscopy, which revealed spherical nanoparticles with diameters of 10-20 nm. The soluble Fe(III)-phosphate complexes also formed as competing reactions during iron loading into ferritin. Elemental analysis on ferritin samples separated from the Fe(III)-phosphate complexes showed that as the phosphate concentration increased, the iron loading into horse ferritin decreased. The composition of the mineral that does form inside horse ferritin has a higher iron/phosphate ratio (~1:1) than ferritin purified from tissue (~10:1). Phosphate significantly inhibited iron loading into L ferritin, due to the lack of the ferroxidase center in this homopolymer. Spectrophotometric assays of iron loading into H ferritin showed identical iron loading curves in the presence of phosphate, indicating that the ferroxidase center of H ferritin efficiently competes with phosphate for the binding and oxidation of Fe(2+). Additional studies demonstrated that H ferritin ferroxidase activity could be used to oxidize Fe(2+) and facilitate the transfer of the Fe(3+) into apo transferrin in the presence of phosphate.
Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA.
Robert J
RJ
Hilton
N
N
David Andros
Richard K
RK
Watt
D002570:Ceruloplasmin / Q000737:chemistry*; D003102:Colloids; D005290:Ferric Compounds / Q000737:chemistry*; D005293:Ferritins / Q000737:chemistry*; D006801:Humans; D007501:Iron / Q000737:chemistry*; D010092:Oximetry; D011994:Recombinant Proteins / Q000737:chemistry; D013053:Spectrophotometry
D003102:Colloids; D005290:Ferric Compounds; D011994:Recombinant Proteins; D005293:Ferritins; D007501:Iron; D002570:Ceruloplasmin