Ferroxidase activity of ferritin: effects of pH, buffer and Fe(II) and Fe(III) concentrations on Fe(II) autoxidation and ferroxidation

It is widely accepted that iron deposition in the iron storage protein ferr itin in vitro involves Fe(II) oxidation, and that ferritin facilitates this oxidation at a ferroxidase site on the protein. However, these views have recently been questioned, with the protein ferroxidase activity instead bei ng attributed to autoxidation from the buffer alone. Ligand exchange betwee n another protein with ferroxidase activity and ferritin has been proposed as an alternative mechanism for iron incorporation into ferritin. In the pr esent work, a pH stat apparatus is used to eliminate the influence of buffe rs on iron(II) oxidation. Here we show that the recent experiments question ing the ferroxidase activity of ferritin were flawed by inadequate pH contr ol, that buffers actually retard rather than facilitate iron(II) oxidation, and that horse spleen ferritin has ferroxidase activity when measured unde r proper experimental conditions. Furthermore, high pH (7.0), a high Fe(II) concentration and the presence of Fe(III) all favour Fe(II) autoxidation i n the presence or absence of ferritin.