Iron (III) can be transferred between ferritin molecules

The iron-storage molecule ferritin can sequester up to 4500 Fe atoms as the mineral ferrihydrite. The iron core is gradually built up when Fe(II) is added to apoferritin and allowed to oxidize. Here we present evidence, from Mossbauer spectroscopic measurements, for the surprising result that iron atoms that are not incorporated into mature ferrihydrite particles, can be transferred between molecules. Experiments were done with both horse spleen ferritin and recombinant human ferritin. Mossbauer spectroscopy responds only to ⁵⁷Fe and not to ⁵⁶Fe and can distinguish chemically different species of iron. In our experiments a small number of ⁵⁷Fe(II) atoms were added to two equivalent apoferritin solutions and allowed to oxidize (1-5 min or 6 h). Either ferritin containing a small iron-core composed of ⁵⁶Fe, or an equal volume of NaCl solution, was added and the mixture frozen in liquid nitrogen to stop the reaction at a chosen time. Spectra of the ferritin solution to which only NaCl was added showed a mixture of species including ⁵⁷Fe(III) in solitary and dinuclear sites. In the samples to which 150 ⁵⁶Fe(III)-ferritin had been added the spectra showed that all, or almost all, of the ⁵⁷Fe(III) was in large clusters. In these solutions ⁵⁷Fe(III) initially present as intermediate species must have migrated to molecules containing large clusters. Such migration must now be taken into account in any model of ferritin iron-core formation.