Fate of oxygen during ferritin iron incorporation

The deposition of Fe(II) into ferritin using 18O2 as the oxidant was investigated. Only 3-4% of the oxygen atoms in the FeOOH core of ferritin were derived from the oxidant. This was true whether large (1200-1900 Fe/molecule) or small (220-240 Fe/molecule) amounts of iron were added to apoferritin or when iron was added to ferritin already containing 1000 Fe/molecule. Experiments using H218O in the solvent showed that nearly all of the oxygen atoms in the core were derived from solvent. The stoichiometry of the reaction was close to 2 Fe(II)/O2, instead of the expected value of 4 Fe(II)/O2. Reactions of 18O2 with Fe(II) in the absence of apoferritin had a similar 18O distribution and stoichiometry. Our results are most consistent with the crystal growth model of ferritin iron deposition.