Iron uptake by ferritin: NMR relaxometry studies at low iron loads

Twenty ferritin samples were prepared at pH 6.5 with average loadings of 0-89 Fe atoms per molecule. Nuclear magnetic relaxation times T₁ and T₂ were measured at 3°C, 23°C, and 37°C and at field strength from 0.025 to 1.5 T. The field dependence, temperature dependence, and approximate equality of T₁ and T₂ at low fields all suggest that nuclear magnetic relaxation in this range is caused primarily by solitary Fe³⁺ ions. The relaxivity (relaxation rate per mM ferritin) increases quickly with initial iron loading, reaches a peak at 13-14 Fe atoms per molecule, and then declines. This provides supportive evidence for the formation of antiferromagnetically- coupled clusters during early stages in iron loading; the failure to see a similar peak in an earlier study may be related to the nonphysiological pH that was used. Above 50 atoms per molecule, the relaxivity remains approximately constant, except that 1/T₂ at high fields increases slightly, consistent with early core growth. The residual ionic relaxivity in this region is consistent with about three solitary Fe³⁺ ions remaining on the protein shell, indicating that spin cancellation is not complete. A similar value is obtained by extrapolating relaxation data at high loadings (up to 3000 Fe atoms per molecule), suggesting that these uncoupled spins persist on the protein shell even after an appreciable core has been built.