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 T-1 a nd T-2 were measured at 3 degrees C, 23 degrees C, and 37 degrees C an d at field strength from 0.025 to 1.5 T. The field dependence, tempera ture dependence, and approximate equality of T-1 and T-2 at low fields all suggest that nuclear magnetic relaxation in this range is caused primarily by solitary Fe3+ ions. The relaxivity (relaxation rate per m M ferritin) increases quickly with initial iron loading, reaches a pea k at 13-14 Fe atoms per molecule, and then declines. This provides sup portive evidence for the formation of antiferromagnetically-coupled cl usters during early stages in iran loading; the failure to see a simil ar peak in an earlier study may be related to the nonphysiological pH that was used. Above 50 atoms per molecule, the relaxivity remains app roximately constant, except that 1/T-2 at high fields increases slight ly, consistent with early core growth. The residual ionic relaxivity i n this region is consistent with about three solitary Fe3+ ions remain ing on the protein shell, indicating that spin cancellation is not com plete. A similar value is obtained by extrapolating relaxation data at high loadings (up to 3000 Fe atoms per molecule), suggesting that the se uncoupled spins persist on the protein shell even after an apprecia ble core has been built. (C) 1998 Published by Elsevier Science Inc.