Metal ion binding to apo, holo, and reconstituted horse spleen ferritin.
The binding of Cd2+, Zn2+, Cu2+, Ni2+, Co2+, Mn2+, and Mg2+ to apo, holo, reconstituted horse spleen ferritin (HoSF), and native holo HoSF with phosphate removed was measured by gel-exclusion chromatography. Three classes of strong binding interactions (Kd < 10(-7) M) with apo HoSF at pH 7.5 were found for the various M2+ studied: high stoichiometric binding (30-54 M2+/HoSF) for Cd2+, Zn2+, Cu2+, with two protons released per metal bound; intermediate binding (16 M2+/HoSF) for Ni2+ and Co2+, with one proton released per metal bound; and low levels of binding (2-12 M2+/HoSF) for Mn2+, Mg2+, and Fe2+, with < 0.5 protons released per metal bound. M2+ binding to apo HoSF was nearly abolished at pH 5.5, except for Fe2+ and Cu2+, which remained unaffected by pH alteration. Holo HoSF bound much higher levels of M2+, a result directly attributable to the presence of phosphate binding sites. This conclusion was confirmed by decreased binding of M2+ to HoSF reconstituted in the absence of phosphate and by native holo HoSF with phosphate chemically removed. The binding of Cd2+ to apo HoSF was 54 per HoSF, but in the presence of developing core, the amount bound decreased to about 30 Cd2+/HoSF. This result indicated that Cd2+ and developing core were competing for the same sites on the HoSF interior, suggesting that 24 of the Cd2+ were bound to the inside surface. No other M2+ studied bound to the interior of HoSF by this criterion. Several of the M2+ appeared to bind strongly to the phosphate-free mineral core surface in reconstituted HoSF.
Pead
S
S
Department of Chemistry Undergraduate Research Program, Brigham Young University, Provo, UT 84602, USA.
Durrant
E
E
Webb
B
B
Larsen
C
C
Heaton
D
D
Johnson
J
J
Watt
G D
GD
D000818:Animals; D001052:Apoferritins / Q000378:metabolism; D001665:Binding Sites; D001667:Binding, Competitive; D002413:Cations, Divalent / Q000378:metabolism*; D002850:Chromatography, Gel; D005293:Ferritins / Q000378:metabolism*; D006736:Horses; D007501:Iron / Q000378:metabolism; D010084:Oxidation-Reduction; D010710:Phosphates / Q000494:pharmacology; D011522:Protons; D013154:Spleen / Q000737:chemistry
D002413:Cations, Divalent; D010710:Phosphates; D011522:Protons; D005293:Ferritins; D001052:Apoferritins; D007501:Iron