Iron exchange between ferritin and transferrin in vitro

Abstract

The transfer of Fe between horse spleen [55Fe]-ferritin and human apotransferrin or [59Fe]transferrin in homogeneous solution was investigated. Transfer between the 2 proteins in the presence of citrate, ATP or ascorbate occurs in both directions, but the net flow is always from ferritin to transferrin. Ferritin which is approximately 1/3 to 1/2 saturated with Fe appears to be most reactive. Chemically prepared apoferritin does not accept Fe from diferric transferrin. Citrate-mediated transfer of Fe from ferritin to apotransferrin is first order with respect to ferritin, zero order with respect to transferrin and has a complex dependence upon citrate concentration. Direct transfer of Fe from native or reconstituted ferritin to apotransferring in the absence of any identifiable mediating agent was observed to occur at about half the rate attained in the presence of 1 mM citrate. No transfer of Fe between the 2 proteins occurs across a dialysis membrane in the absence of a mediating agent. No binding of transferrin and ferritin to each other was demonstrable. One possible explanation for these observations is that Fe from the core of ferritin is in equilibrium with Fe near the outer surface of the protein, where the metal would be available to transferrin.