REDOX REACTIONS OF APO MAMMALIAN FERRITIN

Apo horse spleen ferritin undergoes a 6.3 +/- 0.5 electron redox reaction at -310 mV at pH 6.0-8.5 and 25-degrees-C to form reduced apoferritin (apoMF(red)). Reconstituted ferritin containing up to 50 ferric ions undergoes reduction at the same potential, taking up one electron per ferric ion and six additional electrons by the protein. We propose that apo mammalian ferritin (apoMF) contains six redox centers that can be fully oxidized forming oxidized apoferritin (apoMF(ox)) or fully reduced forming apoMF(red). ApoMF(red) can be prepared conveniently by dithionite or methyl viologen reduction. ApoMF(red) is slowly oxidized by molecular oxygen but more rapidly by Fe(CN)63- to apoMF(ox). Fe(III)-cytochrome c readily oxidizes apoMF(red) to apoMF(ox)). with a stoichiometry of 6 Fe(III)-cytochrome c per apoMF(red), demonstrating a rapid interprotein electron-transfer reaction. Both redox states of apoMF react with added Fe3+ and Fe2+. Addition of eight Fe2+ to apoMF(ox) under anaerobic conditions produced apoMF(red) and Fe3+, as evidenced by the presence of a strong g = 4.3 EPR signal. Subsequent addition of bipyridyl produced at least six Fe(bipyd)3(2+) per MF, establishing the reversibility of this internal electron-transfer process between the redox centers of apoMF and bound iron. Incubation of apoMF(red) with the Fe3+-ATP complex under anaerobic conditions resulted in the formation and binding of two Fe2+ and four Fe3+ by the protein. The various redox states formed by the binding of Fe2+ and Fe3+ to apoMF(ox) and apoMF(red) are proposed and discussed. The yellow color of apoMF appears to be an integral characteristic of the apoMF and is possibly associated with its redox activity.