Electrical conductivity measurements were performed on single apoferritin and holoferritin molecules by conductive atomic force microscopy. Conductivity of self-assembled monolayer films of ferritin molecules on gold surfaces was also measured. Holoferritin was 5-15 times more conductive than apoferritin, indicating that for holoferritin most electron-transfer goes through the ferrihydrite core. With 1 V applied, the average electrical currents through single holoferritin and apoferritin molecules were 2.6 pA and 0.19 pA, respectively.

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(17), and an E m ϭ -790 mV (18) was estimated for the 1ϩ͞0 couple, which is in line with E m estimates of all-ferrous Fe-S clusters by using discrete Fourier transform calculations (19). Indeed, this latter study raised the question of whether [Fe 4 S 4 ] 0 Av2 can even be made in vivo (1). A value of -790 mV is not consistent with turnover potential measured by Ti(III) and other reductants and with reported potentials for the Ti(IV)͞Ti(III) couple (7,20). If an E m ϭ -460 mV for the 1ϩ͞0 couple (16)

Mammalian ferritin from horse spleen undergoes an electrochemical or chemical reduction reaction in which each iron atom present is reduced by one electron (2300 electrons per ferritin molecule containing 2300 Fe3+ ions). Midpoint potentials of -190 mV, -310 mV, and -416 mV were determined at pH 7.0, 8.0, and 9.0. This variation of potential with pH indicates that approximately 2 H+ are transferred to the core for each Fe3+ reduced to Fe2+. Mössbauer measurements of partially reduced ferritin give spectra that consist of a ferric quadrupole doublet with a superposed ferrous quadrupole doublet. The relative intensities of these doublets are consistent with the electrochemically determined degree of reduction.

Both mammalian and bacterial ferritin undergo rapid reaction with small-molecule reductants, in the absence of Fe2 chelators, to form ferritins with reduced (Fe+) mineral cores. Large, low-potential reductants (flavoproteins and ferredoxins) similarly react anaerobically with both ferritin types to quantitatively produce Fe2+ Ferritin is a 24-subunit protein that contains up to 4500 iron atoms in its hollow, nearly spherical interior in the form of a Fe(O)OH-type mineral core. The ubiquitous distribution of ferritin among plant and animal species and its presence in nearly every tissue type of highly differentiated organisms have led to the view that ferritin serves as the universal iron-storage protein in nature. In this role, ferritin provides a means for living systems to gain access to this essential mineral nutrient under conditions that otherwise favor the formation of hydrous ferric oxide, a biologically inert form of iron. Having such a control protein that stores iron and regulates the iron flux within its cells allows an organism to maintain effectively the balance between iron insufficiency and iron toxicity. Thus, the delineation of the mechanism by which ferritin functions in its iron-storage and -release roles is important in understanding the entry of iron into various metabolic cellular activities.Ferritin has been studied extensively (see refs. 1-3 for reviews) with regard to its biochemical characterization as well as its iron-storage and -release function. Of importance in visualizing the overall geometry of the ferritin molecule and details of its subunit arrangements and interactions have been the development and refinement to 2.8 A of a model (4-6) derived from crystallographic x-ray diffraction studies. Detailed analysis has revealed (6) the presence of channels along the threefold and fourfold axes (3-5 A across) that penetrate into the central cavity, through which small molecules (Fe2", reductants, oxidants, and iron chelators) are thought to enter and leave the ferritin interior during the The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. 7457processes of iron deposition and release (7)(8)(9)(10)(11)(12)(13)(14). Iron transport to and from the ferritin interior is a well-established feature offerritin, but the facile movement of other molecules into and out of the ferritin interior is less well documented. In fact, studies of direct transfer of small molecules into the ferritin interior suggest that moderate diffusional impediments exist with neutral molecules such as sucrose (15,16) and that serious transfer limitations occur with small anions such as acetate (17,18), indicating that both charge and size effects are important in channel penetration. Our own recent study (19), using reductants of nearly constant redox potential but of varying charge and cross-sectional area, has shown that the reduction potential is im...

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