Human transferrin is a bilobal protein, with two Fe(III) binding sites displaying different spectroscopic, kinetic, and thermodynamic properties despite identical ligand donor groups, We present a spectroelectrochemical determination of the formal Fe(III/II) redox potentials for all possible permutations of this Fe-binding protein : Fe-saturated diferric (Fe2Tf), monoferric C lobe (FeCTf), and monoferric N-lobe (FeNTf), at endosomal pH (5.8), 20 degrees C, and 0.5 M KCl, The spectroelectrochemical technique utilized an anaerobic optically transparent thin-layer electrochemical (OTTLE) cell and methyl viologen mediator. Correction was made for the (FenTf)-Tf-II (n = 1, 2) dissociation equilibrium using an estimated value for the Fe(II)-transferrin binding constant st pH 7.4 in order to calculate upper limits of the redox potentials at FH 5.8 for Fe2Tf, FeCTf, and FeNTf of -526, -501, and -520 mV (vs NHE), respectively. Reduction of Fe2Tf is by two-electron transfer. This study represents the first direct determination of the Fe(III/II) transferrin redox potentials for all Fe-bound permutations of the protein at physiological conditions relevant to the endosomal release of Fe. Our results suggest that the two redox sites are similar and that the redox potentials an too negative for Fe(III) reduction by a physiological reducing agent to be involved in endosomal release of Fe from transferrin in the absence of a reaction sequence sequestering Fe(II) or effecting a conformational change in transferrin that substantially raises the reduction potential of its Fe(III), No such sequence is known.