Energy-dependent accumulation of iron by isolated rat liver mitochondria. Requirement of reducing equivalents and evidence for a unidirectional flux of Fe(II) across the inner membrane.

The relationship between the rate of endogenous respiration and the energy-dependent accumulation of iron and calcium was studied in rat liver mitochondria energized by external ATP and inhibited to a variable extent by rotenone. In contrast to the uptake of calcium, that of iron revealed an absolute requirement for reducing equivalents supported by the respiratory chain, which indicates that iron crosses the inner membrane only in the ferrous form. Experimental evidence is presented that a primary event of the energy-linked uptake of iron is that Fe(III) is bound to ligands on the C-side of the inner membrane, ligands which have a unique microenvironment giving the metal a half-reduction potential which is sufficiently high to establish a oxidation-reduction equilibrium with the respiratory chain at the level of cytochrome c. In addition, evidence is presented that this accumulation represents a unidirectional flux of Fe(II) from the C-side to the M-side of the inner membrane and the matrix where it is tightly bound to ligands (proteins?) not yet characterized. As expected, the energy-dependent accumulation of iron is accompanied by an internal alkalinization of the mitochondria analogous to that observed for calcium. A schematic model of the energized accumulation of iron by rat liver mitochondria is presented.