Iron transport into erythroid cells by the Na+/Mg2+ antiport

Rabbit erythroid cells can take up non-transferrin-bound iron by a high-affinity and a low-affinity transport mechanism (Hodgson et al. (1995) J. Cell. Physiol. 162, 181-190). The latter process, which is present in mature erythrocytes as well as reticulocytes, was investigated in this study using rabbit reticulocytes and erythrocytes. Iron uptake was optimal in isotonic KCI (pH 7.0), was shown to be much greater for Fe(II) than Fe(III), to be saturable with a Km value of approx. 15 microM Fe(II), temperature-dependent and inhibited by inhibitors of cell energy metabolism, by Na+ and many divalent cations and by several known inhibitors of membrane cation transport mechanisms. Uptake was more rapid with rabbit than with rat or human erythrocytes. The Fe(II) transport process was much more sensitive to inhibition by Mg2+ than by Ca2+ and the inhibition by both Mg2+ and Na+ was of competitive type. Cells depleted of intracellular Mg2+ by the use of the ionophore A23187 had low rates of Fe(II) uptake. High rates of uptake could be achieved by replenishment of intracellular Mg2+, and the Mg(2+)-dependent uptake of Fe(II) was inhibited by the same reagents which reduced the uptake by untreated cells. Many features of the Fe(II) transport process are very similar to those of the previously described Na+/Mg2+ antiport. These features, plus the stimulation of Fe(II) uptake by intracellular Mg2+ and inhibition by extracellular Mg2+ or Na+, strongly suggest that the iron is transported into the cells by the antiport.