Binding and uptake of exogenous isoferritins by cultured human erythroid precursor cells

The interaction of extracellular human isoferritins with normal erythroid precursors developing in a two-phase liquid culture was studied. Cells at the stage of polychromatic normoblasts exhibited substantial specific binding of radio-iodinated placental isoferritins. Considerably more acidic isoferritin was bound than basic isoferritin. The binding of ferritin was significantly higher at 37°C than at 4°C. All of the ¹²⁵I-acidic isoferritin bound at 4°C, but only part of that bound at 37°C, could be dislodged by the addition of a 500-fold excess of non-labelled acidic isoferritin. Acidic isoferritin displaced radio-iodinated acidic isoferritin from the erythroid cells more efficiently than intermediate or basic isoferritins. Kinetic analysis suggests a dissociation constant (K(d)) of 3.9 x 10^-8 M for acidic ferritin and 3.7 x 10^-7 M for basic isoferritin. The average number of binding sites for acidic isoferritin was 1.3 x 10⁵ per cell. The results point to specific binding and receptor-mediated internalization for predominantly acidic isoferritin by developing human erythroid cells.