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 radioiodinated placental isoferritins. Considerably more acidic isoferritin was bound than basic isoferritin. The binding of ferritin was significantly higher at 37 degrees C than at 4 degrees C. All of the 125I-acidic isoferritin bound at 4 degrees C, but only part of that bound at 37 degrees C, could be dislodged by the addition of 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 (Kd) 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 10(5) per cell. The results point to specific binding and receptor-mediated internalization for predominantly acidic isoferritin by developing human erythroid cells.