BINDING AND UPTAKE OF EXOGENOUS ISOFERRITINS BY CULTURED HUMAN ERYTHROID PRECURSOR CELLS

The interaction of extracellular human isoferritins with normal erythr oid precursors developing in a two-phase liquid culture was studied. C ells at the stage of polychromatic normoblasts exhibited substantial s pecific 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 degree s C. All of the I-125-acidic isoferritin bound at 4 degrees C, but onl y part of that bound at 37 degrees C, could be dislodged by the additi on of a 500-fold excess of non-labelled acidic isoferritin. Acidic iso ferritin displaced radio-iodinated acidic isoferritin from the erythro id cells more efficiently than intermediate or basic isoferritins. Kin etic 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 ave rage number of binding sites for acidic isoferritin was 1.3 x 10(5) pe r cell. The results point to specific binding and receptor-mediated in ternalization for predominantly acidic isoferritin by developing human erythroid cells.