TRANSFER OF IRON FROM SERUM IRON-BINDING PROTEIN TO HUMAN RETICULOCYTES*

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Abstract
Iron bound to purified human iron binding protein (IBP) or to conalbumin, the iron binding protein to immature, not to mature, mammalian red cells. Iron transfer is dependent upon the extent of saturation of IBP, and there exists a competition for iron between IBP and iron-binding receptors on the reticulocyte membrane. These cellular receptors are destroyed by certain proteolytic enzymes. Iron appears to be directly transferred to these receptors without existing in a free form, and iron once bound to reticulocyte membranes is not exchangeable with free iron and cannot be eluted with high concentrations of IBP or of ethylene diamine tetra-acetic acid (EDTA). The transfer of IBP-iron to immature red cells, unlike the adsorption of free iron, is related to metabolic energy-producing processes. Thus, it is accelerated by glucose and O2 and is diminished or blocked by low concentrations of metabolic inhibitors, by nonphysiologic temperatures and by hemolysis. The uptake of iron is not primarily linked to heme synthesis. Appropriate concentrations of Pb++ blocked iron incorporation into heme by reticulocytes without reducing iron transfer, thus causing a pile-up of iron on the cell membranes .