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Ratio of transferrin (Tf) to Tf-receptor complex in circulation differs depending on Tf iron saturation.

Serum transferrin receptor (TfR) is mostly derived from bone marrow erythroblasts, and TfR concentrations are increased by both enhanced erythropoiesis and iron deficiency (1)(2)(3)(4)(5). Several studies have shown that serum TfR measurements are especially useful in the differential diagnosis of iron deficiency anemia (IDA) and anemia of chronic disease (ACD) (6)(7)(8). Soluble TfR released by cleavage of membrane receptors between amino acids 100 and 101 (Arg-Lys) just above the cell membrane and purified from human serum is able to rebind with transferrin (Tf) (9)(10)(11). However, details regarding the molecular structure of the soluble TfR present in serum or plasma have not been clarified. Previously it was shown that in serum, truncated TfR exists as a complex with Tf (9)(11), and we have demonstrated by HPLC size fractionation that the predominant form of serum TfR is a dimeric TfR in complex with Tf (12). A study on the binding of radiolabeled Tf to the surface TfR of K562 human myelogenous leukemia cells showed that the binding affinity of monoferric Tf was reduced to approximately one-fourth that of diferric Tf and that the binding affinity of apo-Tf was nearly zero (13)(14). Therefore, it is plausible that in serum, Tf-TfR binding affinity is decreased under conditions associated with decreased Tf iron saturation and that this may affect the ratio of these two substances in complex formation. In the present study, we examined whether there is a difference in the molecular structure of serum TfR between healthy individuals and patients with decreased Tf iron saturation, such as in cases of IDA and ACD.

Blood samples were collected from patients who were diagnosed as having IDA (n = 40; defined as serum ferritin <12 μg/L …