Human transferrin (Tf) very tightly binds two ferric ions to deliver iron to cells. Fe(III)(2)Tf (Fe(2)Tf) binds to the Tf receptor (TfR) at pH 7.4; however, iron-free Tf (apoTf) does not. Iron uptake is facilitated by endocytosis of the Fe(2)Tf-TfR complex. Tf can also bind aluminum ions, which cause toxic effects and are associated with many diseases. Since Al(III)(2)Tf (Al(2)Tf) does not bind to TfR, the uptake of aluminum by the cells does not occur through a TfR-mediated pathway. We have studied the absence of binding between Al(2)Tf and TfR by investigating the physicochemical characteristics of apoTf, Al(2)Tf, Fe(2)Tf, and TfR. The hydrodynamic radius of 38.8 A for Al(2)Tf obtained by dynamic light scattering was between that of 42.6 A for apoTf and 37.2 A for Fe(2)Tf. The zeta potential of -11.3 mV for Al(2)Tf measured by capillary electrophoresis was close to -11.2 mV for apoTf as compared to -11.9 mV for Fe(2)Tf, indicating that the Al(2)Tf surface had a relatively scarce negative charge as the apoTf surface had. These results demonstrated that the structure of Al(2)Tf was a trade-off between the closed and open forms of Fe(2)Tf and apoTf, respectively. Consequently, it is suggested that Al(2)Tf cannot form specific ionic interresidual interactions, such as those formed by Fe(2)Tf, to bind to TfR, resulting in impossible complex formation between Al(2)Tf and TfR.