Receptor recognition of transferrin bound to lanthanides and actinides: a discriminating step in cellular acquisition of f-block metals

Following an internal contamination event, the transport of actinide (An) and lanthanide (Ln) metal ions through the body is facilitated by endogenous ligands such as the human iron-transport protein transferrin (Tf). The recognition of resulting metallo-transferrin complexes (M2Tf) by the cognate transferrin receptor (TfR) is therefore a critical step for cellular uptake of these metal ions. A high performance liquid chromatography-based method has been used to probe the binding of M2Tf with TfR, yielding a direct measurement of the successive thermodynamic constants that correspond to the dissociation of TfR(M2Tf)(2) and TfR(M2Tf) complexes for Fe3+, Ga3+, La3+, Nd3+, Gd3+, Yb3+, Lu3+, Th-232(4+), (UO22+)-U-238, and Pu-242(4+). Important features of this method are (i) its ability to distinguish both 1:1 and 1:2 complexes formed between the receptor and the metal-bound transferrin, and (ii) the requirement for very small amounts of each binding partner (<1 nmol of protein per assay). Consistent with previous reports, the strongest receptor affinity is found for Fe2Tf (K-d1 = 5 nM and K-d2 = 20 nM), while the lowest affinity was measured for Pu2Tf (K-d1 = 0.28 mu M and K-d2 = 1.8 mu M) binding to the TfR. Other toxic metal ions such as Th-IV and U-VI, when bound to Tf, are well recognized by the TfR. Under the described experimental conditions, the relative stabilities of TfR:(MxTf)(y) adducts follow the order Fe3+ >> Th4+ similar to UO22+ similar to Cm3+ > Ln(3+) similar to Ga3+ >>> Yb3+ similar to Pu4+. This study substantiates a role for Tf in binding lanthanide fission products and actinides, and transporting them into cells by receptor-mediated endocytosis.