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

Abstract

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⁺, ²³²Th⁴⁺, ²³⁸UO₂²⁺, and ²⁴²Pu⁴⁺. 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 μM and K_d2 = 1.8 μM) 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:(M_xTf)_y adducts follow the order Fe³⁺ ≫ Th⁴⁺ ∼ UO₂²⁺ ∼ Cm³⁺ > Ln³⁺ ∼ Ga³⁺ ⋙ Yb³⁺ ∼ Pu⁴⁺. This study substantiates a role for Tf in binding lanthanide fission products and actinides, and transporting them into cells by receptor-mediated endocytosis.