The role of transferrin in actinide(IV) uptake: comparison with iron(III).
ABSTRACT The impact of actinides on living organisms has been the subject of numerous studies since the 1950s. From a general point of view, these studies show that actinides are chemical poisons as well as radiological hazards. Actinides in plasma are assumed to be mainly complexed to transferrin, the iron carrier protein. This paper casts light on the uptake of actinides(IV) (thorium, neptunium, plutonium) by transferrin, focusing on the pH dependence of the interaction and on a molecular description of the cation binding site in the protein. Their behavior is compared with that of iron(III), the endogenous transferrin cation, from a structural point of view. Complementary spectroscopic techniques (UV/Vis spectrophotometry, microfiltration coupled with gamma spectrometry, and X-ray absorption fine structure) have been combined in order to propose a structural model for the actinide-binding site in transferrin. Comparison of our results with data available on holotransferrin suggests some similarities between the behavior of Fe(III) and Np(IV)/Pu(IV)/ Np(IV) is not complexed at pH <7, whereas at pH approximately 7.4 complexation can be regarded as quantitative. This pH effect is consistent with the in vivo transferrin "cycle". Pu(IV) also appears to be quantitatively bound by apotransferrin at around pH approximately 7.5, whereas Th(IV) was never complexed under our experimental conditions. EXAFS data at the actinide edge have allowed a structural model of the actinide binding site to be elaborated: at least one tyrosine residue could participate in the actinide coordination sphere (two for iron), forming a mixed hydroxo-transferrin complex in which actinides are bound with transferrin both through An-tyrosine and through An--OH bonds. A description of interatomic distances is provided.
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ABSTRACT: Plutonium can enter the body through different routes and remains there for decades; however its specific biochemical interactions are poorly defined. We, for the first time, have studied plutonium-binding proteins using a metalloproteomic approach with rat PC12 cells. A combination of immobilized metal ion chromatography, 2D gel electrophoresis, and mass spectrometry was employed to analyze potential plutonium-binding proteins. Our results show that several proteins from PC12 cells show affinity towards Pu(4+)-NTA (plutonium bound to nitrilotriacetic acid). Proteins from seven different spots in the 2D gel were identified. In contrast to the previously known plutonium-binding proteins transferrin and ferritin, which bind ferric ions, most identified proteins in our experiment are known to bind calcium, magnesium, or divalent transition metal ions. The identified plutonium interacting proteins also have functional roles in downregulation of apoptosis and other pro-proliferative processes. MetaCore™ analysis based on this group of proteins produced a pathway with a statistically significant association with development of neoplastic diseases.Journal of proteomics 12/2011; 75(5):1505-14. · 5.07 Impact Factor
- Gastroenterology 01/2011; 140(5). · 12.82 Impact Factor
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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 (Kd1 = 5 nM and Kd2 = 20 nM), while the lowest affinity was measured for Pu2Tf (Kd1 = 0.28 μM and Kd2 = 1.8 μM) binding to the TfR. Other toxic metal ions such as ThIV and UVI, 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+ ∼ 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.Metallomics 02/2013; · 4.10 Impact Factor