TITANIUM BINDING PATTERNS TO HUMAN TRANSFERRIN IN UNSPIKED SERUM SAMPLES: PREFERENTIAL BINDING SITES AND EFFECT OF SIALIC ACIDS
Yoana Nuevo-Ordóñez, Maria Montes-Bayón, Elisa Blanco-González, Alfredo Sanz-Medel, Clay Davis
Serum transferrin (Tf) is an iron binding glycoprotein that plays a central role in the metabolism of this essential metal. Taking into account that in serum only 30% of Tf is saturated with Fe, it can be possible to bind other metal ions such as in the case of titanium (Ti).  Previous work involving Ti speciation was carried out in vitro, with a previous addition of Ti into the serum. [2,3] This work represents the first quantitative speciation of Ti bound to Tf in vivo without spiking the serum sample, in spite of the low concentration of this element in real serum samples (<0.5 μg L-1 in control individuals and up to 12 μg L-1 in exposed patients with intramedullary implants) using a double focusing sector field inductively coupled plasma mass spectrometer (SF-ICP-MS). Additionally, given that Tf has two lobes for metal binding (C-lobe or N-lobe), with similar amino acid sequences but with different affinity for binding metal ions, we also report the preferential binding of Ti to the N-lobe of Tf using different gradient conditions, based on the studies carried out by Nagaoka et al. . These results suggest that the species FeC-TiN-Tf might provide a route for Ti entry into cells via the transferring receptors, after the release of the metal from the implants (considering that Fe2-Tf is the species that binds to the cellular receptors). This factor can be of major importance, considering that the presence of Ti inside the cell might induce oxidative stress and could interrupt important phosphorylation cascades (due to its binding with the phosphate groups in proteins).
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