Calorimetric, Spectroscopic, and Model Studies Provide Insight into the Transport of Ti(IV) by Human Serum Transferrin

Abstract

Evidence suggests that transferrin can bind Ti(IV) in an unhydrolyzed form (without bound hydroxide or oxide) or in a hydrolyzed form. Ti(IV) coordination by N,N'-di(o-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED) at different pH values models the two forms of Ti(IV)-loaded transferrin spectrally and structurally. 13C NMR and stopped-flow kinetic experiments reveal that when the metal is delivered to the protein using an unhydrolyzed source, Ti(IV) can coordinate in the typical distorted octahedral environment with a bound synergistic anion. The crystal structure of TiHBED obtained at low pH models this type of coordination. The solution structure of the complex compares favorably with the solid state from pH 3.0 to 4.0, and the complex can be reduced with E1/2 = -641 mV vs NHE. Kinetic and thermodynamic competition studies at pH 3.0 reveal that Ti(citrate)3 reacts with HBED via a dissociative mechanism and that the stability of TiHBED (log beta = 34.024) is weaker than that of the Fe(III) complex. pH stability studies show that Ti(IV) hydrolyzes ligand waters at higher pH but still remains bound to HBED until pH 9.5. Similarly, at a pH greater than 8.0 the synergistic anion that binds Ti(IV) in transferrin is readily displaced by irreversible metal hydrolysis although the metal remains bound to the protein until pH 9.5. Thermal denaturation studies conducted optically and by differential scanning calorimetry reveal that Ti(IV)-bound transferrin experiences only minimal enhanced thermal stability unlike when Fe(III) is bound. The C- and N-lobe transition Tm values shift to a few degrees higher. The stability, competition, and redox studies performed provide insight into the possible mechanism of Ti2-Tf transport in cells.

Cytotoxicity of a Ti(IV) compound is independent of serum proteins

“…At pH 7.4, Tf and SA were dialyzed against [TiO (HBED)] − at micromolar concentrations. After dialyzing against metal-free buffer, the Ti(IV) content in the protein solutions was measured by a colorimetric assay (33). The test revealed the absence of Ti(IV).…”

Binding and hydrolysis studies of antitumoural titanocene dichloride and Titanocene Y with phosphate diesters

“…The intensity of reported IR signals is defined as w = weak, m = medium, s = strong, vs = very strong, br = broad. Quantitative Ti analyses were carried out spectrophotometrically by chelation with 2,3-dihydroxynaphthalene-6-sulfonate according to the literature method [41]. Ti-containing precipitates were dissolved by heating in 7 M nitric acid.…”

Cytotoxic Titanium Salan Complexes: Surprising Interaction of Salan and Alkoxy Ligands

“…Although it is necessary to strip off the lig-A C H T U N G T R E N N U N G ands when Ti IV is trafficked via transferrin, [33][34][35][36][37] a route via albumin could leave the complexes intact. An adduct of the complex stabilized by albumin might enter the cell, which would promote a more active role for these drugs in contrast to the prodrug role proposed for the transferrin delivery mechanism.…”