Binding of Oxovanadium(IV) Complexes to Blood Serum Albumins

Abstract

In this work the binding of V<sup>IV</sup>O<sup>2+</sup> and V<sup>IV</sup>O-complexes to serum albumins {human serum albumin (HSA), bovine serum albumin (BSA) and porcine serum albumin (PSA)} are studied using circular dichroism (CD), electron paramagnetic resonance (EPR) and visible absorption spectroscopy. The results confirm previous findings that V<sup>IV</sup>O<sup>2+</sup> occupies at least two types of binding sites on albumin: ‘the strong vanadium binding site’ (designated by VBS1) and ‘the weak vanadium binding sites’ (designated by VBS2). VBS1 binds 1 mol equivalent of V<sup>IV</sup>O<sup>2+</sup>. On the other hand VBS2 correspond to binding of several mol equivalents of V<sup>IV</sup>O, and studies done with PSA in the presence of excess Zn<sup>II</sup> ions indicate that VSB2 corresponds to two distinct types of sites. The hyperfine coupling constant A<sub>z</sub> for V<sup>IV</sup>O<sup>2+</sup> binding at VBS2 on HSA and BSA are all very similar (~168 × 10-4 cm<sup>-1</sup>) but differ slightly on PSA (~166 × 10-4 cm<sup>-1</sup>) due to differences in the binding sets. When (V<sup>IV</sup>O)-HSA systems are titrated with maltol ternary species of (maltol)<sub>m</sub>(V<sup>IV</sup>O)<sub>m</sub>HSA and (maltol)<sub>2m</sub>(V<sup>IV</sup>O)<sub>m</sub>HSA stoichiometry form which are clearly distinguishable from the binary (V<sup>IV</sup>O)-HSA system by the type and intensity of the CD spectra recorded. Changes are also observable in the intensity of the X-band EPR spectra, but not much in the hyperfine coupling constants A<sub>z</sub>, which are all in the range 166-167 × 10-4 cm<sup>-1</sup>. The results further demonstrate that the presence of maltol may enhance the binding of VIVO to albumin.

Vanadium and proteins: Uptake, transport, structure, activity and function

“…The value of A z is consistent with non-specific interactions with carboxylate and/or imidazole side chains of Asp/Glu and His accessible residues. V IV O 2+ remains bound to defatted BSA (dBSA) when Zn II was added and this confirms the presence of a another type of binding site besides MBS; Costa-Pessoa and coworkers suggested that also the amino acid side chains of the NTS site may be involved in the metal coordination, but with a distinct binding set from that of Cu II and Zn II [146].…”

“…From an examination of the EPR spectra measured at room temperature at pH 5, Chasteen and Francavilla distinguished one "strong" (recently designed with VBS1 [146]) and at least five "weak" binding sites (named VBS2 [146]) having different EPR parameters [143]. These results were subsequently confirmed by Willsky et al [96] and…”

“…Recently, it was shown that at physiological pH the main (V IV OL 2 ) n HSA species can co-exist with a minor (V IV OL) n HSA mixed complex, and CD spectra even suggested more than two types of binding sites [146]; this appears to be in agreement with the observation of Orvig and co-workers [97]. A c c e p t e d M a n u s c r i p t 26 sufficiently long lifetime in the blood stream to allow correlation of its dose-dependent action with blood vanadium content.…”

“…The binary system V IV O 2+ /HSA has been studied by several techniques [84,96,97,99,100,143,144,145] [146]. From an examination of the EPR spectra measured at room temperature at pH 5, Chasteen and Francavilla distinguished one "strong" (recently designed with VBS1 [146]) and at least five "weak" binding sites (named VBS2 [146]) having different EPR parameters [143].…”

Thirty years through vanadium chemistry

“…Several studies involving the binding of V IV O 2+ and V IV O-complexes to human serum albumin (HSA) were also made by using CD, EPR and gel electrophoresis [190,[230][231][232].…”