Binding of Oxovanadium(IV) Complexes to Blood Serum Albumins

Abstract

In this work the binding of V^IVO²⁺ and V^IVO-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^IVO²⁺ 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^IVO²⁺. On the other hand VBS2 correspond to binding of several mol equivalents of V^IVO, and studies done with PSA in the presence of excess Zn^II ions indicate that VSB2 corresponds to two distinct types of sites. The hyperfine coupling constant A_z for V^IVO²⁺ binding at VBS2 on HSA and BSA are all very similar (~168 × 10-4 cm^-1) but differ slightly on PSA (~166 × 10-4 cm^-1) due to differences in the binding sets. When (V^IVO)-HSA systems are titrated with maltol ternary species of (maltol)_m(V^IVO)_mHSA and (maltol)_2m(V^IVO)_mHSA stoichiometry form which are clearly distinguishable from the binary (V^IVO)-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_z, which are all in the range 166-167 × 10-4 cm^-1. The results further demonstrate that the presence of maltol may enhance the binding of VIVO to albumin.