Interaction of human serum albumin with oxovanadium ions studied by FT-IR spectroscopy and gel and capillary electrophoresis

Some oxovanadium compounds have shown potential to inhibit RNase activity, while at the same time not inhibiting DNase activity. Some vanadyl complexe s also inhibit protein synthesis in rabbit reticulocytes, but induce activa tion of protein-tyrosine kinase. To gain an insight into the interaction of oxovanadium ions with proteins, the present study was designed to examine the bindings of VOSO4 and NaVO3 salts with human serum albumin (HSA) in aqu eous solution at physiological pH with metal ion concentrations of 0.0001 t o 1 mM and HSA (fatty acid free) concentration of 2% w/v. Gel and capillary electrophoresis (CE) and Fourier transform infrared (FT-IR) spectroscopic methods were used to determine the metal ion binding mode, association cons tant, and the secondary structure of the protein in the presence of the oxo vanadium compounds. Gel electrophoresis results showed that a maximum of 20 vanadyl cations (VO2+) are bound per HSA molecule with strong (K-1 = 7.0 x 10(7) M-1) and weak (K-2 = 6.5 x 10(5) M-1) bindings. Similarly, capillary electrophoresis showed two major bindings for vanadyl cation with K-1 = 1. 2 x 10(8) M-1 and K-2 = 8.5 x 10(5) M-1, whereas vanadate (VO3-) has only a weak binding affinity (K = 6.0 x 10(3) M-1) with HSA molecule. The VO3- bi nds mainly to the lysine epsilon -amino NH3+ groups, while VO2+ binds possi bly to the histidine nitrogen atom and the N-terminal of the alpha -amine r esidue. Infrared spectroscopic analysis showed metal ion binding results in major protein secondary structural changes from that of the alpha -helix ( 55.0 to 43-44%) to the beta -sheet (22.0 to 23-26%), beta -antiparallel (12 .0 to 13-16%), and turn (11.0 to 17-18%), at high metal ion concentration. The observed spectral changes indicate a partial unfolding of the protein s tructure, in the presence of oxovanadium ions.