A spectroscopic study of nickel(II)-bovine serum albumin binding and reactivity

The pH dependence of the uv/visible and CD spectra of the 1:1 Ni(BSA) complex in aqueous solutions is interpreted in terms of a major square-planar form and an octahedral form. At pH 7.4, the two forms, respectively, account for ca. 70% and 30% of the total Ni(II). The two forms are in rapid equilibrium with each other and so both probably involve Ni(II) binding to the N-terminal region of the albumin protein. The kinetics of the equilibrium reaction of Ni(BSA) with His were studied at 37 degrees C in buffered media of pH 7.4 and 9.3. In line with predictions, the two Ni(BSA) forms show markedly different reactivities, with the square-planar form being the more thermodynamically stable and the less reactive. The octahedral form reacts with an observed zero-order dependence on His concentration while the square-planar form shows both zero-order and first-order dependence, the latter being the more dominant. The significance of the slow equilibrium rate at pH 7.4 to the possible physiological role of Ni-albumin in blood serum is discussed.