Equilibrium dialysis of metal-serum albumin. I. Successive stability constants of Zn(II)-serum albumin and the Zn 2+-induced cross-linking self-association

Abstract

The binding of Zn(II) to human serum albumin (HSA) and bovine serum albumin (BSA) was intensively studied by equilibrium dialysis. The successive stability constants were obtained by least-squares fitting. For both the Zn(II)-HSA and Zn(II)-BSA systems, the successive stability constants are basically similar, though such constants for the latter generally slightly larger than those for the former; the order of magnitude of K 1 and K 2 was found to be ≈ 10 5 M −1. The twelve binding sites found for Zn(II)-HSA and fourteen binding sites for Zn(II)-BSA can be divided into three and two different sets, respectively, but in both systems, there exist approximately three identical strongest binding sites. The binding equilibrium of Zn(II)-HSA markedly depends on the concentration of HSA. The type of Scatchard plots indicates the existence of the Zn 2+-induced cross-linking self-association of HSA, which was found from Wyman plots to be the strongest at a concentration of Zn 2+ in the range (4-26) × 10 −6 M. The maximum of the apparent self-association constants k 2 so obtained was approximately 2.6 × 10 4 M −1. Such a self-association mechanism and the order of magnitude of K 1, and K 2 both to some extent support the inference that the zinc group ions are bound to the cysteinyl sulphur atoms.