The interaction between human and bovine serum albumin and zinc studied by a competitive spectrophotometry

The binding of zinc to human serum albumin (HSA) and bovine serum albumin (BSA) has been studied by a competitive spectrophotometry. The indictor ligand, 5-Br-PAPS, as well as its metal complexes, ML and ML2, having intense colors with sufficient separate absorption peaks permits the analysis of the interaction between zinc and HSA or BSA. The results show that zinc binds to both albumins in the molar ratio of 1:1 at pH 6-8. The apparent association constants of Zn-HSA and Zn-BSA decrease with decreasing pH. The pH dependence of the constants for both Zn-HSA and Zn-BSA indicates the 1:1 competition between metal ions and hydrogen ions for the binding site. The intrinsic association constants of the metal and the proton are found to be log KZn-HSA = 7.1 +/- 0.2 and log KH-HSA = 8.2 +/- 0.3 for HSA and log KZn-BSA = 7.6 +/- 0.2 and log KH-BSA = 8.2 +/- 0.3 for BSA, respectively.