Abstract: The binding of La(Ⅲ) to human serum albumin (HSA) or bovine serum albumin(BSA) has been studied by equilibrium dialysis at pH_6.3. The results from Scatchard plots indicate that there are two strong sites and eight weak sites in HSA and two strong sites and six weak sites in BSA, respectively. The results of La(Ⅲ)competing with Cu(Ⅱ), Zn(Ⅱ), Cd(Ⅱ) for binding to HSAor BS Asuggest that one of the strong binding sites is most probably coordinated with atoms which are almost all oxygen. The successive stability constants which are reported for the first time are obtained by non-linear least square method fitting Bjerrum formula. For both La(Ⅲ)-HSA and La(Ⅲ)-BSA systems, the order of magnitude of K₁ was found to be 104. The analyses of Hill coefficient and free energy coupling show that some negative cooperative effect was found in both La(Ⅲ)-HSA and La(Ⅲ)-BSA systems.

Key words: La(Ⅲ)-serum albumin, Equilibrium dialysis, Coordination analysis, Successive stability constant, Cooperative effect