The binding of Cd(II) to human serum albumin (HSA) or bovine serum albumin (BSA) has been studied by equilibrium dialysis at physiological pH(7.43). The successive stability constants for the Cd (II)-HSA and Cd(II)-BSA systems are determined for the first time by non-linear least-square method fitting Bjerrum formula, the order of magnitude of the K~1-K~3 in both systems is found to be 10^4. The analyses of Hill coefficients and free energy coupling indicate that the strong positive cooperative effect exists in both Cd(II)-HSA and Cd(II)-BSA systems, and the positive cooperative effect in Cd(II)-HSA system is much stronger than that in Cd(II)-BSA system. The Scatchard analysis indicates that there are three strong binding sites for Cd (II) in both HSA and BSA. The probable locations and (or) coordinated atoms of the sites are also suggested based on the results of Cd(II) competing with Cu(II), Zn(II) or Ca(II) to bind to HSA or BSA.

Key words: CADMIUM, SERUM ALBUMIN, SYNERGISTIC EFFECT, DIALYSIS, LEAST SQUARE METHODS