EQUILIBRIUM DIALYSIS OF METAL-SERUM ALBUMIN .1. SUCCESSIVE STABILITY-CONSTANTS OF ZN(II)-SERUM ALBUMIN AND THE ZN2-INDUCED CROSS-LINKING SELF-ASSOCIATION()

The binding of Zn(II) to human serum albumin (HSA) and bovine serum al bumin (BSA) was intensively studied by equilibrium dialysis. The succe ssive stability constants were obtained by least-squares fitting. For both the Zn(II)-HSA and Zn(II)-BSA systems, the successive stability c onstants are basically similar, though such constants for the latter g enerally slightly larger than those for the former; the order of magni tude of K-1 and K-2 was found to be approximate to 10(5) M(-1). The tw elve binding sites found for Zn(II)-HSA and fourteen binding sites for Zn(II)-BSA can be divided into three and two different sets, respecti vely, but in both systems, there exist approximately three identical s trongest binding sites. The binding equilibrium of Zn(II)-HSA markedly depends on the concentration of HSA. The type of Scatchard plots indi cates the existence of the Zn2+-induced cross-linking self-association of HSA, which was found from Wyman plots to be the strongest at a con centration of Zn2+ in the range (4-6) x 10(-6) M. The maximum of the a pparent self-association constants k(2) so obtained was approximately 2.6 x 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 tha t the zinc group ions are bound to the cysteinyl sulphur atoms.