Effects of salts and pH on the binding of thiocyanate with bovine plasma albumin.

The binding of SCNwith bovine plasma albumin has been studied at 4” by equilibrium dialysis in ethylenediaminetetraacetate and N-(carboxymethyl)-N’,-Z-hydroxyethylN, N’-ethylenediglycine solutions. These two supporting electrolytes were used to minimize Donnan effects. At low ionic strengths (0 to 0.04), the association constants of SCNwere found to increase with decreasing concentrations of supporting electrolyte. At high ionic strengths (0.1 to 0.8), binding was independent of supporting electrolyte concentration. In the pH region 4.5 to 10, the binding data were best fitted by two sets of binding constants: k10 = 700, nl = 7, kZo = 10, nz = 90. The electrostatic interaction factor, w, followed the Linderstrplm-Lang model in this pH region. In the pH regions below 4.5 and above 10, binding changed in a manner consistent with a marked reduction in w. Binding at the primary sites was unaltered in 2 M urea and slightly reduced in 4 M urea, but was destroyed in 8 M urea. Binding at the primary sites was reduced in 29% dioxane. The n2 sites were not much affected by urea or dioxane. Oleate inhibited binding at one of the primary sites. Several binding studies were also conducted with Iand Cl-. Our results from dialysis equilibrium in solutions with the supporting electrolytes do not agree in many respects with those reported by Scatchard and co-workers from electromotive force measurements in systems free of supporting electrolytes.