Effects of various salts on the binding of indole compounds with bovine albumin.

Influences of different concentrations of salts (0.002 to 1.25 eq per liter) on the binding of skatole and acetyl-Ltryptophan at the primary binding site of bovine plasma albumin have been studied by equilibrium dialysis. With the exception of Naz-ethylenediaminetetraacetate and ethylenediammonium aspartate, which have little effect, all salts in the concentration range of 0.005 to 0.1 eq per liter inhibit the binding of acetyl-L-tryptophan; their order of effectiveness, which is invariant with salt concentration, is KC104 > KSCN > KI > LiCl > CaClt > KF > KC1 > NaCl > ethylenediammonium chloride > KzS04. With skatole the addition of halide-like salts (except KF) in the concentration range of 0 to 0.05 eq per liter increases the binding markedly. The order at 0.002 eq per liter solute is KC104 > KSCN > KI > ethylenediarnmonium chloride > KC1 > LiCl > CaCls (this order is consistent with increased skatole binding with increased salt anion binding). Between 0.05 and 0.25 eq per liter binding of skatole decreases with further addition of KC104 and KSCN, remains approximately constant with further addition of KI, but increases with further addition of the Clsalts. Other solutes have no effect (Naz-ethylenediaminetetraacetate, ethylenediammonium aspartate, K2S04, and glycine), or a small enhancing effect (KF) on skatole binding. Between 0.25 and 1.25 eq per liter the addition of most salts increases the binding of skatole. The salt concentration binding profile of p-3indolylethanol in KSCN solutions is similar to that of acetylL-tryptophan, implying that the charge of the acetyl-Ltryptophan is not the element which leads to the differences between its binding and that of skatole. Effects of SCNand Clon acetyl-L-tryptophan-albumin binding show direct competitive inhibition as well as anomalous ionic strength effects. It is concluded that the primary influence of salts is against the polar elements of the binding center.