Binding of indole-3-acetic acid to human serum albumin and competition with L-tryptophan

Indole-3-acetic acid (IAA) is a product of tryptophan (Trp) metabolism and is found to be markedly increased in uremic sera. IAA binding to defatted human serum albumin at 37 degrees C and pH 5, 7.4, and 8.5 was studied by equilibrium dialysis, and data were analyzed assuming two independent high affinity binding sites plus a class of low affinity sites. The estimated values of the association constant of dominant site were: 7.96 x 10(3) M-1 at pH 5, 11.57 x 10(3) M-1 at pH 7.4, and 6.30 x 10(3) M-1 at pH 8.5. The competition between IAA and Trp for albumin binding at pH 7.4 was investigated. The results suggest that one specific albumin site is common for IAA and Trp, but the data were not adequately predicted by a purely competitive scheme. A better prediction was achieved assuming that the binding of IAA to a site different from the common site inhibits Trp binding.