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 d efatted 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 s ites. The estimated values of the association constant of dominant sit e were: 7.96 x 10(3) M-1 at pH 5, 11.57 x 10(3) M-1 at pH 7.4, and 6.3 0 x 10(3) M-1 at pH 8.5. The competition between IAA and Trp for album in binding at pH 7.4 was investigated. The results suggest that one sp ecific albumin site is common for IAA and Trp, but the data were not a dequately predicted by a purely competitive scheme. A better predictio n was achieved assuming that the binding of IAA to a site different fr om the common site inhibits Trp binding. (C) 1997 Elsevier Science E.V .