SERUM UREMIC TOXINS FROM PATIENTS WITH CHRONIC-RENAL-FAILURE DISPLACETHE BINDING OF L-TRYPTOPHAN TO HUMAN SERUM-ALBUMIN

The level of free tryptophan (Trp) and its metabolites in serum appear s to be related to some pathologic states, such as chronic renal failu re and neuropsychiatric disorders, so that a precise characterization of tryptophan binding to serum albumin is of interest. Ln the present paper, the binding of L-tryptophan to defatted human serum albumin at 37 degrees C and at pH 7.4 was studied by means of equilibrium dialysi s. The competition between L-tryptophan and serum solutes extracted fr om uremic patients undergoing hemodialysis, before dialysis treatment, was also investigated; Solutes were extracted from uremic pools of se ra using two different deproteinization methods: serum ultrafiltration and heat denaturation of serum proteins followed by ultrafiltration. We found 1.10 +/- 0.03 binding sites for Trp to defatted albumin with an association constant 11.37 +/- 1.03 x 10(3) M(-1). The competition experiments suggested that the number of Trp binding sites were not si gnificantly modified by the addition of solutes obtained with the meth od of ultrafiltration with respect to the binding of L-tryptophan to a lbumin in the absence of competitors, while their affinity constant wa s markedly reduced (2.66 +/- 0.18 x 10(3) M(-1)). Moreover, a signific ant reduction of the affinity constant was observed when competitors f or Trp were obtained using heat deproteinization associated with ultra filtration (1.91 +/- 0.15 x 10(3) M(-1) vs. 2.66 +/- 0.18 x 10(3) M(-1 ); P < 0.005). These results might be ascribed to the fact that the la st procedure has a higher yield with a more complete liberation of ure mic toxins from serum proteins, so that they became probably totally f ree thus competing at higher extent with L-tryptophan for albumin bind ing sites. (C) 1997 Elsevier Science B.V.