Thyroxine binding to human serum albumin immobilized on sepharose and effects of nonprotein albumin-binding plasma constituents.

125I-thyroxine (125I-T4) binding to human serum albumin (HSA) covalently attached onto CNBr-activated Sepharose (HSA-Sepharose) was studied. 125I-T4 binding to HSA-Sepharose was rapid and saturable. Nonlinear curve-fitting analysis of binding isotherms revealed two classes of binding sites. The values of dissociation constants of high and low affinity sites were 2.19 +/- 0.53 x 10(-6) M and 2.69 +/- 0.78 x 10(-5) M, respectively. The number of binding sites of the high and the low affinity sites were 1.28 +/- 0.46 mol/mol and 23.5 +/- 9.7 mol/mol of HSA, respectively. Fatty acids and bilirubin competitively inhibited the high-affinity binding of 125I-T4 to HSA-Sepharose without affecting the low-affinity binding. 8-anilino-1-naphthalene sulfonic acid (ANS) inhibited the high affinity T4 binding via reduction of the binding capacity. Unlabeled T4 showed little inhibition of ANS binding to HSA, as measured by fluorescence intensity. These results suggest that ANS allosterically inhibits the high-affinity T4 binding to HSA-Sepharose.