Effect of divalent metal ions on the binding of thyroxine to bovine serum albumin as measured by fluorescence.

The binding of thyroxine (T4) to bovine serum albumin (BSA) has been studied in the presence and absence of Ca2+, Cu2+ and Zn2+ ions at various pH's in 0.1 M Tris-acetate buffer at 25 degrees C using the fluorescence method. In the presence of 50 microM Ca2+ and Zn2+ and the absence of metal ions, the binding constant (K) increased similarly with increasing pH values from pH 5 to pH 9, and the K value near midpoint, pH 7.4, was 1.66 +/- 0.17 x 10(6) M-1. By contrast, the binding constant remained constant between pH5 and pH9 in the presence of 10 microM Cu2+, with an average value of 1.61 +/- 0.22 x 10(6) M-1, suggesting a significant influence of Cu2+ ions on T4 binding to BSA.