The binding of thyroxine by serum albumin as measured by fluorescence quenching.

The binding of thyroxine by bovine and human serum albumin has been studied by measurements of the quenching of the ultraviolet fluorescence of the protein. The results obtained by this method have been compared with those of equilibrium dialysis measurements. Both approaches agree in assigning to serum albumin a single binding site of high affinity and three or more weak sites. The binding constant for the combination of albumin with a single molecule of thyroxine is 1.6 X lo6 f 25% at pH 7.4. At pH 7.4 the binding constant for triiodothyronine is about one-sixth that for thyroxine.