Circular Dichroism Studies on the Interaction between Human Serum Albumin and Thyroxine

Abstract

The Cotton effects induced by the interaction of thyroxine (T₄) with human serum albumin (HSA) have been examined at various molar ratios of T₄ to HSA. The results suggested the following three distinct modes of interaction, depending on the molar ratio (n).
1. The first mode of interaction appeared at n=0-1. A large difference CD amplitude attributable to T₄ was observed in the region between 280 nm and 325 nm. This suggests that the T₄ molecule binds tightly to a highest affinity site of HSA. The difference CD spectra of HSA in the presence and absence of T₄ bound at the highest affinity site showed that the peak at 292 nm might be induced by an intrinsic dissymmetry owing to an asymmetric carbon atom in the T₄ molecule, while a CD band at 325 nm might be due to perturbation of the β-ring chromophore of T₄ by certain intrinsic loci of HSA at the binding site.
2. As the second mode of interaction at n=1_??_5-8, T₄ molecules may bind loosely to HSA with various conformations, because the difference CD amplitude at 325 nm was decreased.
3. The third mode was detected at n⟩5-8. The difference CD patterns is quite distinct from the others and is very similar to the CD band of T4: L-lysine mixture.