Circular dichroism studies on the interaction between human serum albumin and thyroxine

The Cotten 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 beta-ring chromophore of T4 by certain intrinsic loci of HSA at the binding site. 2. As the second mode of interaction at n=1 approximately 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 greater than 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.