The Binding of Thyroid Hormones to Bovine Serum Albumin as Measured by Circular Dichroism

The circular dichroism (CD) spectra of 3, 5, 3′-triiodothyronine-bovine serum albumin (T3-BSA) and thyroxine-bovine serum albumin (T4-BSA) complexes were measured at the various ratios of T3 or T4 to BSA (T/P) in the wavelength region of 200–350 nm. No spectral change was observed in the wavelength region of 200–250 nm, but induced Cotton effect was observed in the chromophoric wavelength region of 250–350 nm. The value of [ø] at 319 nm of the induced Cotton effect increased with increase of the T/P ratio in both complexes, and in T4-BSA complex, it increased remarkably. The number of thyroid hormone molecules bound to one BSA molecule was estimated to be one in the case of T4-BSA complex and four to five in T3-BSA.