Interaction of levothyroxine with bovine serum albumin: a spectroscopic assay

Abstract

Bovine serum albumin (BSA) acts as a carrier for many endogenous and exogenous compounds, such as thyroid hormones or corresponding drugs. Binding of the hydrophilic compound levothyroxine (LT4) to BSA can significantly alter the pharmacological properties of the compound. Therefore, studying its interaction with BSA could be a difficult issue. In this work, the binding mechanism and affinity of the interaction between LT4 and BSA were investigated, both in solution using UV-Vis, Fourier-transform infrared spectroscopy (FT-IR), fluorescence and fluorescence resonance energy transfer (FRET), as well as by Surface Plasmon Resonance (SPR) with BSA confined to a gold-coated chips, as far as we know for the first time used to study the interactions between LT4 and proteins. Quenching of BSA fluorescence by LT4 combined with UV-Vis spectroscopy shows a ground-state complex formation that may be accompanied by a nonradiative energy transfer process. FT-IR revealed the changes induced by LT4 in the secondary structure of BSA molecules, due to the partial unfolding of BSA native structure upon LT4 binding. Scatchard approach allowed the determination of the binding constant and the thermodynamic parameters, which correspond to an enthalpic process, driven mainly by hydrogen bonds and van der Waals forces. Using SPR, the adsorbed amount of biomolecules was calculated and the binding affinity of LT4 with confined-BSA was characterized using the Hill-Langmuir equation, indicating that the BSA immobilization plays an important role in LT4 binding. As preliminary results, both fluorescence quenching and SPR can be used as a stepping stone for the development of a spectroscopic biosensor for LT4 detection, with a limit of detection as low as 0.23 x 10-6 M.