Binding interaction of isoxsuprine hydrochloride and levothyroxine to milk β-lactoglobulin; from the perspective of comparison.

Isoxsuprine hydrochloride (ISO) and levothyroxine (LEV) are medicines which can be utilized alone or simultaneously by pregnant women. The purpose of this work is to investigate the separate and simultaneous interaction of ISO and LEV with β-LG. The results showed that both drugs can bind to β-LG; the static quenching was suggested for fluorescence quenching mechanism of β-LG.The values of binding constants (Kβ-LG-ISO  = 2.69 × 104  M-1 , Kβ-LG-LEV  = 0.54 × 103  M-1 and Kβ-LG-ISO-LEV  = 2.18 × 103  M-1 at 310 K) suggested that ISO has stronger binding affinity toward β-LG than LEV and affinity of β-LG to LEV is increased in the presence of ISO while the presence of LEV has no significant effect on the affinity of protein to ISO. Thermodynamic parameters showed that the binding of LEV to β-LG are hydrogen bonding and Van der Waals forces but the formation of β-LG-ISO is hydrophobic associations. The results of FT-IR and UV-visible measurements indicated that the binding of both drugs to β-LG may induce conformational changes of protein. In silico molecular docking analyses confirmed that ISO and LEV binds to residues located at site I and site II of β-LG, respectively.