Protein-binding interactions are displacement reactions which have been implicated as the causative mechanisms in many drug-drug interactions. Thus, the aim of presented study was to analyse human serum albumin-binding displacement interaction between two ligands, haloperidol (HPD) and widely distributed plant flavonoid quercetin. Fluorescence analysis was used in order to investigate the effect of substances on intrinsic fluorescence of human serum albumin (HSA) and to define binding and quenching properties of ligand-albumin complexes in binary and ternary systems, respectively. Both ligands showed the ability to bind to HSA, although to a different extent. The displacement effect of one ligand from HSA by the other one has been described on the basis of the quenching curves and binding constants comparison for the binary and ternary systems.
Fluorescent spectroscopic data showed that the fluorescence quenching of HSA results from the formation of the HPD -HSA-quercetin complex. Spectroscopic analyses at different temperatures indicate that the mechanism of extinguishing the human serum albumin and the quercetin dynamic process. Process constants (Ka and Ksv) and connective sites (n), of interaction between HPD and HAS at 303 K were Ka= 4.07 × 103 mol/l, Ksv = 3.5 × 103 and n=1.02. In the presence of a quercetin, Ka = 3.75 x 102 mol/l, Ksv = 9.65 x 102 mol/l and n=0.89.
A decrease in the constants indicates that the binding of quercetin to HSA leads to a lower binding of haloperidol to human serum albumin, which can result in an increase in the free fraction of the drug in the plasma and the occurrence of adverse effects.