Study of the ropinirole hydrochloride interactions with human holo-transferrin in the presence of common metal ions

The binding of ropinirole hydrochloride (REQUIP) to human holo-transferrin (hTf) in the absence and presence of common ions has been investigated by fluorescence spectroscopy combined with fluorescence anisotropy, time-resolved fluorescence and circular dichroism (CD) under simulative physiological conditions. The quenching of the fluorescence intensity and the red shift in the maximum wavelength revealed an increased polarity of the microenvironment of the tryptophan and tyrosine residues. The number of binding sites and the apparent binding constants of REQUIP with hTf in the presence of Co2+, Fe3+, Cr3+, Al3+, Ca2+, Pb2+, Co (3) (2-) , Cu2+, Mg2+ and K+ ions were determined. Time-resolved fluorescence decay profile gives the lifetime components reduce from 4.20 to 3.57 ns. Steady-state and time-resolved fluorescence data illustrated that the fluorescence quenching of complexes are static mechanism. Gradual addition of hTf led to marked increase in fluorescence anisotropy. From the value of anisotropy, it is argued that the REQUIP is located in a restricted environment of hTf. The quantitative analysis of CD spectra indicated that, in the presence of Co2+ and Fe3+ ions, the alpha-helical structure content of hTf increased and for the other common ions, the alpha-helical content decreased. In addition, thermodynamic analysis demonstrated that the van der Waals forces and hydrogen bond interactions stabilized the hTf-REQUIP complex. These experimental results revealed that REQUIP could bind to hTf and those common ions, therefore could be a useful guideline for further therapeutic projects.