Fluorescence Study on the Interaction Between Hypericin and Lens Protein "α-Crystallin"

Hypericin has been reported as a potent photosensitizing agent exhibiting antiviral, antibacterial, antineoplastic activities. Although its photophysics and mode of action are strongly modulated by the binding protein, detailed information about its mechanism of interaction with possible cellular targets, including proteins, is still lacking. Previous in vitro studies demonstrated that hypericin can be uptaken by intact lens and is able to bind to the major lens protein "alpha-crystallin." In this study, the mechanism of interaction of this potent drug with alpha-crystallin was studied using the chemical denaturant guanidine hydrochloride (GdnHCl) and the hydrophobic surface probe, 8-anilino-1-naphthalenesulfonic acid (ANS). Fluorescence measurements showed that the increased exposure of tryptophan resulting from partial unfolding of alpha-crystallin incubated with 1.0 mol L-1 of GdnHCl corresponds to the maximum accessibility of hydrophobic sites to ANS at the same GdnHCl concentration. Interestingly at this additional hydrophobicity of the protein, hypericin exhibited its maximum fluorescence intensity. This in vitro study implied that hydrophobic sites of alpha-crystallin play a significant role in its interaction with hypericin. The binding between alpha-crystallin and hypericin was found to be enhanced by partial perturbation of the protein.