Inactivation of Alpha-2-Macroglobulin by Photo-Illuminated Gallic Acid.

Gallic acid is a naturally occurring plant polyphenol found in green tea and various fruits. Under certain conditions gallic acid exhibits pro-oxidant characteristics rather than its well known antioxidant property. In the present work, we explored the interaction of gallic acid with sheep alpha-2-macroglobulin (αM) in the presence of light and determined the functional alteration and conformational modifications induced in αM structure. αM is a highly abundant homotetrameric antiproteinase glycoprotein having diverse functions. Our result suggests αM loses almost 54% of its proteinase inhibitory activity after 2 h incubation with gallic acid in presence of light. The inactivation of αM was due to photodynamic generation of superoxide radical and hydrogen peroxide by gallic acid. The UV/visible absorption spectra of αM showed increase in absorbance due to complex formation with gallic acid. Intrinsic fluorescence study shows that αM-gallic acid interaction leads to quenching of fluorescence intensity of αM and the mechanism of quenching is found to be static in nature. Synchronous fluorescence measurements reveal that gallic acid interaction leads to change in the microenvironment around tryptophan residues of αM. Moreover, Fourier transform infrared spectroscopy and circular dichroism spectra suggests perturbation in secondary structure of αM. Binding parameters were investigated by spectroscopic as well as calorimetric measurements. Negative value of enthalpy change and Gibbs free energy confirms the binding process to be exothermic and spontaneous.