Spectroscopic studies on the interaction of bilirubin with liver cystatin

Studies on the role of endogenous metabolites such as bilirubin and their interactions with biomolecules have attracted considerable attention over the past several years. In this work, the interaction of bilirubin (BR) with purified goat liver cystatin (LC) was studied using fluorescence and ultraviolet (UV) spectroscopy. The fluorescence data proved that the fluorescence quenching of liver cystatin by BR was the result of BR–cystatin complex formation. Stern–Volmer analysis of fluorescence quenching data showed the binding constant to be 9.27 × 10⁴ M⁻¹ and the number of binding sites to be close to unity. The conformation of the BR–cystatin complex was found to change upon varying the pH of the complex. The BR–cystatin complex was found to have reduced papain inhibitory activity. Photo-illumination of BR–cystatin complex causes perturbation in the micro-environment of goat liver cystatin as indicated by red-shift. This report summarizes our research efforts to reveal the mechanism of interaction of bilirubin with liver cystatin.