Binding of glutathione and melatonin to pepsin occurs via different binding mechanisms.

Glutathione is a hydrophilic antioxidant and melatonin is a hydrophobic antioxidant, thus, the binding mechanism of the two antioxidants interacting with protease may be different. In this study, binding of glutathione and melatonin to pepsin has been studied using isothermal titration calorimetry (ITC), equilibrium microdialysis, UV-Vis absorption spectroscopy, circular dichroism (CD) spectroscopy, and molecular modeling. Thermodynamic investigations reveal that the binding of glutathione/melatonin to pepsin is driven by favorable enthalpy and unfavorable entropy, and the major driving forces are hydrogen bond and van der Waals force. ITC, equilibrium microdialysis, and molecular modeling reveal that the binding of glutathione to pepsin is characterized by a high number of binding sites. For melatonin, one molecule of melatonin combines with one molecule of pepsin. These results confirm that glutathione/melatonin interact with pepsin through two different binding mechanisms. In addition, the UV-Vis absorption and CD experiments indicate that glutathione and melatonin may induce conformational and microenvironmental changes of pepsin. The conformational changes of pepsin may affect its biological function as protease.