Effects of oxidative stress on the model thiol protease- papain: An investigation of changes in activity and structure

Amyloid precursor protein (APP) is believed to be processed abnormally in Alzheimer's disease (AD) leading to the deposition of amyloid plaques in the AD brain. The processing of APP is thought to be mediated by an as yet unidentified protease, possibly a thiol protease. Many laboratories, including ours, have shown that oxidative damage is a common phenomena in aged tissues and age-related diseases. Noting that little is known about the effect of oxidation on the activity and structure of thiol proteases, we have studied the effect of oxidation on a model thiol protease, viz., papain. We have investigated, the effect of oxidation, using Fe²⁺/H₂O₂ generated hydroxyl radicals, on papain using three different techniques (electron paramagnetic resonance, circular dichroism and enzymatic assay). The results indicate that with oxidation loss in activity of the enzyme is accompanied by changes in the active site geometry and secondary structure of papain. These changes are similar to those observed when the enzyme undergoes denaturation. The results suggest that oxidation changes the structure and activity of thiol proteases. Such changes could be important in aberrant processing of APP.