HUMAN CATHEPSIN-B IS A METASTABLE ENZYME STABILIZED BY SPECIFIC IONIC INTERACTIONS ASSOCIATED WITH THE ACTIVE-SITE

The effect of neutral or alkaline pH on cathepsin B activity and structure was investigated. An irreversible loss of activity, accompanied by large structural changes, was observed at pH greater than or equal to 7.0. The high activation energy of 183.5 kJ mol(-1), calculated for the inactivation process, is in good agreement with structural changes observed by circular dichroism. Both the pH-induced inactivation and the pH-induced unfolding of cathepsin B were found to be first-order processes, exponentially increasing with increasing pH of the solution. The good agreement of the rate constants of inactivation and unfolding of the enzyme indicates an important structure-function relationship. Cathepsin B was also found to be destablized both by increasing ionic strength and organic solvent content.