Glutathione Mediation of Papain Inactivation by Hydrogen Peroxide and Hydroxyl Radicals

Glutathione reacts with$\text{papain}{\rm Cys}_{25}{\rm SOH}$, formed by the reaction of papain with hydrogen peroxide, to give$\text{papain}{\rm Cys}_{25}{\rm SSG}$. Subsequent reaction of this mixed disulfide with glutathione is slow ($k<3\ M^{-1}\ {\rm sec}^{-1}$). However, at 30°C it is readily cleaved by cysteine to form active papain, i.e.,$\text{papain}{\rm Cys}_{25}{\rm SH}$. Glutathione resembles cysteine in protecting papain by the scavenging of ·OH radicals, but, unlike cysteine, glutathione gave no evidence for the repair of enzyme radical lesions or for the conversion of$\text{papain}{\rm Cys}_{25}{\rm S}$· radicals to repairable derivatives. Its overall effectiveness for reducing the radiation inactivation of papain in aqueous solution is much less than that of cysteine.