Repairable and Nonrepairable Inactivation of Irradiated Aqueous Papain: Effects of OH, O 2 - ,e aq - , and H 2 O 2

Abstract

Repairable inactivation of papain irradiated in dilute aqueous solutions saturated with air or nitrous oxide is caused predominantly by reversible oxidation of $\text{Cys}_{25}{\rm SH}$ by H2 O2. The same process occurs in nitrogen-saturated solutions but the yield of repairable product decreases at higher doses, probably because of the consumption of H2 O2 by intermediates formed from $e{}_{{\rm aq}}{}^{-}$ and papain. The OH radical produces only nonrepairable damage, with the fraction of the OH radical causing nonrepairable inactivation $(f_{{\rm OH}}{}^{{\rm n.r.}})$ equal to 0.1 and this is accompanied by, if not solely due to, SH loss. The O2- radical with $f_{{\rm O}_{2}}{}^{-{\rm n.r.}}=0.4$ also causes nonrepairable damage resulting from or accompanied by SH loss. In addition, there is evidence that every O2- reacts with papain to produce a hydrogen peroxide molecule, thus causing a marked increase in the repairab...