Oxidative damage to proteolytic enzymes: inactivation of papain and ficin by ferrylmyoglobin

 Ferrylmyoglobin [MbFe(IV)=O], formed by activation of metmyoglobin by hydrogen peroxide, inactivates the cysteine proteinases papain (EC 3.4.22.2) and ficin (EC 3.4.22.3) more efficiently than hydrogen peroxide, but less efficiently than hydroxyl radicals as generated by peroxynitrite or the Fenton reaction. Metmyoglobin and oxymyoglobin could not inactivate papain and ficin. Oxidation of papain and ficin by ferrylmyoglobin occurs in enzyme/haem-protein complexes with binding constants of approximately 10⁵ l · mol^–1; inactivation of proteolysis by papain plateaus at neutral pH to about ¹/₃ whereas the inactivation under acidic conditions was larger. Received: 11 July 1997 / Revised version: 16 September 1997