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

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 Ferrylmyoglobin [MbFe(IV)=O], formed by activation of metmyoglobin by hydrogen peroxide, inactivates the cysteine proteinases papain (EC and ficin (EC 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.