STIMULATION BY NITROXIDES OF CATALASE-LIKE ACTIVITY OF HEMEPROTEINS -KINETICS AND MECHANISM

The ability of stable nitroxide radicals to detoxify hypervalent heme proteins such as ferrylmyoglobin (MbFe(IV)) produced in the reaction o f metmyoglobin (MbFe(III)) and H2O2 was evaluated by monitoring O-2 ev olution, H2O2 depletion, and redox changes of the heme prosthetic grou p, The rate of H2O2 depletion and O-2 evolution catalyzed by MbFe(III) was enhanced by stable nitroxides such as 4-OH-2,2,6,6-tetramethyl-pi peridinoxyl (TPL) in a catalytic fashion, The reduction of MbFe(IV) to MbFe(III) was the rate-limiting step, Excess TPL over MbFe(III) enhan ced catalase-like activity more than 4-fold. During dismutation of H2O 2, [TPL] and [MbFe(IV)] remained constant, NADH caused: (a) inhibition of H2O2 decay; (b) progressive reduction of TPL to its respective hyd roxylamine TPL-H; and (c) arrest/inhibition of oxygen evolution or eli cit consumption of O-2. Following depletion of NADH the evolution of O -2 resumed, and the initial concentration of TPL was re stored, Kineti c analysis showed that two distinct forms of MbFe(IV) might be involve d in the process, In summary, by shuttling between two oxidation state s, namely nitroxide and oxoammonium cation, stable nitroxides enhance the catalase mimic activity of MbFe(III), thus facilitating H2O2 dismu tation accompanied by O-2 evolution and providing protection against h ypervalent heme proteins.