Nitrosylmyoglobin as antioxidant--kinetics and proposed mechanism for reduction of hydroperoxides.

Nitrosylmyoglobin (MbFe(II)NO), which is believed to have a protective role during ischemia and reperfusion injury, was oxidized by tert-butyl hydroperoxide (t-BuOOH), and by hydrogen peroxide (H(2)O(2)) to the nitrite anion and metmyoglobin (MbFe(III)). Further characterization of the reaction of MbFe(II)NO with excess of t-BuOOH was investigated with respect to reaction stoichiometry, temperature and pH dependence. It was found that the reaction between MbFe(II)NO with excess of t-BuOOH followed a simple stoichiometry and had moderate pH and temperature dependence with the activation parameters DeltaH(double dagger) = 57.4 +/- 1.4 kJ mol(- 1) and DeltaS(double dagger) = - 112.0 +/- 5.1 J mol(- 1) K(- 1), which is consistent with an associative reaction mechanism. Moreover, t-BuOOH-induced oxidation of MbFe(II)NO did not result in any detectable formation of the hypervalent myoglobin (Mb) species, i.e. perferrylmyoglobin, (( radical)MbFe(IV) = O) or ferrylmyoglobin (MbFe(IV) = O), and hereby differed from H(2)O(2)-induced oxidation of MbFe(II)NO, which results in the formation of MbFe(IV) = O. Based on the obtained results and on published data, different mechanisms for the reaction of the MbFe(II)NO with t-BuOOH and H(2)O(2) are proposed.