On the molecular mechanism of metmyoglobin-catalyzed reduction of hydrogen peroxide by ascorbate.

Hydrogen peroxide induces rapid oxidation of metmyoglobin with an apparent second order rate constant, k1 = 3.4 x 10(4) M-1 min-1. The product of this interaction is ferrylmyoglobin with an unstable free radical on the globin moiety. This activated form of myoglobin is able: (a) to initiate the peroxidation of erythrocyte membranes and (b) to form intra- and intermolecular covalent crosslinkings. The presence of ascorbic acid in amounts stoichiometric to H2O2 efficiently prevents all the above processes. Moreover, in the presence of ascorbic acid a cyclic process is taking place leading to H2O2 reduction, ascorbic acid oxidation, and unmodified metmyoglobin formation (reaction 1).