Abstract
Metmyoglobin catalyzes the decomposition of H 2O 2 as well as other hydroperoxides by using ascorbic acid as a substrate. The ratio of H 2O 2 reduced to ascorbate oxidized is close to one, whereas the rate of oxidation is directly proportional to both H 2O 2 and metmyoglobin concentrations. Ascorbate also prevents the protein modifications and the O 2 evolution that accompany the reaction of metmyoglobin with hydroperoxides. In the absence of ascorbate, myoglobin and H 2O 2 promote the peroxidation of unsaturated fatty acids and, thus, may cause damage to cellular constituents. However, lipid peroxidation is inhibited in the presence of ascorbate and, for this reason, it is suggested that this heme protein functions in the opposite manner. The redox cycling of myoglobin by ascorbate may act as an important electron “sink” and defense mechanism against peroxides during oxidative challenge to muscle.
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