| Although there is extensive evidence roles of myoglobin (Mb) in oxygen storage and diffusion in skeletal muscle and heart, Mb has also been implicated in cellular antioxidant defense. Upon releasing its oxygen, Mb can be oxidized to peroxidase-active metMb, which can then react with hydrogen peroxide (H2O2) to produce a highly reactive oxoferryl heme iron (IV=O). We hypothesized that Mb can act as a peroxidase using a variety of reducing substrates. Using spectrophotometric techniques, our data show that horse skeletal muscle Mb has peroxidase activity using a variety of biological substrates. In addition, we found that metMb reacts with hypochlorite in a heme-dependent fashion. However, physiological concentrations of hypochlorite did not appreciably affect Mb peroxidase activity, implying that Mb might play a role in hypochlorite detoxification. When cellular reactive oxygen species become overly abundant and exceed the capacity of antioxidant systems, oxidative stress can cause irreversible modifications to protein side chains and eventual loss of protein function. Here, we show that a number of the oxidative modifications in oxidized Mb can be reversed by action of reducing substrates. In particular, formation of dityrosine, which is thought to be an irreversible oxidation event, is formed both between and within Mb molecules. However, we show that Mb possesses dityrosinease activity, as it can reverse its own dityrosine cross-links through action with ascorbate. In addition, we show that high concentrations of ascorbate is sufficient to reverse the heme-protein crosslink formed in H2O2-treated Mb. We also show that heme-protein crosslinked Mb species (HMb) have increased peroxidase activity as well as increased specificity towards ascorbate. Taken together, these data suggest that in the presence of reducing substrates, Mb is able to prevent and reverse oxidative protein damage. |
