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L. A. Brennan, W. Lee, A. Sur, T. Cowell, F. Giblin, M. Kantorow; Critical Role of Msra in Repair of Oxidized Cytochrome C for Mitochondrial Function and Apoptosis in the Lens. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3253. doi: https://doi.org/.
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Methionine sulfoxide reductase A (MsrA) is mitochondrial enzyme that repairs methionine converted to methionine sulfoxide (PMSO) under oxidative stress. Increased PMSO levels occur in the lens upon loss of MsrA activity with damage to mitochondria and apoptosis. Those key mitochondrial components whose function is lost in the absence of MsrA have not been identified in any tissue. Here we show that MsrA functions in lens mitochondria to repair a critical methioinine residue in Cytochrome C (CytC) that is converted to PMSO by oxidative stress. The results suggest a key role for MsrA in lens redox regulation, apoptotic switching and cataract formation.
CytC and MsrA were co localized in human lenses by immunohistochemistry. Specific interactions between CytC and MsrA were evaluated by immune precipitation. Purified cytochrome C was treated with hypochlorous acid (HClO) and CytC oxidation products identified. The ability of MsrA to repair oxidized CytC was evaluated by examining CytC oxidase, reductase and peroxidase activities. The levels of oxidized cytochrome C were evaluated in the lenses of MsrA knockout relative to w+ mice and MsrA knockout verses w+ lenses were examined for lens opacity under normoxic and hyperbaric oxygen conditions.
CytC co-localizes with MsrA throughout the human lens including the lens nuclear fibers. CytC and MsrA form specific complexes in the lens. Oxidation of CytC results in loss of CytC oxidase, reductase and peroxidase activities through conversion of the iron bound methionine 80 of CytC to CytC met80 sulfoxide. MsrA was capable of restoring the function of all three CytC activities through reduction of CytC met80 PMSO to CytC met80. MsrA deficient mice contained high lens levels of CytC met 80 sulfoxide relative to w+ mice and treatment of the MsrA deficient mice with hyperbaric oxygen resulted in increased lens opacity.
Oxidation of CytC is a key factor in mitochondrial redox regulation and apoptotic control. The data presented demonstrate that MsrA repairs CytC damaged upon oxidation. Regulation of CytC activity through MsrA repair of oxidized CytC is likely to mediate the balance between mitochondrial redox activities and apoptosis in the lens, contributing to cataract formation.
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