| Summary: | The mitochondrial amidoxime reducing component (mARC) is a human molybdoenzyme known to catalyze the reduction of various <i>N</i>-oxygenated substrates. The physiological function of mARC enzymes, however, remains unknown. In this study, we examine the reduction of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) by the human mARC1 and mARC2 enzymes. Furthermore, we demonstrate an increased sensitivity toward H<sub>2</sub>O<sub>2</sub> for HEK-293T cells with an <i>MTARC1</i> knockout, which implies a role of mARC enzymes in the cellular response to oxidative stress. H<sub>2</sub>O<sub>2</sub> is a reactive oxygen species (ROS) formed in all living cells involved in many physiological processes. Furthermore, H<sub>2</sub>O<sub>2</sub> constitutes the first mARC substrate without a nitrogen–oxygen bond, implying that mARC enzymes may have a substrate spectrum going beyond the previously examined <i>N</i>-oxygenated compounds.
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