Elevated levels of iodide promote peroxidase-mediated protein iodination and inhibit protein chlorination

At inflammatory sites, immune cells generate oxidants including H₂O₂. Myeloperoxidase (MPO), released by activated leukocytes employs H₂O₂ and halide/pseudohalides to form hypohalous acids that mediate pathogen killing. Hypochlorous acid (HOCl) is a major species formed. Excessive or misplaced HOCl formation damages host tissues with this linked to multiple inflammatory diseases. Previously ( Redox Biology , 2020, 28, 101331) we reported that iodide (I⁻) modulates MPO-mediated protein damage by decreasing HOCl generation with concomitant hypoiodous acid (HOI) formation. HOI may however impact on protein structure, so in this study we examined whether and how HOI, from peroxidase/H₂O₂/I⁻ systems + Cl⁻, modifies proteins. Experiments employed MPO and lactoperoxidase (LPO) and multiple proteins (serum albumins, anastellin), with both chemical (intact protein and peptide mass mapping, LC-MS) and structural (SDS-PAGE) changes assessed. LC-MS analyses revealed dose-dependent iodination of anastellin and albumins by LPO/H2O2 with increasing I⁻. Incubation of BSA with MPO/H2O2/Cl⁻ revealed modest chlorination (Tyr286, Tyr475, ∼4%) and Met modification. Lower levels of these species, and extensive iodination at specific Tyr and His residues (>20% modification with >10 µM I⁻) were detected with increasing I⁻. Anastellin dimerization was inhibited by increasing I⁻, but less marked changes were observed with albumins. These data confirm that I⁻ competes with Cl⁻ for MPO and is an efficient HOCl scavenger. These processes decrease protein chlorination and oxidation, but result in extensive iodination. This is consistent with published data on the presence of iodinated Tyr on neutrophil proteins. The biological implications of protein iodination relative to chlorination require further clarification. ### Competing Interest Statement MJD declares consultancy contracts with Novo Nordisk A/S, and is a Director and major shareholder in the start-up company Seleno Therapeutics plc. These funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. * ACN : acetonitrile 3-ClTyr : 3-chlorotyrosine 3,5-di-ITyr : 3,5-di-iodotyrosine ESI : electrospray ionization FA : formic acid HOCl : the physiological mixture of hypochlorous acid and its conjugate anion −OCl HOI : the physiological mixture of hypoiodous acid and its conjugate anion −OI HPLC : high-performance liquid chromatography 3-ITyr : 3-iodotyrosine LC-MS : liquid chromatography-mass spectrometry LPO : lactoperoxidase MPO : myeloperoxidase MS : mass spectrometry PTM : post-translational modification TFA : trifluoroacetic acid UPLC : ultra high-performance liquid chromatography.