The catalytic effect of peroxidases on the iodination of tyrosine in the presence of hydrogen peroxide

Abstract

Summary 1. Myeloperoxidase, horseradish peroxidase, and a thyroid peroxidase preparation (donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7) were each incubated with [14C]tyrosine (10 μM) and iodide (generally 20 μM) in phosphate buffer (pH 7.4) with continuous addition of H2O2. 2. The catalytic effect of myelo- and horseradish peroxidase on the oxidation of iodide by H2O2 was followed in a spectrophotometer. 3. The results indicated that myelo- and horseradish peroxidase catalyzed the oxidation of iodide, but only myeloperoxidase also catalyzed the formation of the iodo tyrosines 4. Reaction conditions were found by which myeloperoxidase in 30 min catalyzed the formation of 11% monoiodotyrosine and 61% diiodotyrosine 5. The thyroid peroxidase preparation seeemed to be more similar to myeloperoxidase than to horseradish peroxidase 6. The results were discussed with respect to the formation of the iodotyrosines in vivo.