The iodination of tyrosine by myeloperoxidase and beef thyroids. The possible involvement of free radicals

Abstract

• 1.

  1. The oxidation of sulfite, measured by the uptake of oxygen, was employed to detect the formation of free radicals in the iodination of tyrosine by myeloperoxidase in the presence of added H₂O₂.

• 2.

  2. The uptake of oxygen was dependent on sulfite, tyrosine and myeloperoxidase. Tyrosine could be replaced by monoiodotyrosine, diiodotyrosine, 4-hydroxy-3,5-diiodophenylacetic acid or 4-hydroxy-3,5-diiodobenzaldehyde but not by thyroglobulin, phenylacetic acid, phenylalanine or iodide. Iodide was inhibitory.

• 3.

  3. The oxidation of sulfite under these conditions was affected by pH and had an optimal range between pH 5 and 6.5.

• 4.

  4. Sulfite and bisulfite inhibited completely the iodination of tyrosine and of thyroglobulin by myeloperoxidase in the presence of added H₂O₂. Sulfate was without effect.

• 5.

  5. The iodination of tyrosine by myeloperoxidase in the presence of added H₂O₂ showed the same pH optimum as the oxidation of sulfite.

• 6.

  6. A concentrated solubilized preparation of beef thyroid tissues, which catalyzed the peroxidation of $\text{o-}\text{dianisidine}$ and the iodination of tyrosine, could not substitute effectively for myeloperoxidase in the oxidation of sulfite. However, the iodination of tyrosine by this enzyme preparation was inhibited by sulfite and bisulfite.

• 7.

  7. These results are discussed in relation to a free radical reaction mechanism involved in the synthesis of iodotyrosines by myeloperoxidase and by the beef thyroid preparation.