Properties of a peroxidase purified from beef thyroid tissues

• 1.

  1. A peroxidase was purified from beef thyroid tissues by column chromatography on ion-exchange CM-Sephadex. Some of the properties of the purified enzyme were studied.

• 2.

  2. By means of electrophoresis on polyacetate strips the purified enzyme was found to consist of one protein component. This single component demonstrated peroxidase activity when stained with o-dianisidine in the presence of H₂O₂.

• 3.

  3. The molecular weight of the beef thyroid peroxidase was estimated to be approx. 50 000 by sucrose gradient centrifugation using as a reference a purified preparation of dog myeloperoxidase.

• 4.

  4. Protoporphyrin IX was identified as the prosthetic group by spectral analysis of the acid methylethyl ketone extract and the pyridine haemochrome of the enzyme.

• 5.

  5. Spectral properties of the enzyme complexes with CN- and CO, as well as its reduction by sodium dithionite, were distinctively different from those of bovine haemoglobin.

• 6.

  6. Kinetic studies using o-dianisidine as a hydrogen donor showed that the K_m of the enzyme for H₂O₂ was between 1.5 to 2.0 · 10⁻⁵ M.

• 7.

  7. The peroxidase activity of the enzyme was inhibited by SCN-, CN-, N₃-, F-, I-, propylthiouracil and Tapazole but not by p-hydroxymercuribenzoate or ClO₄- as shown by kinetic studies. Thyroid-stimulating hormone did not show any effect.

• 8.

  8. The enzyme catalyzed also the iodination of tyrosine actively in the presence of added H₂O₂ or a H₂O₂-generating system. The iodination of tyrosine by this enzyme occurred optimally at pH 6. Thyroglobulin was iodinated much less actively.

• 9.

  9. The enzymic iodination of tyrosine was inhibited by the inhibitors mentioned above, except iodide, and was not affected by thyroid-stimulating hormone.