Thyroid Peroxidase

Four peroxidase fractions obtained from tryptic digests of bovine thyroid subcellular particles were completely inactivated after extraction with 0.1 n HCl-acetone. Extraction with acetone containing less HCl partially inactivated the peroxidase. Hematin was as effective as, or superior to, the acetone extract in restoring the activity in three of the four apoperoxidases. The pyridine hemochromogen spectra of the acetone extract and hematin were similar, but not identical, and, because other substances in the acetone extract may have altered the spectrum of the native prosthetic group, its identification was inconclusive. However, the data conclusively show that the prosthetic group of thyroid peroxidase is not covalently linked to the apoenzyme and that ferriprotoporphyrin IX, or a closely related iron porphyrin, is essential for enzymatic activity.