Evidence for a Catalytic Role for Thyroid Peroxidase in the Conversion of Diiodotyrosine to Thyroxine*

When thyroglobulin and other proteins are incubated with iodide in the presence of purified thyroid peroxidase and glucose plus glucose oxidase, the iodotyrosines MIT and DIT are formed by iodination of tyrosyl residues, and significant amounts of T4 are also produced. Formation of T4 is greatest when goiter thyroglobulin is the iodine acceptor. The present study was designed to determine whether thyroid peroxidase plays a catalytic role in the coupling reaction to form T4, or whether it serves only to form the precursor of T4, diiodotyrosine, which might then couple non-enzymatically to form T4. Two lines of evidence are presented in support of the view that thyroid peroxidase plays a role in T4 formation beyond that of simply providing the DIT precursor for the coupling reaction. In one group of experiments, thyroglobulin, casein, and fibrinogen were iodinated chemically with molecular iodine and enzymatically with thyroid peroxidase. At any given level of iodination the number of DIT and MIT residues per molecule of protein was generally higher after chemical iodination than after enzymatic iodination, but T4 formation was much greater after enzymatic iodination. In a second group of experiments, advantage was taken of the fact that chemical iodination of goiter thyroglobulin results in efficient formation of DIT, but very poor formation of T4. Goiter thyroglobulin was iodinated with 131I-labeled molecular iodine to yield a protein containing several 131I-DIT residues per molecule, but almost no 131I-T4. Incubation of this 131I-thyroglobulin with thyroid peroxidase and glucose plus glucose oxidase resulted in a marked increase in the content of i3ii-T4 and a marked decrease in the content of 131I-DIT, indicating conversion of DIT to T4. (Endocrinology90: 1417, 1972)