Enzymatic iodination of tyrosine and thyroglobulin with chloroperoxidase.

1. Crystalline chloroperoxidase is effective in catalyzing the iodination of tyrosine or thyroglobulin when supplemented with HzOz, or with the H20z-generating system, glucoseglucose oxidase. The iodination reaction is very rapid at low concentrations of iodide (5 X 10-5 M), and at low concentrations of tyrosine (1 X 10V4 M) or thyroglobulin (0.33 mg per ml). 2. Iodide is rapidly bound as 3-iodotyrosine, 3,5-diiodotyrosine, and thyroxine during the chloroperoxidase-catalyzed iodination. After 60 min of incubation with thyroglobulin as acceptor, chromatography of a Pronase digest showed approximately 45 to 50 % of the added nlI in the form of 3,5-diiadotyrosine, about 20 to 25% as 3-iodotyrosine, and 4 to 5% as thyroxine. 3. Since iodination of thyroglobulin occurred very effectively in the presence of only a single crystalline enzyme, the results suggest that it may not be necessary to postulate the existence of a separate “tyrosine iodinase” in the thyroid. Moreover, the formation of appreciable thyroxine in the chloroperoxidase-catalyzed iodination of thyroglobulin suggests that a peroxidase may also be involved in the coupling reaction in the thyroid. 4. The chloroperoxidase-catalyzed iodination of tyrosine and thyroglobulin is greatly accelerated by 0.1 M chloride and bromide. The mechanism of this stimulatory effect is not yet known, although several possibilities are discussed. 5. Iodination of thyroglobulin and tyrosine in the presence of chloroperoxidase is readily inhibited by low concentrations of antithyroid drugs and by naturally occurring reducing agents such as cysteine, reduced glutathione, and ascorbic acid. Results obtained with these inhibitors support the view that antithyroid compounds act as competitive inhibitors in the formation of organic iodine in the thyroid. 6. Excess iodide inhibits the chloroperoxidase-catalyzed iodination of tyrosine and thyroglobulin. These results suggest that the antithyroid effect of iodide, observed under