Morris Soodak

Iodination by thyroid peroxidase

Publisher Summary The Iodination of tyrosyl moieties in thyroglobulin and other proteins is catalyzed by the thyroid peroxidase (TPO), an integral membrane, heme glycoprotein. Tyrosine and tyrosyl peptides can also be iodinated by TPO. The iodination reaction requires H 2 O 2 , I - , an enzyme-associated iodinating intermediate (TPO-I oxid ), and an iodide acceptor, such as free or protein-bound tyrosine. The TPO-catalyzed reaction produces both free and protein-bound mono and diiodotyrosine. TPO can also catalyze the formation of tetraiodothyronine by a coupling mechanism. In the presence of I - and H 2 O 2 , TPO catalyzes the iodination of tyrosyl moieties on proteins such as thyroglobulin and bovine serum albumin (BSA). For routine assays during purification, BSA is used as an I - acceptor. The protein-bound, iodinated tyrosines and thyronines can be identified by paper chromatography after Pronase digestion in the presence of 4m M 1-methyl-2-mercaptoimidazole (MMI). The tripeptide Glu-Tyr-Glu has also been used in TPO assays and the iodinated product has been identified by thin-layer chromatography and autoradiography.