Mechanisms of thyroid peroxidase- and lactoperoxidase-catalyzed reactions involving iodide.

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Magnusson RP, Taurog A, Dorris ML

Mechanisms of thyroid peroxidase- and lactoperoxidase-catalyzed reactions involving iodide.

J Biol Chem. 1984 Nov 25;259(22):13783-90.

PubMed ID

6094529 [ View in PubMed

]

Abstract

In a previous communication we proposed a reaction scheme to explain our observation that thyroid peroxidase and lactoperoxidase degrade H2O2 catalatically in the presence of low concentrations of iodide. An essential feature of the scheme was the proposal that enzyme-bound hypoiodite, designated [EOI]-, is a common intermediate in various peroxidase-catalyzed reactions involving iodide. In the present investigation, we tested the validity of this scheme by studying the predictions that it makes concerning the formation of OH-, O2, I2, and organically bound iodine. Stoichiometric and kinetic measurements were made to correlate formation of these various products. Three different peroxidase-catalyzed reactions were studied: 1) oxidation of I- to I2; 2) iodide-dependent catalytic degradation of H2O2 to O2; and 3) iodination of tyrosine or thyroglobulin. Reaction 2 was also studied nonenzymatically using I2, for comparison with the enzyme-catalyzed reaction. In all three reactions, both the stoichiometric and kinetic results with thyroid peroxidase agreed closely with the predictions made by the proposed scheme. This was largely the case with lactoperoxidase also. However, in the case of lactoperoxidase-catalyzed iodination of tyrosine or thyroglobulin, we observed a marked discrepancy between initial rates of OH- release and iodination, inconsistent with the mechanism originally proposed for the iodination reaction. As a possible explanation for this kinetic discrepancy, we postulate that lactoperoxidase generates hypoiodous acid and that the latter is the active intermediate in the various reactions involving iodide.

DrugBank Data that Cites this Article

Drug Enzymes

Drug	Enzyme	Kind	Organism	Pharmacological Action	Actions
Iodine	Thyroid peroxidase	Protein	Humans	Unknown	Substrate	Details