Thyroid oxidase

Figure 57.4 The possible link between thyroid hormone synthesis, oxidative stress and mutagenesis in the thyroid gland. This figure illustrates the mechanism, the sequence of steps and the key moleoules that link thyroid hormone synthesis, iodine deficiency, oxidative stress, DNA damage and possible mutagenesis. ThOX, thyroid oxidase TPO, thyroid peroxidase T3, T4, thyroid hormones.

Although shown as a sequential reaction, the iodination and coupling reactions occur simultaneously via TPO and hydrogen peroxide. Hydrogen peroxide is generated by a NADPH/FAD thyroid oxidase (THOX) at the apical membrane. Low plasma levels for T4 cause the iodinated TG to be resorbed into the follicular cell, where complete proteolysis occurs by lysosomal protease to T4, T3, DIT, MIT, and noniodinated amino acids. Both T4 and T3 are secreted by the cell into the blood T4 is deiodinated to active T3. Both DIT and MIT are recycled by a dehalogenase (or deiodinase) to free tyrosine and iodide, both of which are recycled back into iodinated thyroglobulin. 

Vanadium. Vanadium is essential in rats and chicks (85,156). Estimated human intake is less than 4 mg/d. In animals, deficiency results in impaired growth, reproduction, and Hpid metaboHsm (157), and altered thyroid peroxidase activities (112). The levels of coen2yme A and coen2yme Q q in rats are reduced and monoamine oxidase activity is increased when rats are given excess vanadium (157). Vanadium may play a role in the regulation of (NaK)—ATPase, phosphoryl transferases, adenylate cyclase, and protein kinases (112). 

Fig. 1 Thyroid hormone synthesis in a thyroid follicular cell. NIS and TPO (organification and coupling reaction) have been marked in red dashed line as the two main targets for direct thyroid gland function disrupters. DEHALl iodotyrosine dehalogenase 1, DIT diiodotyrosine, DUOX2 dual oxidase 2, MIT monoiodotyrosine, Na/K-ATPase sodium-potassium ATPase, NIS sodium-iodide symporter, PSD pendrin, TG thyroglobulin, TPO thyroperoxidase. Reprinted from [7] with permission from Elsevier...

Drugs that may affect nateglinide include nonsteroidal anti-inflammatory agents (NSAIDs), salicylates, monoamine oxidase inhibitors, rifamycins, MAOIs, and nonselective beta-adrenergic blocking agents, thiazides, corticosteroids, thyroid products, and sympathomimetics. 

THY-R, thyroid hormone receptor TIMP, tissue inhibitor of metalloprotease TK, tyrosine kinase TLC, thin layer chromatography TLRs, Toll-like receptors TMAOX, trimethylamine oxidase TMY tobacco mosaic virus TNF, tumour necrosis factor TNF-a, tumour necrosis factor-a TNF-a-RTK, tumour necrosis factor-a receptor tyrosine kinase TOPI, DNA topoisomerase I TOPII, DNA topoisomerase II t-PA, tissue plasminogen activator TPA, 12-Tetradecanoylphorbol 13-acetate... 

Cobalt most often depresses the activity of enzyme including catalase, amino levulinic acid synthetase, and P-450, enzymes involved in cellular respiration. The Krebs citric acid cycle can be blocked by cobalt resulting in the inhibition of cellular energy production. Cobalt can replace zinc in a number of zinc-required enzymes like alcohol dehydrogenase. Cobalt can also enhance the kinetics of some enzymes such as heme oxidase in the liver. Cobalt interferes with and depresses iodine metabolism resulting in reduced thyroid activity. Reduced thyroid activity can lead to goiter. 

DIAMINE OXIDASE INHIBITORS act on the non-selective enzyme diamine oxidase (histaminase), which has as substrate such diverse substances as histamine, cadaverine and putrescine. As with the monoamine-oxidase enzyme, an intermediate complex is formed to yield the aldehyde, and this is then oxidized. The enzyme has been studied in relation to histamine metabolism, and is found to be released in certain circumstances from eosinophils and other tissues, and can be used as a marker in thyroid and ovarian carcinoma. Blood levels are raised in pregnancy, and heparin raises these levels. Amounts of the enzyme are high in the intestinal mucosa, liver and kidney of most species, A preparation of the enzyme itself (Torantil ) was once available for use in therapeutics for conditions in which a deficiency of histamine was implicated. 

An integrated approach to the use of ADME studies in conjunction with extensive toxicity testing of PBO has been crucial for the development of an optimal regulatory position for the compound. The toxieokinetic studies explain how PBO is metabolized by the body, and how this process in turn leads to its efficacy as synergist for insecticides and also to its activity as a mixed function oxidase inducer in mammals. The latter property accounts for the principal toxicological hndings seen with the compound, namely liver tumours in mice and weak thyroid effects in rats. 

H2O2-generating system which may be an NADPH (NADH) oxidase system similar to that of leukocytes (Chapter 15). Thyroperoxidase is a glycosylated heme enzyme that is bound to the apical plasma membrane of the thyrocyte. Thyroperoxidase exists in two molecular forms (M.W. 105,000 and 110,000) and its catalytic domain faces the colloid space. In thyroid autoimmune disorders, one of the major microsomal antigenic components is thyroperoxidase. 

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