Deiodinase (or "Monodeiodinase") is a

peroxidase Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides. Functionality Peroxidases typically ca ...

enzyme that is involved in the activation or deactivation of

thyroid hormone File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4 rect 376 268 820 433 Thyroid-stimulating hormone rect 411 200 849 266 Thyrotropin-releasing hormone rect 297 168 502 200 Hypothalamus rect 66 216 386 25 ...

Types

Types of deiodinases include:

Iodothyronine deiodinase Iodothyronine deiodinases ( and ) are a subfamily of deiodinase enzymes important in the activation and deactivation of thyroid hormones. Thyroxine (T4), the precursor of 3,5,3'-triiodothyronine (T3) is transformed into T3 by deiodinase activity ...

s catalyze release of iodine directly from the thyronine hormones. They are selenocysteine-dependent membrane proteins with a catalytic domain resembling

Peroxiredoxin Peroxiredoxins (Prxs, ; HGNC root symbol ''PRDX'') are a ubiquitous family of antioxidant enzymes that also control cytokine-induced peroxide levels and thereby mediate signal transduction in mammalian cells. The family members in humans are PRDX ...

s (Prx). Three related isoforms, deiodinase type I, II, and III, contribute to activation and inactivation of the initially released hormone precursor T₄ (

thyroxine File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4 rect 376 268 820 433 Thyroid-stimulating hormone rect 411 200 849 266 Thyrotropin-releasing hormone rect 297 168 502 200 Hypothalamus rect 66 216 386 25 ...

) into T₃ (

triiodothyronine Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. Production of T3 and its prohormone thyroxine ( ...

) or rT₃ (

reverse triiodothyronine Reverse triiodothyronine (3,3′,5′-triiodothyronine, reverse T3, or rT3) is an isomer of triiodothyronine (3,5,3′ triiodothyronine, T3). Reverse T3 is the third-most common iodothyronine the thyroid gland releases into the bloodstream, at 0.9 ...

) in target cells. The enzymes catalyze a reductive elimination of iodine (the different isoforms attack different thyronine positions), thereby oxidizing themselves similar to Prx, followed by a reductive recycling of the enzyme.

Iodotyrosine deiodinase Iodotyrosine deiodinase, also known as iodotyrosine dehalogenase 1, is a type of deiodinase enzyme that scavenges iodide by removing it from iodinated tyrosine residues in the thyroid gland. These iodinated tyrosines are produced during thyroid h ...

contributes to breakdown of thyroid hormones. It releases

iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...

, for renewed use, from iodinated tyrosines resulting from catabolism of iodothyronines. Iodotyrosine deiodinase employs a flavin mononucleotide cofactor and belongs to the NADH oxidase/flavin reductase superfamily.

Starvation response

In starvation or severe stress, deiodinase type 1 is inhibited which lowers circulating levels of T3 (due to it being the main source of peripherally converted T3 from T4 in the plasma), causing a decrease in the metabolic rate. Intuitively, if plasma levels of T3 fall, there would be a compensatory rise in TSH, the secretion of which is inhibited by T3. However, because type 2 deiodinase mediates the conversion of T4 to T3 within the pituitary and CNS, and because caloric restriction does not affect this enzyme, local T3 levels in the pituitary are normal.Thus, the thyrotrophs (endocrine cells in pituitary) in the pituitary continue to have adequate amounts of T3, and no compensatory rise in TSH occurs. This effect of caloric restriction makes sense for someone who is starving because it tends to conserve body stores of fuel. On the other hand, this effect makes it more difficult to lose weight intentionally while dieting.

Selenium

Selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

in iodothyronine deiodinase, as selenocysteine, plays a crucial role in determining the free circulating levels of T₃.

Selenium deficiency Selenium deficiency occurs when an organism lacks the required levels of selenium, a critical nutrient in many species. Deficiency, although relatively rare in healthy well-nourished individuals, can have significant negative results, affecting the ...

can have implications in fall of T₃ levels.

References

External links

* Enzymes Thyroid {{enzyme-stub