Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:1.11.1.8 (thyroid peroxidase)
3,116 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A stable apoprotein has been prepared from a soluble purified bovine thyroid iodotyrosine deiodinase, previously shown to be an FMN-containing flavoprotein requiring dithionite for enzymatic activities. The apoprotein binds FMN (Ka = 1.47 x 10(8) M-1) with an almost complete restoration of enzymatic activity. It can also bind FAD (Ka = 0.58 x 10(8) M-1) with partial restoration of activity, but does not bind riboflavin. Photoreduction of the holoenzyme in presence of excess of its free cofactor, FMN, supported enzyme activity at a level of 50% of that obtained with dithionite; substituting FAD or riboflavin for FMN produced, respectively, 20 and 11% of the dithionite-supported activity. The oxidation-reduction potential (E1) of the couple semiquinone/fully reduced enzyme is -0.412 V at pH 7 and 25 degrees C. The value (E2) for the oxidized/semiquinone couple is -0.190 V at pH 7 and 25 degrees C. Potentiometric titrations with sodium hydrosulfite suggests that the enzyme is reduced in two successive 1-electron oxidation-reduction steps. Effects of pH on E1 suggest ionization of the protonated flavin with an ionization constant of 5.7 x 10(-7). The highly negative oxidation-reduction potential for the fully reduced enzyme species and the apparent requirement for full reduction for enzymatic activity suggests that in NADPH-mediated microsomal deiodination an NADPH-linked electron carrier of suitably negative midpoint potential is a probable intermediate.
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PMID:Characterization of a flavoprotein iodotyrosine deiodinase from bovine thyroid. Flavin nucleotide binding and oxidation-reduction properties. 50 Jul 18

Iodide oxidation and binding to proteins require a thyroperoxidase and an ill defined H2O2-generating system. The NADP+ supply and, thus, NADPH oxidation are the limiting steps of the pentose phosphate pathway. The purpose of this work was to test the hypothesis that H2O2 generation is a limiting step of iodination and NADPH oxidation and, therefore, of the pentose phosphate pathway. H2O2 produced by dog thyroid slices was measured with the homovanillic fluorescence assay. Our data show that H2O2 generation is stimulated by both the cAMP cascade [as activated by TSH, forskolin and (Bu)2cAMP] and the Ca2(+)-phosphatidylinositol cascade (as activated by carbamylcholine, ionomycin, and 12-O-tetradecanoylphorbol-13-acetate). We used several physiological and pharmacological agents that modulate iodide organification. In all cases there was a strict parallelism between effects on H2O2 generation, iodide binding to proteins, and pentose phosphate pathway activity. Moreover, in TSH- or carbamylcholine-stimulated slices, glucose or Ca2+ depletion, which greatly depressed H2O2 generation, also greatly decreased iodide organification and the activity of the pentose phosphate pathway. The glutathione peroxidase-catalyzed H2O2 reduction in the cytosol, which involves NADPH oxidation and, therefore, increases the NADP supply, also enhances the activity of the pentose phosphate pathway. All of these data strongly support the hypothesis that H2O2 generation in dog thyroid controls iodination of proteins; through the NADPH oxidation resulting from H2O2 production and reduction, hydrogen peroxide also regulates the activity of the pentose phosphate pathway.
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PMID:The H2O2-generating system modulates protein iodination and the activity of the pentose phosphate pathway in dog thyroid. 184 88

The thyroid plasma membrane contains a Ca2(+)-regulated NADPH-dependent H2O2 generating system which provides H2O2 for the thyroid peroxidase-catalyzed biosynthesis of thyroid hormones. The plasma membrane fraction contains a Ca2(+)-independent cytochrome c reductase activity which is not inhibited by superoxide dismutase. But it is not known whether H2O2 is produced directly from molecular oxygen (O2) or formed via dismutation of super-oxide anion (O2-). Indirect evidence from electron scavenger studies indicate that the H2O2 generating system does not liberate O2-, but studies using the modified peroxidase, diacetyldeuteroheme horseradish peroxidase, to detect O2- indicate that H2O2 is provided via the dismutation of O2-. The present results provide indirect evidence that the cytochrome c reductase activity is not a component of the NADPH-dependent H2O2 generator, since it was removed by washing the plasma membranes with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid without affecting H2O2 generation. Spectral studies with diacetyldeuteroheme-substituted horseradish peroxidase showed that the thyroid NADPH-dependent H2O2 generator does not catalyze superoxide anion formation. The O2- adduct compound (compound III) was formed but was completely inhibited by catalase, indicating that the initial product was H2O2. The rate of NADPH oxidation also increased in the presence of diacetylheme peroxidase. This increase was blocked by catalase and was greatly enhanced by superoxide dismutase. The O2- adduct compound (compound III) was produced in the presence of NADPH when glucose-glucose oxidase (which does not produce O2-) was used as the H2O2 generator. NADPH oxidation occurred simultaneously and was enhanced by superoxide dismutase. We conclude that O2- formation occurs in the presence of an H2O2 generator, diacetylheme peroxidase and NADPH, but that it is not the primary product of the H2O2 generator. We suggest that O2- formation results from oxidation of NADPH, catalyzed by the diacetylheme peroxidase compound I, producing NADP degree, which in turn reacts with O2 to give O2-.
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PMID:Mechanism of hydrogen peroxide formation catalyzed by NADPH oxidase in thyroid plasma membrane. 199 28

The plasma membrane fraction from porcine thyroid is known to exhibit an NADPH-dependent production of hydrogen peroxide (H2O2), which is utilized for the oxidative biosynthesis of thyroid hormones catalyzed by thyroid peroxidase. The H2O2 formation is cyanide-insensitive, ATP-activatable, and Ca2+-dependent (Nakamura, Y., Ogihara, S., and Ohtaki, S. (1987) J. Biochem. (Tokyo) 102, 1121-1132). It remains unknown, however, whether H2O2 is produced directly from molecular oxygen (O2) or formed via dismutation of superoxide anion (O2-). We therefore attempted to analyze the mechanism of H2O2 formation by utilizing a new method for the simultaneous measurement of O2- and H2O2, in which diacetyldeuteroheme-substituted horseradish peroxidase was employed as the trapping agent for both oxygen metabolites. When NADPH was incubated with the membrane fraction in the presence of the heme-substituted peroxidase, a massive O2 consumption was observed together with the formation of compound III, and O2- adduct of the peroxidase. The amounts of compound III formed and O2 consumed were stoichiometric with each other, while formation of compound II, an indicative of H2O2, was not observed during the reaction. On the other hand, when an excess amount of superoxide dismutase was included in the reaction mixture, compound II was produced with complete suppression of the compound III formation. NADH minimally supported both O2 consumption and formation of compound III or II. These results indicate that the NADPH oxidase in the plasma membrane of thyroid produces O2- as the primary metabolite of O2 and hence that H2O2 required for the thyroid hormone synthesis provided through the dismutation of O2-.
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PMID:Superoxide anion is the initial product in the hydrogen peroxide formation catalyzed by NADPH oxidase in porcine thyroid plasma membrane. 253 59

Certain toxic effects of phenytoin are thought to result from its cytochrome P-450-catalyzed bioactivation to a reactive arene oxide intermediate that binds covalently to proteins. Using an in vitro system, we examined an alternative hypothesis based upon the cooxidation of phenytoin to a reactive free radical intermediate by prostaglandin synthetase (PGS), horseradish peroxidase, or thyroid peroxidase. Microsomes from hepatic, thyroid, seminal vesicular, or pulmonary tissues, or PGS or horseradish peroxidase, were incubated with the appropriate enzymatic cofactors to study activities of cytochromes P-450 (NADPH), PGS (arachidonic acid), thyroid peroxidase (guiaicol, H2O2), and horseradish peroxidase (H2O2). The production of potentially teratogenic, reactive phenytoin intermediates during in vitro incubations was estimated by the amount of radiolabeled phenytoin bound covalently to microsomal protein or bovine serum albumin and by the detection of a free radical intermediate using ESR spectrometry. Arachidonic acid-dependent bioactivation of phenytoin was demonstrated for purified PGS and ram seminal vesicles (RSV), as well as for liver, lung, and kidney. Optimal arachidonate concentrations varied substantially for different tissues. Arachidonate-dependent binding of phenytoin with PGS and RSV was reduced to baseline levels by coincubation with the cyclooxygenase inhibitor indomethacin. Hydrogen peroxide-dependent covalent binding of phenytoin was observed with thyroid peroxidase and horseradish peroxidase, and binding was significantly reduced in these systems and in PGS and RSV by coincubation with the peroxidase inhibitor methimazole. Glutathione, the antioxidants caffeic acid and butylated hydroxyanisole, and the free radical trapping agent alpha-phenyl-N-t-butylnitrone (PBN) all significantly reduced arachidonate-dependent phenytoin binding. Oxygen uptake was increased in a dose-dependent manner by the arachidonate-dependent bioactivation of phenytoin by PGS. ESR spin-trapping techniques using PBN indicated the generation of a free radical intermediate during the metabolism of phenytoin by PGS. These results suggest that the hydroperoxidase component of PGS, as well as thyroid peroxidase and other peroxidases, can bioactivate phenytoin to a reactive free radical intermediate, which may be toxicologically relevant.
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PMID:In vitro bioactivation of phenytoin to a reactive free radical intermediate by prostaglandin synthetase, horseradish peroxidase, and thyroid peroxidase. 253 58

In this study, it is shown that NADPH iodination occurs in a thyroid peroxidase-H2O2 system in presence of thyroglobulin, the normal iodination substrate. Previous data suggested that thyroid H2O2 generation is a NADPH-dependent system. Present results support the concept of a compartmentalization of the sites of NADPH oxidation and peroxidasic iodination.
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PMID:Requirement of a compartmentalization for NADPH oxidizing site and peroxidase-H2O2 in the thyroid iodinating system. 344 May 65

A 71-yr-old man, clinically euthyroid, with a 570-g goiter causing severe mechanical neck compression underwent thyroidectomy. His total serum T4 level was 1.8 micrograms/dL, T3 was 200 ng/dL, and TSH was 35 microU/mL, and a perchlorate test was markedly abnormal. The excised thyroid tissue had normal peroxidase activity in the tyrosine iodinase and guaiacol assays. [131I]Iodide, given 24 h before surgery, was distributed in thyroglobulin isolated in vitro as follows: monoiodotyrosine, 71.6%; diiodotyrosine, 26.7%; T3, 1.05%; and T4, 0.65%. The [131I]iodide content of the whole thyroid homogenate was 59%. The goiter content of thyroglobulin was 94.7 mg/g tissue. The thyroglobulin reacted normally with antihuman thyroglobulin antiserum. Fresh goiter slices and slices from five normal human thyroid specimens were incubated with 10(-6) M KI and [131I]iodide (tracer) containing medium alone (basal), medium plus 1 mg/mL glucose oxidase (GO), and medium plus 10(-4) M NADPH and 10(-5) M vitamin K3 (NA-K3). The percentages of organic iodine in the slices, measured as protein-bound 131I, were: basal: goiter, 0.8%; normal, 6.9 +/- 1.8% (+/- SE); GO: goiter, 15.1%; normal, 17.4 +/- 3.1%; and NA-K3: goiter, 16.7%; normal, 4.6 +/- 1.14%. We conclude that an abnormal H2O2 supply may be the cause of the iodine organification defect in this goiter.
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PMID:Abnormal H2O2 supply in the thyroid of a patient with goiter and iodine organification defect. 359 12

In thyroid gland, iodination takes place on the apical plasma membrane and requires the presence of the thyroid peroxidase and H2O2 generating system. H2O2 generation and NBT (nitro blue tetrazolium) reductase activity (both of which are NADPH-dependent) as well as peroxidase activity were compared for their respective orientations in membrane vesicles. The possible role of NADPH-NBT reductase activity in H2O2 generation was also examined. Results favor the conclusion that thyroid peroxidase is oriented towards the luminal side of the vesicles, whereas the NADPH site of NADPH oxidase-dependent H2O2 generation is located on the external side of the same or of different vesicles. Furthermore, it is shown that different NADPH-NBT reductase activities are present on both the outer and inner surfaces of the membrane vesicles, and that none of these activities is able to produce either H2O2 or O-2. The idea that a multi-component complex is involved in H2O2 generation is discussed, and a model is proposed which takes into account the possible spatial separation of the thyroid peroxidase site from the NADPH site of this H2O2 generation system on the apical membrane of the thyrocyte.
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PMID:Relation between thyroid peroxidase, H2O2 generating system and NADPH-dependent reductase activities in thyroid particulate fractions. 401 97

A NADPH-dependent H2O2 generating system associated with a thyroid particular fraction is described. H2O2 is measured by two different methods: iodination of NADPH itself when the system is supplemented with lactoperoxidase and [125I]iodide, and by the scopoletin method. It is shown that: H2O2 generation is inhibited by catalase and is dependent on NADPH or particulate protein concentration; radical scavengers of OH and of singlet oxygen have no effect while superoxide dismutase has only a marginal effect; disruption of the particular fraction by phospholipase A2 or digitonin treatment completely abolished H2O2 generation activity while thyroid peroxidase activity appears, suggesting different sites for the two activities in the membrane vesicles.
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PMID:NADPH-dependent H2O2 generation and peroxidase activity in thyroid particular fraction. 643 Jul 33

NADPH-ferredoxin reductase and ferredoxin activities have been identified in bovine thyroid particulate fractions (mainly mitochondria) after ultrasonication and DEAE-cellulose chromatography. The proteins were identified by their ability to reconstitute NADPH-cytochrome c reductase activity when used in combination. NADPH ferredoxin reductase and ferredoxin also catalyzed NADPH-dependent deiodination of L-diiodotyrosine; bovine adrenodoxin and adrenodoxin reductase could partially replace the thyroidal components in NADPH-dependent deiodination of L-diiodotyrosine. Both these reconstitutive activities were substantially inhibited by the iron chelators, alpha, alpha'-dipyridyl and o-phenanthroline. Deiodination by the NADPH-ferredoxin reductase-ferredoxin system was inhibited by the addition of a previously characterized dithionite-responsive flavoprotein iodotyrosine deiodinase (preparation F), isolated and purified from bovine thyroid particulate fractions after solubilization with steapsin. Ferredoxin reductase alone showed dithionite-responsive deiodinase activity and elution profiles of this activity on gel filtration before and after steapsin treatment suggest that preparation F may be a form of ferredoxin reductase modified by steapsin.
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PMID:Ferredoxin and ferredoxin reductase activities in bovine thyroid. Possible relationship to iodotyrosine deiodinase. 677 76


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