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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)

Hog thyroid peroxidase (TPO) was highly purified in order to study the spectral properties and catalytic specificities of its H2O2 compounds in iodothyronine biosynthesis. Purified TPO exhibited a Soret spectrum with an absorption maximum at 410 nm and had an A410/A280 value of 0.55. Protein iodination was only catalyzed under conditions which allowed formation of the transient TPO compound I (Fe(IV)-pi o+). On addition of an equimolar amount of H2O2, TPO formed a stable compound with an absorption maximum at 417 nm. This compound efficiently catalyzed the coupling reaction, but was unable to iodinate proteins. It catalyzed the formation of 1 mol iodothyronines/mol TPO, and therefore retained two oxidizing equivalents per molecule. It is proposed that this compound constitutes a second form of compound I whose structure might be Fe(IV)-Ro, analogous to that of cytochrome c peroxidase compound I. In the presence of an excess of H2O2, it formed TPO-compound III with an absorption maximum at 420 nm. TPO-compound III catalyzed neither the iodination nor the coupling reaction.
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PMID:Spectral characteristics and catalytic properties of thyroid peroxidase-H2O2 compounds in the iodination and coupling reactions. 299 35

Thyroglobulin iodination and thyroxine synthesis in vitro require the presence of peroxidase, H2O2 and iodide. H2O2 is usually continuously generated by glucose oxidase (GO) and glucose. The aim of this study was to investigate whether the two enzymes could possibly be inactivated by a particular concentration of H2O2 or iodide present during incubation. The results revealed that both enzymes were indeed inactivated under two distinct conditions: Lactoperoxidase and thyroid peroxidase were inactivated by modest concentrations of H2O2 accumulating during incubation. Glucose oxidase was inactivated by an oxidized species of iodine or singlet oxygen produced in the catalytic cycle. The results may explain some hitherto unsolved discrepancies between different iodination procedures. Moreover they may have an impact on the regulation of in vivo thyroglobulin iodination and hormone synthesis.
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PMID:Inactivation of peroxidase and glucose oxidase by H2O2 and iodide during in vitro thyroglobulin iodination. 301 6

Glutathione (GSH) was oxidized to GSSG in the presence of H2O2, tyrosine, and peroxidase. During the GSH oxidation catalyzed by lactoperoxidase, O2 was consumed and the formation of glutathione free radical was confirmed by ESR of its 5,5'-dimethyl-1-pyrroline-N-oxide adduct. When lactoperoxidase was replaced by thyroid peroxidase in the reaction system, the consumption of O2 and the formation of the free radical became negligibly small. These results led us to conclude that, in the presence of H2O2 and tyrosine, lactoperoxidase and thyroid peroxidase caused the one-electron and two-electron oxidations of GSH, respectively. It was assumed that GSH is oxidized by primary oxidation products of tyrosine, which are phenoxyl free radicals in lactoperoxidase reactions and phenoxyl cations in thyroid peroxidase reactions. When tyrosine was replaced by diiodotyrosine or 2,6-dichlorophenol, the difference in the mechanism between lactoperoxidase and thyroid peroxidase disappeared and both caused the one-electron oxidation of GSH. Iodides also served as an effective mediator of GSH oxidation coupled with the peroxidase reactions. In this case the two peroxidases both caused the two-electron oxidation of GSH.
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PMID:Characterization of one- and two-electron oxidations of glutathione coupled with lactoperoxidase and thyroid peroxidase reactions. 302 21

Previous work from our laboratory has shown that 14-iodo-15-hydroxy-5,8,11-eicosatrienoic acid (I-HO-A) is a potent inhibitor of iodine organification in calf thyroid slices. The present studies were performed in order to clarify the mechanism of this action. Incubation of thyroid slices with 10(-4)M I-HO-A caused a 47 and 53% decrease in PB125I formation after 30 and 60 min incubation, respectively. In a series of experiments an inverse relationship between the degree of inhibition caused by I-HO-A and total iodine content and basal iodoprotein formation was observed. Chromatographic analysis of the labeled compounds showed a significant decrease in 125I incorporation into MIT, DIT, T3 and total iodolipid. The site of the inhibitory effect of I-HO-A was then sought. TPO was measured by three different methods. When TPO was solubilized from I-HO-A treated slices, no change in enzymatic activity was observed. Moreover, the same lack of action was found when solubilized TPO was incubated with I-HO-A. The production and release of H2O2 into the incubation medium was measured by chemiluminiscence technique. In control slices the values increased during the first 10 min and reached a plateau. Pretreatment of the slices with 10(-4)M KI caused a 51% inhibition, while the same concentration of I-HO-A produced a 59% inhibition. The possibility that I-HO-A might exert its action through a putative protein inhibitor was also explored. Incubation of slices with 10(-5)M I-HO-A caused a 46% decrease in PB125I formation and addition of actinomycin D or puromycin failed to alter this effect.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:The inhibition of PB125I formation in calf thyroid caused by 14-iodo-15-hydroxy-eicosatrienoic acid is due to decreased H2O2 availability. 313 Dec 25

Spectral scans in both the visible (650-450 nm) and the Soret (450-380 nm) regions were recorded for the native enzyme, Compound II, and Compound III of lactoperoxidase and thyroid peroxidase. Compound II for each enzyme (1.7 microM) was prepared by adding a slight excess of H2O2 (6 microM), whereas Compound III was prepared by adding a large excess of H2O2 (200 microM). After these compounds had been formed it was observed that they were slowly reconverted to the native enzyme in the absence of exogenous donors. The pathway of Compound III back to the native enzyme involved Compound II as an intermediate. Reconversion of Compound III to native enzyme was accompanied by the disappearance of H2O2 and generation of O2, with approximately 1 mol of O2 formed for each 2 mol of H2O2 that disappeared. A scheme is proposed to explain these observations, involving intermediate formation of the ferrous enzyme. According to the scheme, Compound III participates in a reaction cycle that effectively converts H2O2 to O2. Iodide markedly affected the interconversions between native enzyme, Compound II, and Compound III for lactoperoxidase and thyroid peroxidase. A low concentration of iodide (4 microM) completely blocked the formation of Compound II when lactoperoxidase or thyroid peroxidase was treated with 6 microM H2O2. When the enzymes were treated with 200 microM H2O2, the same low concentration of iodide completely blocked the formation of Compound III and largely prevented the enzyme degradation that otherwise occurred in the absence of iodide. These effects of iodide are readily explained by (i) the two-electron oxidation of iodide to hypoiodite by Compound I, which bypasses Compound II as an intermediate, and (ii) the rapid oxidation of H2O2 to O2 by the hypoiodite formed in the reaction between Compound I and iodide.
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PMID:Spectral studies with lactoperoxidase and thyroid peroxidase: interconversions between native enzyme, compound II, and compound III. 340 Oct 9

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

Peroxidases are known to be involved in the intracellular metabolism of H2O2 coupled with various physiological functions. Apart from the thyroid gland, the enzyme has been isolated from various extrathyroidal sources of which salivary gland is one of the richest sources of the enzyme. The enzyme from bovine and goat submaxillary gland has been extensively studied in terms of their molecular, spectral, kinetic, catalytic and immunological properties and compared with the lactoperoxidase which is similar to the salivary peroxidase. The modulation of the salivary peroxidase by various factors and the probable mechanism of the modulation has been described. The enzyme has also been compared with the thyroid peroxidase as regards their physicochemical properties as well as on the immunological and functional aspects. The similarities and dissimilarities have been incorporated. The possible function of the enzyme in iodine metabolism and in bactericidal action has been discussed.
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PMID:Salivary peroxidases. 352 Feb 91

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

The coupling of iodotyrosine (coupling reaction) is one of the least studied in the formation of thyroid hormone, particularly in human thyroid diseases. This paper describes a method of measuring iodotyrosine coupling catalyzed by human thyroid peroxidase (TPO) in vitro. There were two important requirements to demonstrate the coupling reaction: 1) thyroglobulin with a low thyroid hormone content, and 2) partially purified TPO. Thyroglobulin with low thyroid hormone content was obtained from Grave's and follicular adenoma tissues after propylthiouracil (PTU) therapy and L-T4 therapy, respectively. TPO was prepared from Graves' thyroid by solubilizing the 100,000 X g pellet of thyroid homogenate with sodium deoxycholate and trypsin, followed by Sephacryl S-300 gel filtration. Before the coupling reaction, thyroglobulin was iodinated with chloramine-T and potassium iodide, followed by dialysis. The coupling reaction was carried out by incubating newly iodinated thyroglobulin with TPO, diiodotyrosine, a coupling stimulator, and a H2O2-generating system (glucose and glucose oxidase) for 20 min at 37 C. After thyroglobulin was digested with Pronase, the thyroid hormone content of the thyroid digest was measured by RIA. Coupling activity was measured by the amount of newly formed T3 (nanograms of T3 per mg thyroglobulin). The time course of coupling reaction showed a progressive increase in coupling activity up to 30 min, and the reaction was temperature and pH dependent, with a pH optimum of 7.0. Coupling activity in the presence of H2O2 and TPO was 43 +/- 5.0 ng T3/mg thyroglobulin (mean +/- SD of triplicate samples), and addition of diiodotyrosine to the H2O2-TPO system caused a nearly 3-fold increase in coupling activity. This method has potential utilization for measurement of peroxidase coupling activity, since there was a linear relationship between the measured coupling activity and the amount of added TPO when the TPO concentration was over 3 micrograms/300 microliter. Methimazole (MMI) and PTU had similar potencies in inhibiting the TPO-catalyzed coupling reaction, whereas MMI was distinctly more potent than PTU as an inhibitor of TPO-mediated iodination in vitro. The different potencies of MMI in the two reactions suggest that different inhibitory mechanisms may be involved in iodination and coupling. The reducing agent, sodium metabisulfite, was also found to be a more potent inhibitor of the TPO-mediated coupling reaction than of the TPO-mediated iodination reaction. The method of iodotyrosine coupling described here may be useful to investigate the coupling step of thyroid hormone formation in human thyroid diseases.
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PMID:Coupling of iodotyrosine catalyzed by human thyroid peroxidase in vitro. 383 97

The present study was undertaken to investigate degradation of thyroxine (T4) mediated by thyroid peroxidase in man. A particulate fraction (1,000-100,000 x g) of normal human thyroid tissue was prepared and used as crude enzyme. 125I-T4 and unlabeled T4 were incubated with the particulate fraction in buffer containing glucose and glucose oxidase for generation of H2O2. After incubation, iodoamino acids were extracted with ethanol and the products of T4 degradation were analyzed by thin layer chromatography. In this system, T4 was degraded in time-, temperature- and pH-dependent manners, but not in the absence of the H2O2-generating system. The rate of degradation was related to concentration of the particulate fraction. The reaction was inhibited by methimazole, propylthiouracil and catalase. When [3',5'-125I] T4 was used as a tracer, major labeled products of T4 degradation were inorganic iodide and ethanol-unextracted fraction and no detectable labeled 3,5,3'-triiodothyronine (T3) or 3,3',5'-triiodothyronine (rT3) was generated. From a kinetic study by adding various doses of unlabeled T4, the apparent Km value for T4 was 30 microM and the Vmax value was 230 pmol/mg protein/min. When [3,5-125I] T4 was incubated with enzyme preparation, one third of degraded T4 was recovered as diiodotyrosine (DIT) and half of 125I-DIT was degraded in parallel incubation. No formation of radiolabeled DIT was observed in incubation with Na- 125I done in tandem. These findings suggest that thyroid hormones can be metabolized by peroxidase in human thyroid by pathways that include cleavage of ether linkage.
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PMID:Peroxidatic degradation and ether link cleavage of thyroxine in a particulate fraction of human thyroid. 397 5


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