Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.11.1.7 (peroxidase)
 65,474 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Free diiosotyrosine exerts two opposite effects on the reactions catalyzed by thyroid peroxidase, thyroglobulin iodination and thyroid hormone formation. 1. Inhibition of thyroglobulin iodination catalyzed by thyroid peroxidase was observed when free diiodotyrosine concentration was higher than 5 muM. This inhibition was competitive, suggesting that free diiodotyrosine interacts with the substrate site(s) of thyroid peroxidase. Free diiodotyrosine also competively inhibited iodide peroxidation to I2. 2. Free diiodotyrosine, when incubated with thyroid peroxidase in the absence of iodide was recovered unmodified; in the presence of iodide an exchange reaction was observed between the iodine atoms present in the diiodotyrosine molecule and iodide present in the medium. Using 14C-labelled diiodotyrosine, 14C-labelled non-iodinated products were also observed, showing that deiodination occurred as a minor degradation pathway. However, no monoiodo[14C]tyrosine or E114C]tyrosine were observed. Exchange reaction between free diiototyrosine and iodide is therefore direct and does not imply deiodination-iodination intermediary steps. Thyroglobulin inhibits diiodotyrosine-iodide exchange and vice versa, again suggesting competition for both reactions. These results support, by a different experimental approach, the two-site model for peroxidase previously described by us in this journal. 3. Free diiodotyrosine when present at a very low concentration, 0.05 muM, exerts a stimulatory effect on throid hormones synthesis. The relationship between diiodotyrosine concentration and thyroid hormone synthesis give an S-shaped curve, suggesting that free diiodotyrosine acts as a regulatory ligand for thyroid peroxidase. Evidence is also presented that free diiodotyrosine is not incorporated into thyroid hormones. Therefore, thyroid peroxidase catalyzes only intra-molecular coupling between iodotyrosine hormonogenic residues. 4. Finally, although no direct proof exists that these free diiodotyrosine effects upon thyroglobulin iodination and thyroid hormone synthesis are physiologically significant, such a possibility deserves further investigation.
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PMID:Free diiodotyrosine effects on protein iodination and thyroid hormone synthesis catalyzed by thyroid peroxidase. 114 35

A 37-yr-old woman with nontoxic goiter is presented. The thyroid 131I uptake at 3 and 24 hr were, respectively, 77.1% and 81.4% dose. Thiocyanate discharged 65.5% of the accumulated 131I in 30 min. In vitro organification of iodine in the thyroid homogenate from the patient was impaired and it was restored to normal by the addition of H2O2, glucose, and glucose oxidase system, FAD, or reduced cytochrome b5. Riboflavin, FMN, oxidized cytochrome b5, oxidized or reduced cytochrome c, NAD(H), and NADP(H) were ineffective in the reaction. The microsomal NADH-cytochrome b5 reductase activity was definitely low in the patient's thyroid. It was augmented to a normal level by incubation of the microsomes with FAD for 30 min or more. The activities of thyroid peroxidase, G6-PD, 6-PGD, catalase, protease, and NADPH-cytochrome c reductase were within normal limits. The major thyroid protein was normal thyroglobulin which could be readily iodinated in the presence of H2O2 and horse radish peroxidase. These findings suggest the correlation of an iodide organification defect with a cytochrome b5 reductase deficiency. Administration of high doses of FAD led to the restoration of thyroidal iodide organification mechanism associated with an increased thyroid hormone production and to a marked decrease of the goiter. Riboflavin was given without effect even at a high dosage level. Consequently, it seems likely that the deficient cytochrome b5 reductase activity in this patient is due to a defect in the biosynthesis of FAD, the coenzyme of the reductase, from riboflavin.
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PMID:Deficient cytochrome b5 reductase activity in nontoxic goiter with iodide organification defect. 116 26

Iodination of a non halogenated goiter thyroglobulin and the resulting thyroxinogenesis was studied in vitro with purified thyroid peroxidase, H2O2 generating system and various concentrations of iodide. The rate of iodination was linear during the first minutes of incubation but thyroxine synthesis only began after a lag period whatever the iodide concentration in the incubation medium was. With high iodide concentrations a highly iodinated thyroglobulin (40-50 iodine atoms) containing no thyroxine was obtained after 3 minutes of incubation. If this highly iodinated goiter thyroglobulin was purified and reincubated with peroxidase and H2O2, thyroxine synthesis was again observed only after a lag period (2-3 min). In the absence of iodide the enzyme to elicit thyroxine synthesis. Depending of its concentration free diiodotyrosine exerts two opposite effects on the reaction catalyzed by thyroid peroxidase : at high concentration (10(-4) M) in inhibition of thyroglobulin iodination, and at low concentration (10(-7), 10(-8) M) a stimulating effect on thyroid hormones biosynthesis.
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PMID:[Proceedings: In vitro thyroid hormone formation (author's transl)]. 119 Jul 29

Two patients (G2, G3) with iodine organification defect were studied. The first patient (G2), a 25-year-old women with no clinical hypothyroidism, had had her goiter for 10 years; 62% of the thyroidal iodine was released by perchlorate indicating iodine organification defect. The thyroid tissue obtained at thyroidectomy contained a normal concentration of thyroid peroxidase (I2 formation from I-) when tested after solubilization of the enzyme by trypsin and digitonin treatment of the particulate material. 1. The enzymatic activity (G2-TPO) behaved on DEAE cellulose chromatography very differently from those of hog (P-TPO) or another human goiter peroxidase (G1-TPO) (Pommier, et al., J Clin Endocrinol Metab 39: 69, 1974): the molarity of elution was 2M NaCl instead of 0.15 mM. 2. Both P-TPO and G2-TPO catalyzed iodide peroxidation (I- leads to I2) but the Km (iodide) value for G2-TPO was much lower (2.3 x 10(-2) M) when compared with that of P-TPO (3.7 x 10(-3) M) or G1-TPO (3.5 x 10(-3) M). In addition, the optimum pH for this reaction differed markedly (pH 6.1 instead of 7.9). 3. G2-TPO was poorly efficient in catalyzing the oxidation of gaiacol to tetragaiacol. 4. G2-TPO was unable to perform the iodination of non-iodinated goiter thyroglobulin whatever the pH and the iodide concentration. 5. Thyroglobulin from this goiter (G2) was almost not iodinated (0.0014%), i.e., 0.07 atoms iodine/mole thyroglobulin), and its total content in the gland was very low (0.3-4 g/1000 g wet tissue instead of 25 g). A clear discrepancy was thus shown between the euthyroid state of this patient and the total lack of iodinating activity of the isolated peroxidase. The second patient (G3), a 17-year-old man with clinical hypothyroidism, had had his goiter for 5 years. 100% of the thyroidal iodine was released by perchlorate indicating a complete iodine organification defect. The thyroid tissue obtained at thyroidectomy contained no peroxidase activity when tested before and after treatment of the particulate material by trypsin and digitonin and even in the presence of hematin. Thyroglobulin from this goiter, which was almost non-iodinated (0.0014%), was present in normal amounts in the gland (congruent to 25 g/1000 g).
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PMID:Thyroid iodine organification defects: a case with lack of thyroglobulin iodination and a case without any peroxidase activity. 126 32

A human-mouse hybridoma has been produced by fusion of Hashimoto thyroid lymphocytes with the mouse myeloma line X63-Ag8.653. The cloned hybridoma secreted 2.5 micrograms per 10(6) cells per day of an IgG kappa thyroid peroxidase (TPO) autoantibody (2G4) with high affinity (2.5 x 10(9) molar-1) and specificity for human TPO. 2G4 did not react with lactoperoxidase, horseradish peroxidase or human myeloperoxidase or with porcine TPO or with human thyroglobulin. Plastic tubes coated with 2G4 bound about 50% of 125I-labelled human TPO added and the binding was inhibited by IgGs prepared from 18/18 TPO autoantibody-positive sera. This indicated that all 18 sera contained autoantibodies which recognised the same (or closely related) epitope as 2G4. Plastic tubes coated with IgGs from different TPO autoantibody-positive patient sera also bound 125I-labelled TPO but inhibition by 2G4 in this system was not complete. This suggested that the sera contained at least 2 types of TPO autoantibodies, with only one type of autoantibody reactive with the same epitope as 2G4.
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PMID:Production and characterisation of a human monoclonal thyroid peroxidase autoantibody. 128 77

Myeloperoxidase (MPO), which displays considerable amino acid sequence homology with thyroid peroxidase (TPO) and lactoperoxidase (LPO), was tested for its ability to catalyze iodination of thyroglobulin and coupling of two diiodotyrosyl residues within thyroglobulin to form thyroxine. After 1 min of incubation in a system containing goiter thyroglobulin, I-, and H2O2, the pH optimum of MPO-catalyzed iodination was markedly acidic (approximately 4.0), compared to LPO (approximately 5.4) and TPO (approximately 6.6). The presence of 0.1 N Cl- or Br- shifted the pH optimum for MPO to about 5.4 but had little or no effect on TPO- or LPO-catalyzed iodination. At pH 5.4, 0.1 N Cl- and 0.1 N Br- had a marked stimulatory effect on MPO-catalyzed iodination. At pH 4.0, however, iodinating activity of MPO was almost completely inhibited by 0.1 N Cl- or Br-. Inhibition of chlorinating activity of MPO by Cl- at pH 4.0 has been previously described. When iodination of goiter thyroglobulin was performed with MPO plus the H2O2 generating system, glucose-glucose oxidase, at pH 7.0, the iodinating activity was markedly increased by 0.1 N Cl-. Under these conditions iodination and thyroxine formation were comparable to values observed with TPO. MPO and TPO were also compared for coupling activity in a system that measures coupling of diiodotyrosyl residues in thyroglobulin in the absence of iodination. MPO displayed very significant coupling activity, and, like TPO, this activity was stimulated by a low concentration of free diiodotyrosine (1 microM). The thioureylene drugs, propylthiouracil and methimazole, inhibited MPO-catalyzed iodination both reversibly and irreversibly, in a manner similar to that previously described for TPO-catalyzed iodination.
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PMID:Myeloperoxidase-catalyzed iodination and coupling. 131 92

The three-dimensional structure of the enzyme myeloperoxidase has been determined by X-ray crystallography to 3 A resolution. Two heavy atom derivatives were used to phase an initial multiple isomorphous replacement map that was subsequently improved by solvent flattening and non-crystallographic symmetry averaging. Crystallographic refinement gave a final model with an R-factor of 0.257. The root-mean-square deviations from ideality for bond lengths and angles were 0.011 A and 3.8 degrees. Two, apparently identical, halves of the molecule are related by local dyad and covalently linked by a single disulfide bridge. Each half-molecule consists of two polypeptide chains of 108 and 466 amino acid residues, a heme prosthetic group, a bound calcium ion and at least three sites of asparagine-linked glycosylation. There are six additional intra-chain disulfide bonds, five in the large polypeptide and one in the small. A central core region that includes the heme binding site is composed of five alpha-helices. Regions of the larger polypeptide surrounding this core are organized into locally folded domains in which the secondary structure is predominantly alpha-helical with very little organized beta-sheet. A proximal ligand to the heme iron atom has been identified as histidine 336, which is in turn hydrogen-bonded to asparagine 421. On the distal side of the heme, histidine 95 and arginine 239 are likely to participate directly in the catalytic mechanism, in a manner analogous to the distal histidine and arginine of the non-homologous enzyme cytochrome c peroxidase. The site of the covalent linkage to the heme has been tentatively identified as glutamate 242, although the chemical nature of the link remains uncertain. The calcium binding site has been located in a loop comprising residues 168 to 174 together with aspartate 96. Myeloperoxidase is a member of a family of homologous mammalian peroxidases that includes thyroid peroxidase, eosinophil peroxidase and lactoperoxidase. The heme environment, defined by our model for myeloperoxidase, appears to be highly conserved in these four mammalian peroxidases. Furthermore, the conservation of all 12 cysteine residues involved in the six intra-chain disulfide bonds and the calcium binding loop suggests that the three-dimensional structures of members of this gene family are likely to be quite similar.
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PMID:X-ray crystal structure of canine myeloperoxidase at 3 A resolution. 132 Jan 28

2-Thiazoline-2-thiol is an antithyroid agent that strongly reduces thyroid hormone levels. Synthesis of these hormones is catalyzed in vivo by thyroid peroxidase. The interaction of this drug with molecular iodine and its effect on peroxidase activity were investigated. Iodine and 2-thiazoline-2-thiol form a complex of the charge transfer type of 1:1 stoichiometry characterized by a formation constant of 2,527 l.mole-1 at 20 degrees C. This drug was found to inhibit both horseradish peroxidase and lactoperoxidase (used as a model of thyroid peroxidase) in a competitive manner, giving inhibition constants of 5.7 mM and 0.13 mM, respectively. T3 and T4 levels were reduced significantly after a three-week administration of this drug to a group of 10 rats. Histological examination of the thyroid gland showed the presence of a cylindrical epithelium, which is indicative of hyperactivity of the gland. The results indicated that 2-thiazoline-2-thiol acts on both molecular iodine and thyroid peroxidase.
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PMID:Sites of action of 2-thiazoline-2-thiol on biogenesis of thyroid hormones. 138 Sep 99

The coupling activity of thyroid peroxidase (TPO) in thyroid glands from patients with benign adenoma, papillary carcinoma, and diffuse goiter (Graves' disease) was measured for the first time, in addition to the peroxidase activity of these tissues. The peroxidase activity of TPO in the mitochondria-microsomes fraction was measured with guaiacol or iodide as the second substrate. In the case of papillary carcinoma, the mean protein-based specific activity obtained by the guaiacol assay was about 1/7 of that of diffuse goiter. The iodide oxidation activity of carcinoma was very low, about 1/25 [corrected] of that in diffuse goiter and 1/70 of that in adenoma. The peroxidase activity in adenoma was almost similar in the guaiacol oxidation assay and approximately one half in the iodide oxidation assay as compared with that in diffuse goiter. There was a close correlation between the guaiacol and iodide oxidation assays in individual patients with adenoma and diffuse goiter, but not in patients with papillary carcinoma. The coupling activity of TPO was measured with thyroglobulin purified from pooled toxic diffuse goiters and chemically iodinated to contain little additional T3 and T4. The specific coupling activity of TPO in mitochondria-microsomes from carcinoma was significantly lower (about 1/5) than that of diffuse goiter, and the activity in adenoma was not significantly different (about 1/2) from that of diffuse goiter. The data of coupling activities has a close correlation with that of peroxidase activities in individual patients with adenoma but not in patients with carcinoma. Based on these findings, the qualitative abnormality of TPO and its relation to the cold 123I scintigram in thyroid tumors are discussed.
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PMID:Peroxidase and coupling activities of thyroid peroxidase in benign and malignant thyroid tumor tissues. 142 30

In routine guaiacol assays for thyroid peroxidase and lactoperoxidase employing a newly purchased bottle of guaiacol from Aldrich Chemical Co., we were surprised to find the formation of a blue color instead of the expected amber color classically associated with this assay. This was observed also with horseradish, myelo-, and cytochrome c peroxidase. The blue color (Amax approximately 650 nm) was not formed with guaiacol reagents obtained from two other chemical companies, nor was it seen with a bottle of old Aldrich guaiacol that had been in use in the laboratory for more than 10 years. In the present investigation we provide evidence that formation of the blue color is closely associated with the presence of a low concentration of catechol (approximately 0.5 mol%) in the new Aldrich guaiacol reagent. Catechol itself, even in much higher concentration, is a very weak donor for peroxidase, forming a light pink color. The blue color in Aldrich new guaiacol is not formed to the exclusion of 470-nm-absorbing product(s). Formation of the latter is, however, inhibited, and use of Aldrich new guaiacol for assay leads to low values for peroxidase activity. Other dihydroxyphenols (resorcinol and hydroquinone) do not mimic the action of catechol in formation of the blue color. Resorcinol is a very potent inhibitor of peroxidation of guaiacol. Possible schemes are proposed for formation of the products that may be associated with the amber and blue colors.
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PMID:An unexpected side reaction in the guaiacol assay for peroxidase. 144 72


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