Abstract:
:Thyroid peroxidase (TPO) simultaneously catalyzes two very different types of reaction in the thyroid gland- iodination and coupling. The present study addresses the mechanism of this simultaneous dual activity. Compound I, the two-electron oxidation product of TPO, exists in two different forms--an oxoferryl porphyrin pi-cation radical and an oxoferryl protein radical. It has been proposed that iodination is mediated by the porphyrin pi-cation radical form of TPO compound I, while coupling is mediated by the protein radical form. However, results obtained in the present study favor the view that both iodination and coupling are mediated by the porphyrin pi-cation radical form of compound I. In the first part of the study, we compared coupling and iodination activities of two peroxidases with very similar crystal structures--cytochrome c peroxidase (CcP) and lignin peroxidase (LiP). Although these two peroxidases have very similar three-dimensional structures, CcP forms a compound I only of the protein radical type, whereas compound I of LiP exists only as a porphyrin pi-cation radical. Comparison of the catalytic activities of the two enzymes showed that diiodotyrosine (DIT)-stimulated coupling activity of LiP was significantly greater than that of CcP. Moreover, lignin peroxidase displayed very significant iodinating activity at acid pHs, whereas iodination with CcP was negligible at all pHs tested. Our findings with these two structurally similar peroxidases suggested that TPO-catalyzed iodination and coupling could both be mediated by the porphyrin pi-cation radical form of compound I. More direct evidence in support of this view was obtained in the second part of this study, employing TPO and lactoperoxidase (LPO) model systems in which iodination and coupling occurred simultaneously. Heme spectral analysis was used to correlate formation of the protein radical form of compound I with the kinetics of the iodination and coupling reactions. Formation of the compound I protein radical was not observed until the iodination and coupling reactions had almost been completed. In separate experiments it was shown that the spontaneous conversion of the porphyrin pi-cation radical form of TPO or LPO compound I to the protein radical form was markedly inhibited by a low concentration of iodide, especially in the presence of an iodide acceptor. These studies provide compelling evidence that both iodination and coupling are mediated by the porphyrin pi-cation radical form of compound I. This was further substantiated by the finding that coupling was inhibited in the presence of excess iodide, an observation readily explained by competition between iodide and DIT residues in thyroglobulin for oxidation by the porphyrin pi-cation radical.
journal_name
Arch Biochem Biophysjournal_title
Archives of biochemistry and biophysicsauthors
Taurog A,Dorris ML,Doerge DRdoi
10.1006/abbi.1996.0222subject
Has Abstractpub_date
1996-06-01 00:00:00pages
24-32issue
1eissn
0003-9861issn
1096-0384pii
S0003-9861(96)90222-2journal_volume
330pub_type
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journal_title:Archives of biochemistry and biophysics
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journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章
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pub_type: 杂志文章
doi:10.1006/abbi.1998.0847
更新日期:1998-09-15 00:00:00
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journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章
doi:10.1006/abbi.1999.1402
更新日期:1999-10-15 00:00:00
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pub_type: 杂志文章
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更新日期:1985-09-01 00:00:00
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journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章
doi:10.1016/0003-9861(88)90450-x
更新日期:1988-01-01 00:00:00
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journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章,评审
doi:10.1016/j.abb.2016.08.018
更新日期:2016-12-01 00:00:00
abstract::Protein synthesis and lipid peroxidation were evaluated in rat liver slices incubated in the presence of oxidants and protein synthesis inhibitors. Protein synthesis by rat liver slices was evaluated by [3H]leucine incorporation into the trichloroacetic acid (TCA)-insoluble material, and lipid peroxidation was evaluat...
journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章
doi:10.1016/0003-9861(89)90010-6
更新日期:1989-04-01 00:00:00
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doi:10.1016/0003-9861(83)90388-0
更新日期:1983-01-01 00:00:00
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journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章
doi:10.1006/abbi.1997.0523
更新日期:1998-02-15 00:00:00
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journal_title:Archives of biochemistry and biophysics
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doi:10.1016/0003-9861(92)90523-y
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journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章
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更新日期:1987-08-15 00:00:00
abstract::The plant-derived flavonoid, quercetin (QCT), has many biological actions, including cardioprotective actions, resulting from its antioxidant and anti-inflammatory effects. In this study, effects of QCT and its metabolites on the contraction and Ca2+ transients (CaT) of mouse single cardiomyocytes were simultaneously ...
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pub_type: 杂志文章
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pub_type: 杂志文章
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pub_type: 杂志文章
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journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章
doi:10.1016/j.abb.2008.01.004
更新日期:2008-08-15 00:00:00
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pub_type: 杂志文章
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journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章
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更新日期:2002-09-15 00:00:00
abstract::The effects of racemic (+/-) and (+)- and (-)-stereoisomers of isoflurane on depolarization-induced (45)Ca(2+) fluxes mediated by voltage-dependent Ca(2+) channels were investigated in transverse tubule membrane vesicles from rabbit skeletal muscle. In the concentration range 0.5 to 2 mM, (+/-)-isoflurane inhibited (4...
journal_title:Archives of biochemistry and biophysics
pub_type: 杂志文章
doi:10.1006/abbi.2001.2726
更新日期:2002-02-15 00:00:00