Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:1.17.3.2 (xanthine oxidase)
 8,383 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Thyroid hormone formation requires the coincident presence of peroxidase, H2O2, iodide, and acceptor protein at one anatomic locus in the cell. The peroxidase enzyme appears to be a protoporphyrin lX containing heme protein, with binding sites for both iodide and tyrosine. It is probable that both iodide and tyrosine are oxidized to free radical forms which unite to form iodotyrosine. The peroxidase is also involved through an uncertain mechanism in iodotyrosine coupling and probably in oxidation of sulfhydryl bonds in thyroglobulin. H2O2 may be supplied by microsomal NADPH-cytochrome c reductase or NADH-cytochrome b5 reductase. Other possible intracellular H2OI generating systems include monoamine oxidase and xanthine oxidase. The usual acceptor for iodide is thyroglobulin, which is currently believed to be iodinated within apical secretory vesicles at the cell border just prior to liberation into the colloid, or possibly after liberation into the colloid. Other soluble an insoluble proteins are also iodinated within the gland. The peroxidase is present in numerous cellular structures, but iodination activity occurs primarily, if not only, at the apical cell border. The controls of iodination are imperfectly known. Thyrotrophin modulation of iodide uptake, H2O2 generation, thyroglobulin synthesis, and peroxidase enzyme level obviously are the main regulations. Many of these actions are thought to involve mediation of adenyl cyclase and subsequent activation of intracellular phosphokinases. Antithyroid drugs of the thiocarbamide group are competitive inhibitors of iodination under some circumstances, but if much iodide is present, they react with the oxidized iodine intermediate and are irreversibly inactivated themselves. Clinical problems involving defective peroxidase function are among the most frequent hereditary defects of thyroid hormone formation. Recognized abnormalities include deficient peroxidase, abnormality in binding of the peroxidase apoprotein to its prosthetic group, and other less well-identified abnormalities in peroxidase structure and function. Peroxidase is typically elevated in thyroid tissue from patients with hyperthyroidism sometimes deficient in cold thyroid nodules, and frequently diminished in tissue from patients with Hashimoto's thyroiditis.
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PMID:Biosynthesis of thyroid hormone: basic and clinical aspects. 6 47

To examine the possibility of luteolin as a whitening agent, we measured antioxidant activity using DPPH assay, NBT/XO assay and intracellular ROS scavengning assay and depigmenting activity using tyrosinase assay, alpha-MSH-induced melanin production in B-16 cells. Luteolin showed dose-dependent anti-oxidant activity in DPPH, NBT/XO and intracellular ROS assay. Also, luteolin directly inhibited xanthine oxidase activity in a dose-dependent manner. Although luteolin did not directly inhibit tyrosinase activity, it dose-dependently inhibited both tyrosinase activity and melanin production in B16 melanoma cells stimulated by 1 microM alpha-MSH. Luteolin dose-dependently inhibited cAMP levels in B16 melanoma cells stimulated by 1 microM alpha-MSH and 1 microM forskolin, which suggest that luteolin directly inhibits adenyl cyclase in B16 melanoma cells. Therefore, these results suggest that whitening activity of luteolin may be due to the inhibition of adenyl cyclase involved in the signal pathway of alpha-MSH in B16 melanoma cells.
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PMID:Whitening activity of luteolin related to the inhibition of cAMP pathway in alpha-MSH-stimulated B16 melanoma cells. 1880 60