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Inhibition of thyroid peroxidase by Myrcia uniflora flavonoids.

Ferreira AC, Neto JC, da Silva AC, Kuster RM, Carvalho DP.

Chem Res Toxicol. 2006 Mar;19(3):351-5.

[abstract only]

 

"Thyroid peroxidase (TPO), the key enzyme in thyroid hormone biosynthesis, is inhibited by dietary flavonoids; thus, a high consumption of plants containing inhibitory flavonoids may affect thyroid function and lead to hypothyroidism. In this work, TPO inhibition by the aqueous partition of Myrcia uniflora and its isolated compounds has been evaluated. The aqueous partition of the methanolic extract of M. uniflora is able to inhibit TPO activity in vitro. Two known flavonoids were isolated and characterized by mass spectrometry and (1)H NMR from plant extracts: mearnsitrin and myricitrin. The degree of TPO inhibition produced by the aqueous solution of the flavonoids was very high, with a 50% inhibition of the original TPO activity (IC(50)) obtained at 1.97 microM mearnsitrin and at 2.88 microM myricitrin. These results suggest that the indiscriminated consumption of M. uniflora pharmaceutical products allied to the nutritional deficiency of iodine might contribute to the development of hypothyroidism and goiter."

 

 

Inhibition of thyroid type 1 deiodinase activity by flavonoids.

Ferreira AC, Lisboa PC, Oliveira KJ, Lima LP, Barros IA, Carvalho DP.

Food Chem Toxicol. 2002 Jul;40(7):913-7.

[abstract only]

 

"Some dietary flavonoids inhibit thyroperoxidase and hepatic deiodinase activity, indicating that these compounds could be classified as anti-thyroid agents. In this study, we evaluated the in vitro effect of various flavonoids on thyroid type 1 iodothyronine deiodinase activity (D1). D1 activity was measured in murine thyroid microsome fractions by the release of 125I from 125I-reverse T3. D1 activity was significantly inhibited by all the flavonoids tested; however, the inhibitory potencies on thyroid D1 activity differed greatly among them. A 50% inhibition of D1 activity (IC(50)) was obtained at 11 microM baicalein, 13 microM quercetin, 17 microM catechin, 55 microM morin, 68 microM rutin, 70 microM fisetin, 72 microM kaempferol and 77 microM biochanin A. Our data reinforce the concept that dietary flavonoids might behave as antithyroid agents, and possibly their chronic consumption could alter thyroid function."

 

 

Thyroid peroxidase activity is inhibited by amino acids.

Carvalho DP, Ferreira AC, Coelho SM, Moraes JM, Camacho MA, Rosenthal D.

Braz J Med Biol Res. 2000 Mar;33(3):355-61.

 

"Normal in vitro thyroid peroxidase (TPO) iodide oxidation activity was completely inhibited by a hydrolyzed TPO preparation (0.15 mg/ml) or hydrolyzed bovine serum albumin (BSA, 0.2 mg/ml). A pancreatic hydrolysate of casein (trypticase peptone, 0.1 mg/ml) and some amino acids (cysteine, tryptophan and methionine, 50 microM each) also inhibited the TPO iodide oxidation reaction completely, whereas casamino acids (0.1 mg/ml), and tyrosine, phenylalanine and histidine (50 microM each) inhibited the TPO reaction by 54% or less. A pancreatic digest of gelatin (0.1 mg/ml) or any other amino acid (50 microM) tested did not significantly decrease TPO activity. The amino acids that impair iodide oxidation also inhibit the TPO albumin iodination activity. The inhibitory amino acids contain side chains with either sulfur atoms (cysteine and methionine) or aromatic rings (tyrosine, tryptophan, histidine and phenylalanine). Among the amino acids tested, only cysteine affected the TPO guaiacol oxidation reaction, producing a transient inhibition at 25 or 50 microM. The iodide oxidation inhibitory activity of cysteine, methionine and tryptophan was reversed by increasing iodide concentrations from 12 to 18 mM, while no such effect was observed when the cofactor (H2O2) concentration was increased. The inhibitory substances might interfere with the enzyme activity by competing with its normal substrates for their binding sites, binding to the free substrates or reducing their oxidized form."
 

 

Thyroid peroxidase inhibition by Kalanchoe brasiliensis aqueous extract.

Ferreira AC, Rosenthal D, Carvalho DP.

Food Chem Toxicol. 2000 May;38(5):417-21.

[abstract only]

 

"Flavonoids are known inhibitors of thyroid peroxidase (TPO) and some are components of Kalanchoe brasiliensis, a plant used in popular medicine to treat tissue injuries, enlarged ganglia and peptic ulcer. As K. brasiliensis extract is currently used, the present study was designed to evaluate the effects of K. brasiliensis aqueous extract on TPO activity. We show here that TPO iodide-oxidation activity was significantly inhibited by K. brasiliensis aqueous extract and that TPO inhibition seems to be competitive, since the enzyme V(max) was unchanged and K(m) for iodide was significantly increased in the presence of the plant extract. Furthermore, TPO inhibitory activity produced by K. brasiliensis extract was unchanged after boiling or by incubation with hepatic enzymes (activated S9 fraction), suggesting that at least the antithyroid component of this plant infusion could probably reach systemic circulation. We also report that K. brasiliensis aqueous extract is able to scavenge H(2)O(2), in vitro. As H(2)O(2) is an essential TPO cofactor, it is possible that the H(2)O(2) trapping effect of K. brasiliensis may be responsible, at least in part, for the inhibition of the iodide-oxidation reaction catalysed by this enzyme. Thus, K. brasiliensis aqueous extract has antithyroid effects in vitro, suggesting that its chronic consumption could contribute to the development of goitre and hypothyroidism, mainly in areas of low iodine intake."

 
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