Thyroid peroxidase activity is inhibited by amino acids

Abstract

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 µM each) also inhibited the TPO iodide oxidation reaction completely, whereas casamino acids (0.1 mg/ml), and tyrosine, phenylalanine and histidine (50 µM each) inhibited the TPO reaction by 54% or less. A pancreatic digest of gelatin (0.1 mg/ml) or any other amino acid (50 µM) 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 µM. 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 (H 2 O 2 ) 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.

Flavonoid Rutin Increases Thyroid Iodide Uptake in Rats

“…TPO extraction and activity measurement were performed as previously described [30–32]. Rat thyroids were minced and homogenized in 0.5 ml of 50 mM Tris-HCl buffer, pH 7.2, containing 1 mM KI, using an Ultra-Turrax homogenizer (Staufen, Germany).…”

Effects of Dietary Protein on Thyroid Axis Activity

“…The source of consumed protein has a significant impact on the thyroid hormone status. It was shown that some of the sulphur and aromatic amino acids derived from acidic hydrolysis of casein were able to inhibit in vitro the iodide oxidation reaction catalysed by thyroid peroxidase (TPO) [ 40 ]. Although cysteine and methionine or tyrosine and tryptophan were demonstrated to impair TPO iodination activity, their inhibitory potential was reversible with higher iodide concentrations [ 40 ].…”

“…It was shown that some of the sulphur and aromatic amino acids derived from acidic hydrolysis of casein were able to inhibit in vitro the iodide oxidation reaction catalysed by thyroid peroxidase (TPO) [ 40 ]. Although cysteine and methionine or tyrosine and tryptophan were demonstrated to impair TPO iodination activity, their inhibitory potential was reversible with higher iodide concentrations [ 40 ]. There is also evidence that the excitatory amino acids (EAA) such as glutamate and aspartate are able to modify hormone secretion from the pituitary-thyroid axis [ 41 ].…”

Retinoic acid effects on thyroid function of female rats

“…The TPO iodide-oxidation activity was measured as previously described (Nakashima and Taurog 1978;Carvalho et al 1994Carvalho et al , 2000. The assay mixture contained: 1.0 mL of freshly prepared 50 mM sodium phosphate buffer, pH 7.4, containing 24 mM KI and 11 mM glucose, and increasing amounts of solubilized TPO.…”