Thyroid peroxidase activity is inhibited by amino acids

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 per cent 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 (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.

Molecular genetics of hereditary thyroid disease due to a defect in the thyroglobulin or thyroperoxidase synthesis

1. Hereditary goiter and the various degrees of thyroid hypofunction are the result of structural changes in the thyroglobulin (Tg) or thyroperoxidase (TPO) proteins, the inability to couple iodotyrosines or defective iodination, impairing or substantially altering the synthesis of T4 and T3. 2. The first nmutations in the Tg and TPO genes responsable for human cases of dys-hormonogenesis have been described. The mutation in two siblings with hereditary goiter and marked impairment of Tg synthesis was a cytosine to thymine transition creating a stop codon at postion 1510. The point mutation is removed by the preferential accumulation of a 171-nt deleted Tg mRNA. In another subject, molecular studies revealed that exon 4 was missing from the major Tg transcript due to a cytosine to guanine transversion at postion minus 3 in the acceptor splice site of intron 3. 3. Genomic DNA studies identified a duplication of a 4-base sequence in the eight exon of the TPO gene. Interestingly, besides abolishing the enzymatic activity by disrupting the reading frame of the messenger RNA and introducing stop codons, the GGCC duplication also unmasks a cryptic acceptor splice site in exon 9. 4. In conclusion, the identification of different molecular defects provied evidence that hereditary goiter associated with abnormal Tg or TPO synthesis is caused by heterogeneous genetic alterations

Serum rhodanese in goitre and calcific pancreatitis of tropics.

Rhodanese is one of the enzymes concerned in the detoxification of cyanide. Cassava intake and consequent cyanide toxicity are incriminated in the pathogenesis of goitre and calcific pancreatitis of tropics. So we studied the activity of rhodanese in these patients. 14 controls, 13 patients with pancreatitis and 12 with goitre were studied. The median (and range) of rhodanese in these groups were 82 (50-144), 110 (64-180) and 71 (22-160) units respectively. The serum rhodanese was significantly higher (P less than 0.05) in patients with pancreatitis when compared to the other groups. There was no significant difference between the serum rhodanese in patients with goitre and the controls. The presence of adequate amounts of rhodanese indicates that goitre and chronic pancreatitis are not produced by impaired cyanide detoxification.