Opposite effects of thiocyanate on tyrosine iodination and thyroid hormone synthesis

A high throughput screening assay for human Thyroperoxidase inhibitors

Rapid, human relevant assays are needed to assess potential hazards of the many chemicals in commerce. An assay of thyroid peroxidase (TPO) inhibition, using the substrate Amplex Ultra Red, was recently adapted for human TPO (AUR-hTPO). We tested a large number (788) of chemicals through this AUR-hTPO assay and compared performance with published results from an assay using enzyme from rat thyroid microsomes (AUR-rTPO). Coded chemicals, from the US EPA ToxCast Inventory, were tested in a tiered approach: 1) Initial screening at a single concentration; 2) Potency estimation for active chemicals with multiple concentrations; 3) Screening active chemicals for the non-specific activity. The assay gave consistent results for positive chemical methimazole and several positive and negative reference chemicals. hTPO inhibition was observed for 190 chemicals reported as positive in rTPO. Of these, 158 showed no confounding activity (interference due to fluorescence or non-specific protein inhibition). Comparison of all result with rTPO data and with evidence of TPO inhibition found in the literature suggest that the current assay has a higher rate of false negative but a much lower rate of false positive compared with the rTPO screen. These findings underscore the effectiveness of the AUR assay, using hTPO enzyme from engineered cell lines, to identify moderate to strong inhibitors but some improvements may be needed to detect weak TPO inhibitors.

Reactive Halogen Species: Role in Living Systems and Current Research Approaches

Reactive halogen species (RHS) are highly reactive compounds that are normally required for regulation of immune response, inflammatory reactions, enzyme function, etc. At the same time, hyperproduction of highly reactive compounds leads to the development of various socially significant diseases - asthma, pulmonary hypertension, oncological and neurodegenerative diseases, retinopathy, and many others. The main sources of (pseudo)hypohalous acids are enzymes from the family of heme peroxidases - myeloperoxidase, lactoperoxidase, eosinophil peroxidase, and thyroid peroxidase. Main targets of these compounds are proteins and peptides, primarily methionine and cysteine residues. Due to the short lifetime, detection of RHS can be difficult. The most common approach is detection of myeloperoxidase, which is thought to reflect the amount of RHS produced, but these methods are indirect, and the results are often contradictory. The most promising approaches seem to be those that provide direct registration of highly reactive compounds themselves or products of their interaction with components of living cells, such as fluorescent dyes. However, even such methods have a number of limitations and can often be applied mainly for in vitro studies with cell culture. Detection of reactive halogen species in living organisms in real time is a particularly acute issue. The present review is devoted to RHS, their characteristics, chemical properties, peculiarities of interaction with components of living cells, and methods of their detection in living systems. Special attention is paid to the genetically encoded tools, which have been introduced recently and allow avoiding a number of difficulties when working with living systems.

Perchlorate, nitrate, and thiocyanate: Environmental relevant NIS-inhibitors pollutants and their impact on thyroid function and human health

Thyroid disruptors are found in food, atmosphere, soil, and water. These contaminants interfere with the thyroid function through the impairment of thyroid hormone synthesis, plasma transport, peripheral metabolism, transport into the target cells, and thyroid hormone action. It is well known that iodide uptake mediated by the sodium-iodide symporter (NIS) is the first limiting step involved in thyroid hormones production. Therefore, it has been described that several thyroid disruptors interfere with the thyroid function through the regulation of NIS expression and/or activity. Perchlorate, nitrate, and thiocyanate competitively inhibit the NIS-mediated iodide uptake. These contaminants are mainly found in food, water and in the smoke of cigarettes. Although the impact of the human exposure to these anions is highly controversial, some studies indicated their deleterious effects in the thyroid function, especially in individuals living in iodine deficient areas. Considering the critical role of thyroid function and the production of thyroid hormones for growth, metabolism, and development, this review summarizes the impact of the exposure to these NIS-inhibitors on thyroid function and their consequences for human health.

Assessment of Iodine Nutritional Status of School-Age Children in Kolkata District of West Bengal State in Post-Iodation Scenario

To evaluate the state of iodine nutrition in post-iodation scenario, 3500 children were examined clinically for endemic goitre. Iodine and thiocyanate were measured in 240 urine samples; iodine content in 210 salt samples was measured. Total goitre prevalence was 6.1%. Median urinary iodine level was 21.80 µg/dl, and mean (±SD) urinary thiocyanate was 0.89 ± 0.49 mg/dl. Iodine content of only 11.9% salt samples was below recommended level of 15 ppm, 25.2% was between 15 and 30 ppm and 62.9% was >30 ppm. Iodine deficiency disorders are thus clinically mild public health problem of the studied population; however, they have no biochemical iodine deficiency. Studied population found exposed to thiocyanate load that might be the possible cause for persistence of endemic goitre. People of Kolkata should be advised to eat commonly consumed goitrogenic foods after boiling and decanting the water. Periodical monitoring and evaluation of iodine status should be mandatory.

Population Survey of Iodine Deficiency and Environmental Disruptors of Thyroid Function in Young Children in Haiti

CONTEXT

Iodine deficiency is the leading cause of preventable neurodevelopmental delay in children worldwide and a possible public health concern in Haiti.

OBJECTIVE

To determine the prevalence of iodine deficiency in Haitian young children and its influence by environmental factors.

DESIGN

Cross-sectional study, March through June 2015.

SETTING

Community churches in 3 geographical regions in Haiti.

PARTICIPANTS

299 healthy Haitian children aged 9 months to 6 years; one-third each enrolled in a coastal, mountainous, and urban region.

MAIN OUTCOME MEASURES

Urinary iodide, serum thyrotropin (TSH), goiter assessment, and urinary perchlorate and thiocyanate.

RESULTS

Mean age was 3.3±1.6 years, with 51% female, median family income USD 30/week, and 16% malnutrition. Median urinary iodide levels were normal in coastal (145 μg/L, interquartile range [IQR] 97 to 241) and urban regions (187 μg/L, IQR 92 to 316), but revealed mild iodine deficiency in a mountainous region (89 μg/L, IQR 56 to 129), P < 0.0001. Grade 1 goiters were palpated in 2 children, but TSH values were normal. Urinary thiocyanate and perchlorate concentrations were not elevated. Predictors of higher urinary iodide included higher urinary thiocyanate and perchlorate, breastfeeding, and not living in a mountainous region.

CONCLUSIONS

Areas of mild iodine deficiency persist in Haiti's mountainous regions. Exposure to two well-understood environmental thyroid function disruptors is limited.

Thiocyanate status does not play a role in the etiology of residual goiter in school children of Isfahan, Iran

BACKGROUND

Despite long standing iodine supplementation in Iran the prevalence of goiter remains high in some areas. In the present study we investigated the possible role of thiocyanate as a goitrogen in the etiology of goiter in Isfahan, Iran.

METHODS

A total of 2331 (6-13 year old) school children were selected by multistage random sampling. Thyroid size was estimated in each child by inspection and palpation. Urinary iodine concentration (UIC) and urinary thiocyanate (USCN) were measured.

RESULTS

Overall, 32.9% of the 2331 students had goiter. The median UIC was 195.5 μg/L. The mean ± SD of USCN in goitrous and nongoitrous subjects was 0.42 ± 0.28 mg/dL and 0.41 ± 0.32 mg/dL, respectively (P=0.86). USCN level in goitrous and nongoitrous boys was 0.41 ± 0.32 mg/dL and 0.43 ± 0.37 mg/dL, respectively (P=0.67). USCN level in goitrous and nongoitrous girls was 0.43 ± 0.26 mg/dL and 0.40 ± 0.28 mg/dL, respectively (P=0.43).

CONCLUSIONS

Thiocyanate overload does not play a role in high prevalence of goiter in the studied population. We suggest the role of other goitrogenic factors should be investigated in this region.

Iodine deficiency persists in the Zanzibar Islands of Tanzania

BACKGROUND

Iodine is an essential micronutrient for normal human growth and development. It is estimated that more than 1.6 billion people live in iodine-deficient environments, yet there are still some countries and areas where the prevalence of iodine-deficiency disorders is unknown.

OBJECTIVE

To establish the prevalence of iodine-deficiency disorders in the Zanzibar Islands, a community assumed to have ready access to iodine-rich seafoods.

METHODS

In a cross-sectional study, 11,967 schoolchildren were palpated for goiter prevalence, a subsample was evaluated for urinary iodine concentration, and the availability of iodated salt was assessed at the household and retail levels.

RESULTS

The mean total goiter prevalence was 21.3% for Unguja and 32.0% for Pemba. The overall median urinary iodine concentration was 127.5 microg/L. For Unguja the median was 185.7 microg/L, a higher value than the median of 53.4 microg/L for Pemba (p < .01). The household availability of iodated salt was 63.5% in Unguja and 1.0% in Pemba. The community was not aware of the iodine-deficiency problem and had never heard of iodated salt.

CONCLUSIONS

The inadequate intake of iodine documented in the Zanzibar Islands belies the common assumption that an island population with access to seafood is not at risk for iodine-deficiency disorders. We urge health planners to implement mandatory salt iodation and education efforts to alleviate the situation.

Thiocyanate overload and thyroid disease

Thiocyanate [SCN-] is a complex anion which is a potent inhibitor of iodide transport. It is the detoxification product of cyanide and can easily be measured in body fluids. Consumption of naturally occurring goitrogens, certain environmental toxins and cigarette smoke can significantly increase SCN- concentrations to levels potentially capable of affecting the thyroid gland. Goiter endemics were reported to develop when the critical urinary iodine/ SCN- ratio decreases below 3 microg iodine per mg SCN-. Iodine supplementation completely reverses the goitrogenic influence of SCN-. SCN- is also generated from cigarette smoking as a detoxifying product of cyanide. During the past two decades many reports dealt with the possible effects of cigarette smoking on thyroid hormone synthesis, thyroid gland size and thyroid autoimmunity including infiltrative ophtalmopathy of Graves' disease. In this mini-review, issues regarding thiocyanate overload and thyroid disease will be summarized.

Postpartum thyroiditis and autoimmune thyroiditis in women of childbearing age: recent insights and consequences for antenatal and postnatal care

Postpartum thyroiditis is a syndrome of transient or permanent thyroid dysfunction occurring in the first year after delivery and based on an autoimmune inflammation of the thyroid. The prevalence ranges from 5-7%. We discuss the role of antibodies (especially thyroid peroxidase antibodies), complement, activated T cells, and apoptosis in the outbreak of postpartum thyroiditis. Postpartum thyroiditis is conceptualized as an acute phase of autoimmune thyroid destruction in the context of an existing and ongoing process of thyroid autosensitization. From pregnancy an enhanced state of immune tolerance ensues. A rebound reaction to this pregnancy-associated immune suppression after delivery explains the aggravation of autoimmune syndromes in the puerperal period, e.g., the occurrence of clinically overt postpartum thyroiditis. Low thyroid reserve due to autoimmune thyroiditis is increasingly recognized as a serious health problem. 1) Thyroid autoimmunity increases the probability of spontaneous fetal loss. 2) Thyroid failure due to autoimmune thyroiditis-often mild and subclinical-can lead to permanent and significant impairment in neuropsychological performance of the offspring. 3) Evidence is emerging that as women age subclinical hypothyroidism-as a sequel of postpartum thyroiditis-predisposes them to cardiovascular disease. Hence, postpartum thyroiditis is no longer considered a mild and transient disorder. Screening is considered.

Production of immunoreactive thyroglobulin C-terminal fragments during thyroid hormone synthesis

Here, we studied the fragmentation of the prothyroid hormone, thyroglobulin (Tg), which occurs during thyroid hormone synthesis, a process which involves iodide, thyroperoxidase, and the H2O2-generating system, consisting of glucose and glucose oxidase. Various peptides were found to be immunoreactive to autoantibodies to Tg from patients and monoclonal antibodies directed against the immunodominant region of Tg. The smallest peptide (40 kDa) bore thyroid hormones and was identified at the C-terminal end of the Tg molecule, which shows homologies with acetylcholinesterase. Similar peptides were obtained by performing metal-mediated oxidation of Tg via a Fenton reaction. It was concluded that the oxidative stress induced during hormone synthesis generates free radicals, which, in turn, cleave Tg into immunoreactive peptides.

Role of multimerized porcine thyroglobulin in iodine storage

Thyroglobulin (Tg), the prothyroid hormone, is stored in the lumen of the thyroid follicles as soluble dimers and tetramers and insoluble multimers, Soluble Tg is well characterized with regards to structure and role, but insoluble Tg (i-Tg) is not. Here we show that i-Tg, multimerized through formation of disulfide and dityrosine bonds, has a higher iodine content than soluble Tg and no thyroid hormones. Furthermore, the size and the resistance of i-Tg to proteolytic enzymes implied a new mechanism by which thyrocytes may degrade this form of Tg. Using peroxidase and H2O2 generating system, we found that about 80% of i-Tg was degraded and 24% of its iodine content was released. Our data point to a role for i-Tg in iodine storage and the involvement of TPO in i-Tg degradation and iodide release.

Magnetic resonance spectroscopy, calcium content, and anion coordination studies of bovine and goat lactoperoxidase

Bovine lactoperoxidase from two purebred strains and a commercial source as well as lactoperoxidase isolated from Alpine goat milk were examined by proton NMR spectroscopy for structural comparison of the heme site. Hyperfine shifted proton NMR spectra for both the native enzymes and cyanide complexes were equivalent for the protein obtained from the four separate sources. Activity assays (guaiacol and iodide ion oxidations) were also employed to compare the enzyme from various sources. Bovine lactoperoxidase was shown to contain 1.5 +/- 0.1 calcium ions per heme unit. Lactoperoxidase complexes with nitrite ion and thiocyanate ion were characterized for comparison with the cyanide complex. The nitrite complex exhibits a proton NMR hyperfine shift pattern at ambient temperature consistent with a low-spin ferric formulation. Interaction of lactoperoxidase with thiocyanate ion was monitored by NMR and EPR spectroscopy. Proton NMR spectra of lactoperoxidase in the presence of excess thiocyanate ion illustrated the retention of a high-spin ferric configuration consistent with predominant binding of the physiological thiocyanate substrate at a non-heme site at room temperature. However, EPR spectroscopy at cryogenic temperatures revealed the existence of a low-spin lactoperoxidase thiocyanate complex. This result may be explained by low-affinity ambient temperature thiocyanate heme binding that is greatly enhanced at liquid helium temperature.

Influence of cigarette smoking on thyroid hormone levels

1 Serum triiodothyronine (T3), thyroxine (T4) and thyrotropin (TSH) concentrations and urinary thiocyanate levels were examined in healthy smokers and non-smokers as an indicator of smoking behaviour. Smokers were subdivided into moderate and heavy. 2 Significant differences in urinary thiocyanate levels were apparent between all three groups. For heavy smokers, serum T3 concentrations were significantly above the values found in non-smokers. Increased serum T3 levels were not accompanied by a substantial change in serum T4 and TSH concentrations.

[Effect of iodine, copper and zinc supplements to rations with a high quota of rapeseed extract meal on the growth and thyroid function of fattening swine. 3. The effect on weight and the histomorphometric findings in the thyroid and on the serum T3 and T4 concentrations]

Studies were made with a total of 272 fattening pigs. Iodine deficiency both in rations of soya bean meal and rape seed meal increased the weight of the thyroid gland and the height of the epithelium cells and significantly reduced the T4 and T3 level in the serum. Iodine supplements to the rape seed meal rations distinctly diminished the goitrogenic effect, did not, however, cancel it out. Stage of goiter and depression of consumption were--different shown by the T3 and T4 level in the serum. Highly significant relations were calculated between live weight gain, weight and the height of the epithelium cells on the one hand and the T4 level in the serum on the other. Iodine supplementations of rations without thyrostatic components should amount to greater than or equal to 0.1 mg/kg. Iodine supplementations of 0.5 mg/kg feed are at present recommended to rations with rape seed meal quotas of less than 8%.

Spectral characteristics and catalytic properties of thyroid peroxidase-H2O2 compounds in the iodination and coupling reactions

Hog thyroid peroxidase (TPO) was highly purified in order to study the spectral properties and catalytic specificities of its H2O2 compounds in iodothyronine biosynthesis. Purified TPO exhibited a Soret spectrum with an absorption maximum at 410 nm and had an A410/A280 value of 0.55. Protein iodination was only catalyzed under conditions which allowed formation of the transient TPO compound I (Fe(IV)-pi o+). On addition of an equimolar amount of H2O2, TPO formed a stable compound with an absorption maximum at 417 nm. This compound efficiently catalyzed the coupling reaction, but was unable to iodinate proteins. It catalyzed the formation of 1 mol iodothyronines/mol TPO, and therefore retained two oxidizing equivalents per molecule. It is proposed that this compound constitutes a second form of compound I whose structure might be Fe(IV)-Ro, analogous to that of cytochrome c peroxidase compound I. In the presence of an excess of H2O2, it formed TPO-compound III with an absorption maximum at 420 nm. TPO-compound III catalyzed neither the iodination nor the coupling reaction.

NADPH oxidation catalyzed by the peroxidase/H2O2 system. Iodide-mediated oxidation of NADPH to iodinated NADP

Oxidation of NADPH catalyzed by the peroxidase/H2O2 system is known to require the presence of mediating molecules. Using either lactoperoxidase or horseradish peroxidase, we demonstrated that in the peroxidase/H2O2 system, NADPH oxidation was mediated by iodide. The oxidation product was the iodinated NADP. This product was shown to possess spectral characteristics different from those of NADP+ and NADPH, since for iodinated NADP, increased absorbance was observed in the 280-nm region and was directly proportional to the rate of iodination. It is suggested that oxidation and iodination of NADPH proceed via a single reaction between the intermediary iodide oxidation species and NADPH. Experiments with different molecules of NADPH analogues indicated that iodination occurred in the nicotinamide part of the NADPH molecule.

Thyroid hormone synthesis and thyroglobulin iodination related to the peroxidase localization of oxidizing equivalents: studies with cytochrome c peroxidase and horseradish peroxidase

Cytochrome c peroxidase (CcP) and horseradish peroxidase (HRP), when combined with a stoichiometric amount of H2O2, form stable compounds I which are known as FeIV Ro and FeIV o pi + structures, respectively. These compounds were assayed in the catalysis of thyroid hormone synthesis and the iodination reaction. As previously shown for the lactoperoxidase FeIV Ro compound, the CcP FeIV Ro compound was involved in the coupling and not in the iodination reactions. In contrast, the HRP FeIV o pi + compound catalyzed both iodination and hormone formation. The possible role of the different peroxidase-H2O2 compounds in the two sequential reactions, thyroglobulin iodination and thyroid hormone formation, is discussed.

NADPH-dependent H2O2 generation and peroxidase activity in thyroid particular fraction

A NADPH-dependent H2O2 generating system associated with a thyroid particular fraction is described. H2O2 is measured by two different methods: iodination of NADPH itself when the system is supplemented with lactoperoxidase and [125I]iodide, and by the scopoletin method. It is shown that: H2O2 generation is inhibited by catalase and is dependent on NADPH or particulate protein concentration; radical scavengers of OH and of singlet oxygen have no effect while superoxide dismutase has only a marginal effect; disruption of the particular fraction by phospholipase A2 or digitonin treatment completely abolished H2O2 generation activity while thyroid peroxidase activity appears, suggesting different sites for the two activities in the membrane vesicles.

Thiocyanates and iodine in endemic goiter in Italy

Urinary iodine (I) and serum and urinary thiocyanates (SCN-) were determined in sample groups from 12 areas of endemic goiter in Italy. The mean urinary I level of 637 subjects with thyroid 0 was 67 +/- 31 (mean +/- SD) microgram/liter, that of 648 with goiter 54 +/- 29 micrograms/liter. Mean serum and urinary SCN- were 2.44 +/- 1.36 mg/liter and 2.58 +/- 1.36 mg/liter in 887 and 1531 subjects, respectively. The thiocyanates data enabled a distinction to be drawn between two groups (populations). Population I included 73% of the adults and 92% of the school-children (6-16 yr). Its thiocyanate values were logarithmically spread around means of 1.24 +/- 0.6 mg/liter (serum) and 1.24 +/- 0.57 mg/liter (urine), whereas those of population II were widely dispersed around means of 6.1 +/- 3.0 mg/liter and 8.08 +/- 5.5 mg/liter respectively. The boundary between the two populations was set at 3 mg/liter urine SCN-. This distinction was substantiated by the fact that 90% of those in population II smoked 10 or more cigarettes a day, whereas population I comprised occasionally smokers only. It is believed that only the values in population I can be regarded as representative of thiocyanates either endogenous or due to dietary dietary intake: these values never differed more than 60% regardless of the areas, seasons, or dietary habits. As urinary SCN- levels rose, there was also an increase in urinary iodine excretion within certain limits. This, however, did not interfere with thyroid secretion. The urinary I/SCN- ratio was lower in subjects with goiter. This was because their iodine levels were lower, whereas SCN- values were much the same in subjects with and without goiter. We have found no correlation between thiocyanate itself and goiter.