Inhibition of iodothyronine 5'-deiodinase by thioureylenes; structure--activity relationship

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

The thyroid hormone converting enzyme human deiodinase 1 is inhibited by gold ions from inorganic salts, organic substances, and by small-size nanoparticles

The selenocysteine-containing enzyme class deiodinases (DIO) consists of three isoforms. DIOs play a role in regulation of thyroid hormone (TH) signaling through the removal of iodide from TH leading to TH that interact with the hypothalamic-pituitary-thyroid (HPT) axis with differing potency. Some gold-containing organic substances are known to inhibit many selenoenzymes, including DIOs. It is, however, unclear whether the Au-containing substances or the Au ions are causing the inhibition. In this study, five organic and inorganic gold substances as well as three gold nanoparticles (AuNPs) were tested for their potential to inhibit DIO1. The enzyme activity was tested using human liver microsomes as an enzyme source and reverse T3 as a substrate; iodide release was measured by the Sandell-Kolthoff method. The three organic gold substances aurothioglucose, auranofin and sodium aurothiomalate inhibited DIO1 with IC₅₀ between 0.38 and 0.75 μM while their structural analogues lacking the gold ion did not. Likewise, the two tested gold salts, Au(I) and Au(III) chloride, showed a concentration-dependent inhibition of the DIO1 with IC₅₀ values of 0.95 and 0.57 μM. Further, AuNPs of different sizes (100, 30 and 5 nm diameter) were tested with only the 5 nm AuNPs leading to inhibition with an IC₅₀ of 8 × 10¹⁴ AuNP/L. This inhibition was not caused by the Au ions released by the AuNP into the incubation media. The exact mechanism of inhibition of DIO1 by 5 nm AuNPs should be further examined. In conclusion, the microsomal DIO1 assay demonstrated the inhibition of DIO1 by gold ions originating from different gold-containing substances, but not by Au released from AuNPs; rather DIO1 is inhibited by 5 nm, but not larger, AuNPs.

Adversity Considerations for Thyroid Follicular Cell Hypertrophy and Hyperplasia in Nonclinical Toxicity Studies: Results From the 6th ESTP International Expert Workshop

The European Society of Toxicologic Pathology organized an expert workshop in May 2018 to address adversity considerations related to thyroid follicular cell hypertrophy and/or hyperplasia (FCHH), which is a common finding in nonclinical toxicity studies that can have important implications for risk assessment of pharmaceuticals, food additives, and environmental chemicals. The broad goal of the workshop was to facilitate better alignment in toxicologic pathology and regulatory sciences on how to determine adversity of FCHH. Key objectives were to describe common mechanisms leading to thyroid FCHH and potential functional consequences; provide working criteria to assess adversity of FCHH in context of associated findings; and describe additional methods and experimental data that may influence adversity determinations. The workshop panel was comprised of representatives from the European Union, Japan, and the United States. Participants shared case examples illustrating issues related to adversity assessments of thyroid changes. Provided here are summary discussions, key case presentations, and panel recommendations. This information should increase consistency in the interpretation of adverse changes in the thyroid based on pathology findings in nonclinical toxicity studies, help integrate new types of biomarker data into the review process, and facilitate a more systematic approach to communicating adversity determinations in toxicology reports.

An AOP-based alternative testing strategy to predict the impact of thyroid hormone disruption on swim bladder inflation in zebrafish

The adverse outcome pathway (AOP) framework can be used to help support the development of alternative testing strategies aimed at predicting adverse outcomes caused by triggering specific toxicity pathways. In this paper, we present a case-study demonstrating the selection of alternative in chemico assays targeting the molecular initiating events of established AOPs, and evaluate use of the resulting data to predict higher level biological endpoints. Based on two AOPs linking inhibition of the deiodinase (DIO) enzymes to impaired posterior swim bladder inflation in fish, we used in chemico enzyme inhibition assays to measure the molecular initiating events for an array of 51 chemicals. Zebrafish embryos were then exposed to 14 compounds with different measured inhibition potentials. Effects on posterior swim bladder inflation, predicted based on the information captured by the AOPs, were evaluated. By linking the two datasets and setting thresholds, we were able to demonstrate that the in chemico dataset can be used to predict biological effects on posterior chamber inflation, with only two outliers out of the 14 tested compounds. Our results show how information organized using the AOP framework can be employed to develop or select alternative assays, and successfully forecast downstream key events along the AOP. In general, such in chemico assays could serve as a first-tier high-throughput system to screen and prioritize chemicals for subsequent acute and chronic fish testing, potentially reducing the need for long-term and costly toxicity tests requiring large numbers of animals.

Sulfur as hydrogen-bond acceptor in cocrystals of 2-thio-modified thymine

Doubly and triply hydrogen-bonded supramolecular synthons are of particular interest for the rational design of crystal and cocrystal structures in crystal engineering since they show a high robustness due to their high stability and good reliability. The compound 5-methyl-2-thiouracil (2-thiothymine) contains an ADA hydrogen-bonding site (A = acceptor and D = donor) if the S atom is considered as an acceptor. We report herein the results of cocrystallization experiments with the coformers 2,4-diaminopyrimidine, 2,4-diamino-6-phenyl-1,3,5-triazine, 6-amino-3H-isocytosine and melamine, which contain complementary DAD hydrogen-bonding sites and, therefore, should be capable of forming a mixed ADA–DAD N—H...S/N—H...N/N—H...O synthon (denoted synthon 3s N·S;N·N;N·O), consisting of three different hydrogen bonds with 5-methyl-2-thiouracil. The experiments yielded one cocrystal and five solvated cocrystals, namely 5-methyl-2-thiouracil–2,4-diaminopyrimidine (1/2), C5H6N2OS·2C4H6N4, (I), 5-methyl-2-thiouracil–2,4-diaminopyrimidine–N,N-dimethylformamide (2/2/1), 2C5H6N2OS·2C4H6N4·C3H7NO, (II), 5-methyl-2-thiouracil–2,4-diamino-6-phenyl-1,3,5-triazine–N,N-dimethylformamide (2/2/1), 2C5H6N2OS·2C9H9N5·C3H7NO, (III), 5-methyl-2-thiouracil–6-amino-3H-isocytosine–N,N-dimethylformamide (2/2/1), (IV), 2C5H6N2OS·2C4H6N4O·C3H7NO, (IV), 5-methyl-2-thiouracil–6-amino-3H-isocytosine–N,N-dimethylacetamide (2/2/1), 2C5H6N2OS·2C4H6N4O·C4H9NO, (V), and 5-methyl-2-thiouracil–melamine (3/2), 3C5H6N2OS·2C3H6N6, (VI). Synthon 3s N·S;N·N;N·O was formed in three structures in which two-dimensional hydrogen-bonded networks are observed, while doubly hydrogen-bonded interactions were formed instead in the remaining three cocrystals whereby three-dimensional networks are preferred. As desired, the S atoms are involved in hydrogen-bonding interactions in all six structures, thus illustrating the ability of sulfur to act as a hydrogen-bond acceptor and, therefore, its value for application in crystal engineering.

6-Propyl-2-thiouracil versus 6-methoxymethyl-2-thiouracil: enhancing the hydrogen-bonded synthon motif by replacement of a methylene group with an O atom

The understanding of intermolecular interactions is a key objective of crystal engineering in order to exploit the derived knowledge for the rational design of new molecular solids with tailored physical and chemical properties. The tools and theories of crystal engineering are indispensable for the rational design of (pharmaceutical) cocrystals. The results of cocrystallization experiments of the antithyroid drug 6-propyl-2-thiouracil (PTU) with 2,4-diaminopyrimidine (DAPY), and of 6-methoxymethyl-2-thiouracil (MOMTU) with DAPY and 2,4,6-triaminopyrimidine (TAPY), respectively, are reported. PTU and MOMTU show a high structural similarity and differ only in the replacement of a methylene group (-CH2-) with an O atom in the side chain, thus introducing an additional hydrogen-bond acceptor in MOMTU. Both molecules contain an ADA hydrogen-bonding site (A = acceptor and D = donor), while the coformers DAPY and TAPY both show complementary DAD sites and therefore should be capable of forming a mixed ADA/DAD synthon with each other, i.e. N-H...O, N-H...N and N-H...S hydrogen bonds. The experiments yielded one solvated cocrystal salt of PTU with DAPY, four different solvates of MOMTU, one ionic cocrystal of MOMTU with DAPY and one cocrystal salt of MOMTU with TAPY, namely 2,4-diaminopyrimidinium 6-propyl-2-thiouracilate-2,4-diaminopyrimidine-N,N-dimethylacetamide-water (1/1/1/1) (the systematic name for 6-propyl-2-thiouracilate is 6-oxo-4-propyl-2-sulfanylidene-1,2,3,6-tetrahydropyrimidin-1-ide), C4H7N4(+)·C7H9N2OS(-)·C4H6N4·C4H9NO·H2O, (I), 6-methoxymethyl-2-thiouracil-N,N-dimethylformamide (1/1), C6H8N2O2S·C3H7NO, (II), 6-methoxymethyl-2-thiouracil-N,N-dimethylacetamide (1/1), C6H8N2O2S·C4H9NO, (III), 6-methoxymethyl-2-thiouracil-dimethyl sulfoxide (1/1), C6H8N2O2S·C2H6OS, (IV), 6-methoxymethyl-2-thiouracil-1-methylpyrrolidin-2-one (1/1), C6H8N2O2S·C5H9NO, (V), 2,4-diaminopyrimidinium 6-methoxymethyl-2-thiouracilate (the systematic name for 6-methoxymethyl-2-thiouracilate is 4-methoxymethyl-6-oxo-2-sulfanylidene-1,2,3,6-tetrahydropyrimidin-1-ide), C4H7N4(+)·C6H7N2O2S(-), (VI), and 2,4,6-triaminopyrimidinium 6-methoxymethyl-2-thiouracilate-6-methoxymethyl-2-thiouracil (1/1), C4H8N5(+)·C6H7N2O2S(-)·C6H8N2O2S, (VII). Whereas in (I) only an AA/DD hydrogen-bonding interaction was formed, the structures of (VI) and (VII) both display the desired ADA/DAD synthon. Conformational studies on the side chains of PTU and MOMTU also revealed a significant deviation for cocrystals (VI) and (VII), leading to the desired enhancement of the hydrogen-bond pattern within the crystal.

Exposure to non-persistent pesticides and thyroid function: A systematic review of epidemiological evidence

Numerous pesticides are recognized for their endocrine-disrupting properties. Non-persistent pesticides such as organophosphates, dithiocarbamates and pyrethroids may interfere with thyroid function as suggested by animal studies. However, the influence of chronic exposure to these compounds on thyroidal functions in humans remains to be determined. The present study aimed to review epidemiological evidence for an association between exposure to non-persistent pesticides and circulating levels of thyroid hormones (thyroxin [T4] and triiodothyronine [T3]) and thyroid-stimulating hormone (TSH). A systematic review was conducted using MEDLINE, SCOPUS and Virtual Health Library (BVS) databases. Articles were limited to original studies and reports published in English, Portuguese or Spanish. Nineteen epidemiological studies were identified, 17 of which were cross-sectional, 14 were of occupationally exposed workers and 11 used exposure biomarkers. Fungicides and organophosphates (OP) insecticides were the most studied pesticides. Although methodological heterogeneity between studies was noted, particularly regarding study design, exposure assessment, and control of confounding, most of them showed associations with changes in T3 and T4, and/or TSH levels, while results from a few of these are consistent with experimental data supporting the findings that non-persistent pesticide exposure exerts hypothyroid-like effects. However, reporting quality was moderate to poor in 50% of the studies, particularly regarding method of selection of participants and discussion of external validity. Overall, current knowledge regarding the impact of non-persistent pesticides on human thyroid function is still limited. Given the widespread use of pesticides, future research should assess effects of exposure to currently-used pesticides in cohort studies combining comprehensive questionnaire-based assessment and biomarkers. Investigators need to pay particular attention to exposure during critical windows of brain development and exposure in agricultural populations.

Identification of thyroid hormone receptor active compounds using a quantitative high-throughput screening platform

To adapt the use of GH3.TRE-Luc reporter gene cell line for a quantitative high-throughput screening (qHTS) platform, we miniaturized the reporter gene assay to a 1536-well plate format. 1280 chemicals from the Library of Pharmacologically Active Compounds (LOPAC) and the National Toxicology Program (NTP) 1408 compound collection were analyzed to identify potential thyroid hormone receptor (TR) agonists and antagonists. Of the 2688 compounds tested, eight scored as potential TR agonists when the positive hit cut-off was defined at ≥10% efficacy, relative to maximal triiodothyronine (T₃) induction, and with only one of those compounds reaching ≥20% efficacy. One common class of compounds positive in the agonist assays were retinoids such as all-trans retinoic acid, which are likely acting via the retinoid-X receptor, the heterodimer partner with the TR. Five potential TR antagonists were identified, including the antiallergy drug tranilast and the anxiolytic drug SB 205384 but also some cytotoxic compounds like 5-fluorouracil. None of the inactive compounds were structurally related to T₃, nor had been reported elsewhere to be thyroid hormone disruptors, so false negatives were not detected. None of the low potency (>100µM) TR agonists resembled T₃ or T₄, thus these may not bind directly in the ligand-binding pocket of the receptor. For TR agonists, in the qHTS, a hit cut-off of ≥20% efficacy at 100 µM may avoid identification of positives with low or no physiological relevance. The miniaturized GH3.TRE-Luc assay offers a promising addition to the in vitro test battery for endocrine disruption, and given the low percentage of compounds testing positive, its high-throughput nature is an important advantage for future toxicological screening.

N—H...S and N—H...O hydrogen bonds: `pure' and `mixed'R22(8) patterns in the crystal structures of eight 2-thiouracil derivatives

The preferred hydrogen-bonding patterns in the crystal structures of 5-propyl-2-thiouracil, C7H10N2OS, (I), 5-methoxy-2-thiouracil, C5H6N2O2S, (II), 5-methoxy-2-thiouracil–N,N-dimethylacetamide (1/1), C5H6N2O2S·C4H9NO, (IIa), 5,6-dimethyl-2-thiouracil, C6H8N2OS, (III), 5,6-dimethyl-2-thiouracil–1-methylpyrrolidin-2-one (1/1), C6H8N2OS·C5H9NO, (IIIa), 5,6-dimethyl-2-thiouracil–N,N-dimethylformamide (2/1), 2C6H8N2OS·C3H7NO, (IIIb), 5,6-dimethyl-2-thiouracil–N,N-dimethylacetamide (2/1), 2C6H8N2OS·C4H9NO, (IIIc), and 5,6-dimethyl-2-thiouracil–dimethyl sulfoxide (2/1), 2C6H8N2OS·C2H6OS, (IIId), were analysed. All eight structures containR22(8) patterns. In (II), (IIa), (III) and (IIIa), they are formed by two N—H...S hydrogen bonds, and in (I) by alternating pairs of N—H...S and N—H...O hydrogen bonds. In contrast, the structures of (IIIb), (IIIc) and (IIId) contain `mixed'R22(8) patterns with one N—H...S and one N—H...O hydrogen bond, as well asR22(8) motifs with two N—H...O hydrogen bonds.

Se- and s-based thiouracil and methimazole analogues exert different inhibitory mechanisms on type 1 and type 2 deiodinases

The thioamide anti-thyroid drugs methimazole (MMI) and propylthiouracil (PTU) play a pivotal role in the treatment of hyperthyroidism. MMI exerts its effect via inhibiting one of the key enzymes involved in synthesis of thyroid hormones (TH), thyroid peroxidase (TPO). PTU is both an inhibitor of TPO and type 1 deiodinase (D1), which catalyzes TH deiodination at both aromatic rings. In contrast, no selective inhibitors are known for type 2 deiodinase (D2) or type 3 deiodinase, which deiodinate TH at the phenolic or tyrosyl ring, respectively. We aimed to identify specific inhibitors for D1 or D2. New Se- and S-based PTU and MMI-like compounds have been generated. The D1 and D2 inhibiting capacity of several compounds was tested in vitro. Our data show that compounds based on a PTU and MMI backbone can differentially influence the reaction kinetics of deiodinases. For inhibition of D1, the addition of a phenyl group to the PTU backbone increases potency by at least 10-fold over PTU. For inhibition of D2, the addition of an aromatic ring structure to MMI and its Se isomer increases inhibitory potency by an order of magnitude. Furthermore, S-methylation of the MMI changes its reaction kinetics from non-competitive to uncompetitive with respect to the cofactor dithiothreitol. These results open perspectives for further investigations on identifying specific inhibitors of the deiodinase isoenzymes, potentially based on the addition of aromatic ring structures or alkyl groups to PTU and MMI.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.

Effects of methylthiouracil treatment on the growth and moult of cashmere fibre in goats

The effect of long-term treatment of goats with methylthiouracil on the timing, amount and quality of secondary fibre (cashmere) growth and timing of cashmere moult in goats was investigated. From early June, groups of 10 Icelandic × Scottish feral goats were dosed orally each day, for a 15-month period, with 5 mg methylthiouracil per kg live weight in 30 ml water (treated; T) or with water only (control; C). Treatment with methylthiouracil resulted in a significant reduction (P < 0·05) in the proportion of active secondary hair follicles present during March. This was associated with a delayed onset of moult of cashmere in T compared with C goats at both the head (11 March v. 23 February; s.e. 3·33 days; P< 0·05) and mid side (27 March v. 26 February; s.e. 3·58 days; P < 0·001). There was no effect on the time of onset (C, 19 July; T, 19 July; s.e. 5·84 days) or cessation of cashmere fibre growth (C, 9 December; T, 8 December; s.e. 1·69 days) or the mean growth rate (C, 0·473 mm/day; T, 0·451 mm/day; s.e. 0·025) and fibre diameter (C, 16·9 μm; T, 15·4 jim; s.e. 0·266). Wlien present in the fleece, the mean weight and proportion of cashmere was higher in C than in T goats (P < 0·05). It is concluded that methylthiouracil treatment altered secondary follicle activity and the time of onset of the moult of cashmere and that these changes may be a result of reduced triiodothyronine production from thyroxine and associated secondary changes in profiles of insulin and IGF-1.

Thyroid-disrupting chemicals: interpreting upstream biomarkers of adverse outcomes

BACKGROUND

There is increasing evidence in humans and in experimental animals for a relationship between exposure to specific environmental chemicals and perturbations in levels of critically important thyroid hormones (THs). Identification and proper interpretation of these relationships are required for accurate assessment of risk to public health.

OBJECTIVES

We review the role of TH in nervous system development and specific outcomes in adults, the impact of xenobiotics on thyroid signaling, the relationship between adverse outcomes of thyroid disruption and upstream causal biomarkers, and the societal implications of perturbations in thyroid signaling by xenobiotic chemicals.

DATA SOURCES

We drew on an extensive body of epidemiologic, toxicologic, and mechanistic studies.

DATA SYNTHESIS

THs are critical for normal nervous system development, and decreased maternal TH levels are associated with adverse neuropsychological development in children. In adult humans, increased thyroid-stimulating hormone is associated with increased blood pressure and poorer blood lipid profiles, both risk factors for cardiovascular disease and death. These effects of thyroid suppression are observed even within the "normal" range for the population. Environmental chemicals may affect thyroid homeostasis by a number of mechanisms, and multiple chemicals have been identified that interfere with thyroid function by each of the identified mechanisms.

CONCLUSIONS

Individuals are potentially vulnerable to adverse effects as a consequence of exposure to thyroid-disrupting chemicals. Any degree of thyroid disruption that affects TH levels on a population basis should be considered a biomarker of adverse outcomes, which may have important societal outcomes.

Mode of Action: Developmental Thyroid Hormone Insufficiency—Neurological Abnormalities Resulting From Exposure to Propylthiouracil

Because thyroid hormone is essential for normal brain development before and after birth, environmental chemicals that interfere with thyroid hormone signaling can adversely affect brain development. Adverse consequences of thyroid hormone insufficiency depend both on severity and developmental timing, indicating that environmental antithyroid factors may produce different effects at different developmental windows of exposure. Mechanistic studies can provide important insight into the potential impact of chemicals on human thyroid function, but relevance to humans must be systematically evaluated. This kind of analysis depends on data sets that include information about animals and humans. The drug 6-n-propyl-2-thiouracil (PTU) is used in animals to experimentally manipulate serum thyroid hormone levels, and in humans to treat patients, including pregnant women, with Graves' disease. A systematic analysis of the mode of action (MOA) of PTU in rats and in humans discloses similar modes of action. While the analysis predicts that PTU doses that produce thyroid hormone insufficiency in humans would adversely affect the developing brain, careful monitoring of PTU administration in pregnant and lactating humans keeps infant serum thyroid hormone levels within the normal range.

Characterization of recombinant Xenopus laevis type I iodothyronine deiodinase: substitution of a proline residue in the catalytic center by serine (Pro132Ser) restores sensitivity to 6-propyl-2-thiouracil

In frogs such as Rana and Xenopus, metamorphosis does not occur in the absence of a functional thyroid gland. Previous studies indicated that coordinated development in frogs requires tissue and stage-dependent type II and type III iodothyronine deiodinase expression patterns to obtain requisite levels of intracellular T(3) in tissues at the appropriate stages of metamorphosis. No type I iodothyronine deiodinase (D1), defined as T(4) or reverse T(3) (rT3) outer-ring deiodinase (ORD) activity with Michaelis constant (K(m)) values in the micromolar range and sensitivity to 6-propyl-2-thiouracil (6-PTU), could be detected in tadpoles so far. We obtained a X. laevis D1 cDNA clone from brain tissue. The complete sequence of this clone (1.1 kb, including poly A tail) encodes an ORF of 252 amino acid residues with high homology to other vertebrate D1 enzymes. The core catalytic center includes a UGA-encoded selenocysteine residue, and the 3' untranslated region (about 300 nt) contains a selenocysteine insertion sequence element. Transfection of cells with an expression vector containing the full-length cDNA resulted in generation of significant deiodinase activity in the homogenates. The enzyme displayed ORD activity with T(4) (K(m) 0.5 microm) and rT3 (K(m) 0.5 microm) and inner-ring deiodinase activity with T(4) (K(m) 0.4 microm). Recombinant Xenopus D1 was essentially insensitive to inhibition by 6-PTU (IC(50) > 1 mm) but was sensitive to gold thioglucose (IC(50) 0.1 mum) and iodoacetate (IC(50) 10 microm). Because the residue 2 positions downstream from the selenocysteine is Pro in Xenopus D1 but Ser in all cloned PTU-sensitive D1 enzymes, we prepared the Pro132Ser mutant of Xenopus D1. The mutant enzyme showed strongly increased ORD activity with T(4) and rT3 (K(m) about 4 microm) and was highly sensitive to 6-PTU (IC(50) 2 microm). Little native D1 activity could be detected in Xenopus liver, kidney, brain, and gut, but significant D1 mRNA expression was observed in juvenile brain and adult liver and kidney. These results indicate the existence of a 6-PTU-insensitive D1 enzyme in X. laevis tissues, but its role during tadpole metamorphosis remains to be defined.

Interaction of anti-thyroid drugs with iodine: the isolation of two unusual ionic compounds derived from Se-methimazole

The inhibition of lactoperoxidase (LPO)-catalyzed iodination of l-tyrosine by the anti-thyroid drug methimazole (MMI) and its selenium analogue (MSeI) is described. MSeI inhibits LPO with an IC(50) value of 12.4 microM, and this inhibition could be completely reversed by increasing the peroxide concentration. In addition to the inhibition, MSeI reacts with molecular iodine to produce novel ionic diselenides, and the nature of the species formed in this reaction appear to be solvent-dependent. The formation of ionic species in the reaction is confirmed by single-crystal X-ray studies, FT-IR and FT-Raman spectroscopic investigations. This study provides the first experimental evidence that MSeI not only effectively inhibits the LPO-catalyzed iodination of tyrosine, but also reacts with I(2) to produce novel ionic diselenides. These results also suggest that MSeI reacts with iodine, even in its oxidized form, to form ionic diselenides containing iodide or polyiodide anions, which might be effective intermediates in the inhibition of thyroid hormones.

Bioinorganic chemistry in thyroid gland: effect of antithyroid drugs on peroxidase-catalyzed oxidation and iodination reactions

Propylthiouracil (PTU) and methimazole (MMI) are the most commonly used antithyroid drugs. The available data suggest that these drugs may block the thyroid hormone synthesis by inhibiting the thyroid peroxidase (TPO) or diverting oxidized iodides away from thyroglobulin. It is also known that PTU inhibits the selenocysteine-containing enzyme ID-1 by reacting with the selenenyl iodide intermediate (E-SeI). In view of the current interest in antithyroid drugs, we have recently carried out biomimetic studies to understand the mechanism by which the antithyroid drugs inhibit the thyroid hormone synthesis and found that the replacement of sulfur with selenium in MMI leads to an interesting compound that may reversibly block the thyroid hormone synthesis. Our recent results on the inhibition of lactoperoxidase (LPO)-catalyzed oxidation and iodination reactions by antithyroid drugs are described.

Anti-Thyroid Drugs and Thyroid Hormone Synthesis: Effect of Methimazole Derivatives on Peroxidase-Catalyzed Reactions

Syntheses and characterization of the selenium analogue (MSeI) of anti-thyroid drug methimazole and a series of organoselenium compounds bearing N-methylimidazole pharmacophore are described. In contrast to the sulfur compound that exists predominantly in its thione form, the selenium analogue exists in a selenol form, which spontaneously oxidizes in air to produce the corresponding diselenide. The reduction of the diselenide by GSH or NaBH(4) affords the biologically active selenol, which effectively inhibits the lactoperoxidase (LPO) activity in vitro. The monoselenides having N-methylimidazole moiety are found to be much less active than the selenol, suggesting that the presence of a selenol moiety is important for the LPO inhibition. The kinetic and mechanistic studies reveal that MSeI inhibits the LPO activity by reducing the H(2)O(2), providing a novel method to reversibly inhibit the enzyme. Although MSeI strongly inhibits LPO, the enzyme's activity can be completely recovered by increasing the H(2)O(2) concentration. On the other hand, the inhibition by methimazole (MMI), the sulfur analogue, cannot be reversed by increasing the H(2)O(2) concentration, leading to a complete inactivation of the enzyme. The reversible inhibition of LPO by some of the selenium derivatives is correlated with their glutathione peroxidase (GPx) activity, and the high GPx activity of the selenium compounds as compared with their sulfur analogues suggests that the selenium derivatives may protect the thyroid gland from oxidative damage.

Toxicity study of a rubber antioxidant, mixture of 2-mercaptomethylbenzimidazoles, by repeated oral administration to rats

2-Mercaptobenzimidazole (2-MBI), a rubber antioxidant, is known to exhibit potent antithyroid toxicity in rats and is a candidate as an environmental endocrine disrupter. 2-Mercaptomethylbenzimidazoles (a 1:1 mixture of 4-methyl and 5-methyl isomers, MMBIs), are also employed industrially as rubber antioxidants and are suspected to exert antithyroid toxicity such as 2-MBI. In this investigation, acute and subacute oral toxicity studies of MMBIs in Wistar rats were conducted. The clinical signs of acute oral toxicity were observed including decreased spontaneous movement, a paralytic gait, salivation and lacrimation, and adoption of prone and lateral positions. The LD50 was estimated to be 330 mg/kg. In the subacute oral toxicity study, male and female rats were treated with MMBIs by gavage at doses of 0 (corn oil), 4, 20 and 100 mg/kg for 28 consecutive days followed by a 2-week recovery period for the control and highest dose groups. Body weight and food consumption, clinical signs, organ weights, clinical biochemistry and haematological parameters including clotting times and micronuclei induction in bone marrow erythropoeitic cells, and histopathology were examined. Relative organ weights of lung, liver and kidney, and serum cholesterol and phospholipid significantly increased in male rats treated with MMBIs at doses of 20 and 100 mg/kg. Male rats administered 100 mg/kg MMBIs exhibited a 1.8-fold increase in thyroid weight associated with histopathological changes but not altered serum thyroid hormone levels. Female rats administered 100 mg MMBIs/kg exhibited significant increases of liver and kidney but not thyroid weights, and serum cholesterol level. The antithyroid toxicity of MMBIs in rats was estimated to be one-tenth that of 2-MBI. No-observed-effect levels for male and female rats were found to be 4 and 20 mg/kg, respectively, in this subacute oral toxicity study.

The selenium analog of 6-propylthiouracil. Measurement of its inhibitory effect on type I iodothyronine deiodinase and of its antithyroid activity

6-Propylthiouracil (PTU), a widely used antithyroid drug for the treatment of Graves' disease, is also a potent inhibitor of Type I iodothyronine deiodinase (ID-1). Inhibition of ID-1 was attributed initially to the formation of a mixed disulfide between PTU and a putative cysteine residue at the active site. It has been demonstrated recently that ID-1 is a selenium-containing enzyme, with selenocysteine, rather than cysteine, at the active site. It seemed possible, therefore, that the selenium analog of PTU (PSeU) might be a more potent inhibitor of ID-1 than PTU. To test this possibility, we developed a procedure for the synthesis of PSeU, and we compared PSeU and PTU as inhibitors of ID-1 in a test system containing 125I-rT3, rat liver microsomes, and dithiothreitol. Deiodinase activity was measured by the increase in 125I-iodide. PTU and PSeU were tested at 0.1, 0.3, 1 and 3 microM. Based on results of four separate experiments, the drugs were essentially equipotent as inhibitors of ID-1, although statistical analysis suggested that PSeU may be slightly more potent than PTU. PTU and PSeU were also compared for antithyroid activity in vivo and in vitro. As inhibitors of the catalytic activity of thyroid peroxidase (TPO), the two drugs were essentially equipotent in iodination and guaiacol assays involving measurements made shortly after the addition of H2O2. However, in in vivo experiments with rats, PSeU showed no appreciable inhibition of organic iodine formation in the thyroid, whereas PTU, as expected, was a potent inhibitor. The lack of inhibition of organic iodine formation in vivo by PSeU suggests that, unlike PTU, it is not concentrated by the thyroid gland. In an iodination system in which H2O2 was generated by glucose-glucose oxidase, both PTU and PSeU, when present at 10 microM, acted as reversible inhibitors of iodination. However, when the drug concentration was raised to 50 microM, TPO was inactivated and iodination was irreversibly inhibited. These results suggest that PTU and PSeU inhibit TPO-catalyzed iodination by similar mechanisms. Under the same conditions, the selenium analog of methimazole (another widely used antithyroid drug) does not inactivate TPO. It acts primarily as a reversible inhibitor of TPO-catalyzed iodination.

The selenium analog of methimazole. Measurement of its inhibitory effect on type I 5'-deiodinase and of its antithyroid activity

Methimazole (MMI), unlike propylthiouracil (PTU) is a poor inhibitor of type I iodothyronine deiodinase (ID-1). Inhibition of the enzyme by PTU was attributed initially to formation of a mixed disulfide between PTU and a cysteine residue at the active site. Presumably, MMI was unable to form a stable mixed disulfide and thus did not inhibit the enzyme. However, it has been demonstrated recently that ID-1 is a selenium-containing enzyme, with selenocysteine, rather than cysteine, at the active site. This observation raised the possibility that the selenium analog of MMI, methyl selenoimidazole (MSeI), might be a better inhibitor of ID-1 than MMI itself, as formation of the Se-Se bond with the enzyme would be expected to occur more readily than formation of the S-SE bond. To test this possibility, we developed a procedure for the synthesis of MSeI and compared MSeI with MMI and PTU for inhibition of ID-1 and for antithyroid activity. For inhibition of ID-1, MMI and MSeI were tested at concentrations of 10-300 microM. No significant inhibition was observed with MMI. MSeI showed slight but significant inhibition only in the 100-300 microM range. PTU, on the other hand, showed marked inhibition at 1 microM. Thus, replacement of the sulfur in MMI with selenium only marginally increases its inhibitory effect on ID-1. As an inhibitor of ID-1, MSeI is much less than 1% as potent as PTU. MMI and MSeI were also compared for antithyroid activity, both in vivo and in vitro. As an inhibitor of the catalytic activity of thyroid peroxidase, MMI was 4-5 times more potent than MSeI in a guaiacol assay, but only twice as potent in an iodination assay. In in vivo experiments with rats, MMI was at least 50 times more potent than MSeI in inhibiting thyroidal organic iodine formation. The relatively low potency of MSeI in vivo suggests that it is much less well concentrated by the thyroid than in MMI.

Pituitary and thyroid gland lesions induced by 2-mercaptobenzimidazole (2-MBI) inhalation in male Fischer-344 rats

2-Mercaptobenzimidazole (2-MBI) is a member of the thioureylene compound family known for their potent antithyroid activity. Male Fischer-344 rats were exposed to 0.0, 3.13, 12.5, and 50.0 mg/m3 of 2-MBI via inhalation for 13 wk. Follicular cell hyperplasia occurred in the thyroid glands of rats from the 3 2-MBI treatment groups. Thyrotrophs in the pituitary glands from rats in these 3 groups were hyperplastic, and they had varying numbers of hypertrophic cells with either eosinophilic stippled cytoplasm or with eosinophilic globules within 1 or more large vacuoles that displaced the nucleus. These cells were compared by immunohistochemistry and ultrastructural examination to "thyroidectomy cells" within the anterior pituitary glands of thyroparathyroidectomized rats and were determined to be identical to them. Immunohistochemical staining for the beta-chain of thyroid-stimulating hormone confirmed that the hyperplastic and hypertrophic cells were thyrotrophs. Ultrastructurally, hypertrophic cells in 2-MBI-treated rats and thyroidectomy cells in thyroparathyroidectomized rats had expanded cytoplasm containing either increased profiles of rough endoplasmic reticulum (RER) or one or more widely dilated cisternae of RER, which displaced other cellular organelles. 2-MBI appears to act comparably to other thioureylene compounds that have been shown to produce low serum concentrations of triiodothyronine and thyroxine, which results in thyrotroph hypertrophy and hyperplasia, resultant thyroid hyperplasia, and subsequent goiter.

Selenouracil derivatives are potent inhibitors of the selenoenzyme type I iodothyronine deiodinase

Type I iodothyronine deiodinase (ID-I) is a selenoenzyme, which is important for the conversion of the prohormone thyroxine (T4) to the bioactive thyroid hormone 3,3',5-triiodothyronine (T3). 2-Thiouracil derivatives inhibit ID-I by interaction with an enzyme form generated during catalysis. We have now tested the potential inhibitory effects of the selenocompounds 6-methyl- (MSU) and 6-propyl-2-selenouracil (PSU) in comparison with their thioanalogs 6-methyl- (MTU) and 6-propyl-2-thiouracil (PTU) on rat liver ID-I activity using 3,3',5-triiodothyronine (reverse T3, rT3) as substrate and dithiothreitol (DTT) as cofactor. All compounds showed dose-dependent inhibition of ID-I with IC50 values of 1, 0.5, 0.4 and 0.2 microM for MTU, MSU, PTU and PSU, respectively. Our results further suggest that these inhibitions are uncompetitive with substrate and competitive with cofactor. The high potency of selenouracils may be due to reaction with a substrate-induced enzyme selenenyl iodide intermediate under formation of a stable enzyme-selenouracil diselenide.

Leaching of antioxidants and vulcanization accelerators from rubber closures into drug preparations

A thin-layer chromatographic method was used to highlight the leaching into drug preparations of several constituents of elastomeric closures. Among the 150 preparations analysed, the twenty-eight local anaesthetics presented in single-dose delivery syringe-cartridges, one Epinephrine injection in prefilled syringes, eight insulin preparations and two Prednisolone acetate suspensions in the form of small volume flasks (less than or equal to 20 ml) were contaminated by one or more of the following: 2-mercaptobenzothiazole (MBT), 2-mercaptobenzothiazole disulphide and 2-mercaptobenzimidazole (MBI). Prednisolone acetate suspensions also contained 2,2'-methylene-bis(4-methyl-6-alpha-methylcyclohexylphenol). No contamination was found in drug preparations presented in large volume flasks (250-1000 ml). 2-(2-Hydroxyethylthio)-benzothiazole was not present, which indicated that the rubbers had not been sterilized with ethylene oxide. Elastomeric parts of drug closures analysed in the same way contained the same compounds as those found in drugs, one case excepted, which confirms the origin of the contamination. The lowest and the highest concentrations were found in syringe-cartridges; they ranged from 8.3 to 13.8 micrograms ml-1 for MBT, from 2.9 to 9.3 micrograms ml-1 for MBTS and from 2.8 to 11.1 micrograms ml-1 for MBI. Variable results were obtained, for a same preparation, depending upon the batches analysed, which indicates that rubber formulations and/or vulcanization conditions differed. The allergenic, toxic, embryotoxic and mutagenic properties of the compounds leached are discussed.

Crystal structure of phlorizin and the iodothyronine deiodinase inhibitory activity of phloretin analogues

Phloretin, a 7,8-dihydrochalcone of plant origin, and the high molecular weight (less than 15,000) polyphloretinphosphate (PPP) polymers are potent inhibitors of iodothyronine monodeiodinase activity from rat liver microsomal preparations, whereas phlorizin, the 2'-O-glucoside of phloretin, is inactive. The polymers, differing in degree of phosphorylation-dependent polymerization, exhibited a concentration-dependent, and ultimately complete, inhibition of deiodinase activity with an IC50 between 0.2 and 0.5 micrograms PPP/ml. Phloretin inhibition, on the other hand, was cofactor (DTE) competitive, with a Ki = 0.75 microM. 2',4',6',3,4- Pentahydroxychalcone, which has a substitution pattern in the A-ring identical to that of phloretin, was the only active inhibitor (IC50 = 8 microM) among several derivatives tested. The phloretin biodegradation products, phloretic acid and phloroglucinol, and its biosynthetic precursors, monomeric cinnamic acid and cinnamic acid derivatives, were inactive in concentrations up to 100 microM. The X-ray crystal structure analysis of phlorizin dihydrate showed that the molecule is planar and fully extended, similar to the conformation observed in chalcone structures that are characterized by an alpha, beta-unsaturated bond between phenol rings. Comparison of the planar phlorizin crystal structure with a skewed or antiskewed thyroid hormone conformation revealed that the beta-D-glucose moiety does not share any of the thyroid hormone's conformational space, and that the best structural homology is found with the antiskewed conformation of 3',5',3-triiodothyronine, the natural deiodinase substrate that also inhibits further deiodination.

Structural requirements of thiol compounds in the inhibition of human liver iodothyronine 5'-deiodinase

2-Thiouracil and a number of its alkyl derivatives are known to inhibit the enzymic 5'-deodination of thyroxine to 3,5,3'-tri-iodothyronine. The structural requirements for inhibition of iodothyronine 5'-deiodinase were investigated by using a washed postmitochondrial particulate fraction of human liver. A series of sulphur-containing derivatives of pyrimidine, pyridine, imidazole, benzene and urea, capable of existing in a thiol form, were incubated at several concentrations with the enzyme preparation in the presence of thyroxine and dithioerythritol (cofactor). The degree of inhibition by the respective compounds of the production of 3,5,3'-tri-iodothyronine was studied in relation to their structural features. The major observations were: (i) a free thiol group is essential; (ii) compounds that do not possess a polar hydrogen atom spatially configured so that it is proximal to the thiol group are poor inhibitors; (iii) aromatic characteristics in the presence of requirements (i) and (ii) lead to the expression of potent inhibitory properties; (iv) modification of potent inhibitors by the introduction of hydrophilic substituents reduces the inhibitory potency.

Disposition of 2-mercaptobenzimidazole in rats dosed orally or intravenously

The disposition of [14C]-labeled 2-mercaptobenzimidazole (MBI) in male Fischer-344 rats dosed orally (49 or 0.5 mg/kg) or intravenously (0.5 mg/kg) was determined. Absorption of the oral dose was evident, since, in 72 h, most of the radioactivity administered by either route appeared in the urine. Smaller amounts appeared in the feces. In 4 h, 12% of the radioactivity from an intravenous dose of 0.5 mg/kg was excreted in the bile of rats with biliary cannulas. For rats dosed intravenously, the half-life for disappearance of unchanged MBI from plasma was 125 min. In contrast, the terminal half-life for loss of radioactivity from blood was 83 h. The concentration of total radioactivity was higher in liver and kidney tissue than in blood. One of the major urinary metabolites was identified as benzimidazole, and a minor component was tentatively identified as unchanged MBI. Neither of these could be detected in bile.

Properties of detergent-dispersed iodothyronine 5- and 5'-deiodinase activities from rat liver

In order to obtain more knowledge about the regulation and mechanism of thyroid hormone deiodination, some properties of detergent-solubilized iodothyronine deiodinase have been studied. Moreover, a starting point for its purification has been made. Several chromatography media were tested for their ability to purify the deiodinases. In some instances, a 4-fold purification was obtained. Treatment of cholate-solubilized microsomes with 35% ammonium sulphate resulted in quantitative precipitation of the deiodinase activities and concomitant removal of phospholipid. The pellet could be solubilized with 0.3% W-1 ether and the deiodinase in this ammonium sulphate extract exhibited approximately 10-fold higher apparent Km and Vmax values for its substrate compared with the cholate extract. Readdition of some phospholipids was shown to decrease enzyme activity. Isoelectric focusing of W-1 ether-solubilized microsomes resulted in a major activity peak around pH 6.4 and a minor peak at pH 5.2, while in the ammonium sulphate extract the deiodinase had an isoelectric point at pH 9.3. Refocusing of this activity peak yielded a preparation with a specific activity only 3-times higher than in the ammonium sulphate extract. However, after sodium dodecyl sulphate polyacrylamide gel electrophoresis only five bands could be detected. The elevated kinetic parameters as well as the higher isoelectric point of the deiodinase after ammonium sulphate treatment were caused by delipidation of the enzyme. Both the change in isoelectric point and the behaviour on several column materials were found to be similar for the 5- and 5'-deiodinase activities. These results suggest that a single enzyme is operative in the deiodination of iodothyronines in rat liver and that its activity may be regulated by phospholipids.

Solubilization of a phospholipid-requiring enzyme, iodothyronine 5'-deiodinase, from rat kidney membranes

Enzymic activities catalyzing the reductive 5'-deiodination of thyroxine and 3,3',5'-triiodothyronine were solubilized from rat kidney microsomes by treatment with 0.2% deoxycholate. Deoxycholate reversibly inhibited the enzyme(s); removal of detergent restored activity and resulted in the formation of enzymatically active aggregates with a buoyant density of 1.17 g/ml resembling that of membranes. Fractionation of the solubilized membrane components in the presence of 0.2% deoxycholate by either gel filtration or sucrose gradient centrifugation inactivated the enzyme(s) and activity could be restored by the addition of partially purified soybean phospholipids; this allowed some of the physical properties of the enzyme(s) to be determined. 5'-Deiodinating activity of both thyroxine and 3,3',5'-triiodothyronine was associated with protein(s) with S20,W of 3.5 S, Stokes' radius of 32 A, and a calculated molecular weight of 49 900. A partial specific volume of 0.74 cm3/g was calculated from sedimentation in 2H2O and H2O sucrose gradients. Phospholipid reactivation of lipid-depleted enzyme preparations was concentration-dependent, with near maximal restoration when sufficient phospholipid was added to restore the phospholipid:protein ratio to that of thyroxine and 3,3',5'-triiodothyronine could not be resolved by sedimentation or molecular sieving and showed similar behavior toward deoxycholate solubilization and phospholipid reconstitution.

Substrate requirement for inactivation of iodothyronine-5'-deiodinase activity by thiouracil

Preincubation of rat liver microsomal fraction with 1 microM 2-thiouracil and 0.01-1 microM 3,3',5'-triiodothyronine or 3',5'-diiodothyronine, 0.1-10 microM thyroxine or 3,5-diiodothyronine led to a progressive, irreversible and concomitant decrease in subsequently assayed 3,3',5'-triiodothyronine- and 3',5'-diiodothyronine-5'-deiodinase activity. Preincubation with thiouracil alone, with iodothyronines alone or with thiouracil and 10 microM thyronine or 3,5-diiodotyrosine had no or virtually no effect. The results indicate that (1) a previously proposed ping-pong mechanism for thyroid hormone deiodination, involving the formation of an enzyme-sulphenyl iodide intermediate, is correct; (2) thyroxine, 3,3',5'-triiodothyronine and 3',5'-diiodothyronine are substrates for a common 5'-deiodinase; (3) this 5'-deiodinase is not fully specific as regards the position of the iodine substituents in the substrate, since it also appears to catalyse the 5-deiodination of 3,5-diiodothyronine.

Mechanism of inhibition of iodothyronine-5'-deiodinase by thioureylenes and sulfite

Previous studies have demonstrated that thiouracil inhibits the 5'-deiodination of 3,3',5'-triiodothyronine uncompetitively with respect to substrate and competitively with respect to cofactor (thiol compounds). This paper shows that sulfite is also a strong inhibitor of this reaction showing a dose-dependent effect between 1 microM and 1 mM. The mode of inhibition is similar to that described for thiouracil. Dose-dependent inhibition was also observed with thiosulfate (0.01-1 mM), iodide and thiocyanate (both greater than 1 mM). No effect was exerted by up to 10 mM cyanide and up to 100 mM azide. Methimazole and thiourea were weak inhibitors above 0.1 mM but inhibition did not reach completion. These experiments were carried out in the presence of 1 mM dithiothreitol. The effect of thiouracil was found to be competitively obviated by methimazole and thiourea. However, the effect of sulfite and that of methimazole or thiourea were additive. It is proposed that an enzyme-sulfenyl iodide is formed during deiodination (ping-pong mechanism). This sulfenyl iodide may be reduced by cofactor to yield native enzyme. It may also react with thioureylenes, yielding mixed disulfides, or with sulfite, yielding a thiosulfate. The enzyme-methimazole disulfide is apparently less stable than the enzyme-thiouracil complex. It is suggested that sulfite also reacts with the enzyme-thioureylene disulfide.