Determination of 3-monoiodotyrosine and 3,5-diiodotyrosine in newborn urine and dried urine spots by isotope dilution tandem mass spectrometry.

High levels of 3-mono- and 3,5-diiodotyrosine (MIT and DIT, respectively) in urine have been related to iodotyrosine dehalogenase 1 deficiency, a type of congenital hypothyroidism. However, the determination of MIT and DIT in urine is not included in newborn screening programs performed in clinical laboratories to detect inborn errors of metabolism. We report here on the development of an analytical method for the determination of MIT and DIT in newborn urine and dried urine spots (DUS) by Liquid Chromatography Isotope Dilution tandem Mass Spectrometry (LC-IDMSMS). The development included the synthesis of 15N-monoiodotyrosine and 13C2-diiodotyrosine through the iodination of 15N-tyrosine and 13C2-tyrosine, respectively, using bis(pyridine)iodonium(I) tetrafluoroborate (IPy2BF4). Both labelled analogues were added at the beginning of the sample preparation procedure and used to develop both single- and double-spike LC-IDMS methods for the determination of MIT and DIT. The developed double spike methodology was able to quantify and correct possible MIT ↔ DIT interconversions throughout the sample preparation, which was observed for concentrated urine samples but not for DUS. Suppression matrix effects on the absolute signals of MIT and DIT were observed in urine samples but did not affect the IDMS results as recoveries on urine samples at different dilution factors could be considered quantitative. Method detection limits were 0.018 and 0.046 ng g-1 (limits of quantification 0.06 and 0.15 ng g-1) by single-spike IDMS, for MIT and DIT, respectively, in the analysis of urine samples and 0.07 and 0.05 ng g-1 (limits of quantification 0.23 and 0.17 ng g-1) for MIT and DIT, respectively, in the analysis of DUS. No significant differences were obtained for MIT concentrations in the analysis of the same newborn samples stored as liquid urine or DUS when the results were corrected for the creatinine content. Finally, 36 DUS samples from healthy newborns were analyzed and MIT was detected in all samples at low ng mg-1creatinine levels.

[The study of the metabolism of iodotyrosines included in the iodized milk protein in rats].

In the course of evolution in animals and humans, a complex and effective system for providing the body with iodine in the form of various organic and inorganic compounds was developed. The metabolism of inorganic iodine has been studied quite well, in contrast to the mechanism of assimilation of its organic compounds. Among the latter, iodotyrosines, which are part of iodinated milk proteins, are of particular interest. To distinguish the peculiarities of the biotransformation of iodotyrosines in the animals' organism, their concentration and the concentration of tyrosine in blood plasma of rats after single administration of iodinated milk proteins were determined. For comparison, in parallel a group of animals received potassium iodide. The tested preparations were administered intragastrically with a probe in the form of aqueous solutions at a dose equivalent to 30 µg iodine per 1 kg of body weight. The level of mono- and diiodotyrosine in rat blood plasma was determined by HPLC with a mass spectrometer detector. The tyrosine content was determined on an automatic amino acid analyzer. The registration of the indices was carried out before the administration and 1, 4 and 24 hours after the administration of the substances. In the course of the conducted studies it was found that when iodinated milk proteins are once administered, a significant increase in the concentrations of monoiodotyrosine and diiodotyrosine is observed. The maximum level of iodinated amino acids, exceeding the control values by more than 6 fold, was recorded 4 hours after the ingestion of iodine-containing organic compounds into the body. At the same time interval, an increase in the concentration of tyrosine was observed in one of the experimental groups receiving iodinated milk protein. The simultaneous presence of tyrosine and its iodinated derivatives in blood plasma may indicate that monoiodotyrosine and diiodotyrosine are capable of being absorbed into the systemic bloodstream without metabolic transformations in the liver. Under introduction of potassium iodide, an increase in blood plasma concentration of monoiodotyrosine by 35% compared to the control was observed only after 24 hours, which may be a consequence of the activation of the thyroid gland due to the intake of an increased amount of iodine.

Rapid Identification of Unknown Organic Iodine in Small-Volume Complex Biological Samples Based on Nanospray Mass Spectrometry Coupled with in-Tube Solid Phase Microextraction.

A new method for rapid screening of unknown organic iodine (OI) in small-volume complex biological samples was developed using in-tube solid phase microextraction (SPME) nanospray mass spectrometry (MS). The method proposed a new identification scheme for OI based on nanospray high-resolution mass spectrometry (HR-MS). The mass ranges of OI ions were confirmed using the t-MS2 scan mode first; then, the possible precursor ions of OI were selected and identified orderly in full MS/ddMS2 and t-MS2 scan modes. Besides, in-tube SPME was used for the pretreatment of small-volume biological samples, and it was the first time in-tube SPME combined with nanospray MS for OI identification. The whole analysis procedure took only 8 min and consumed 50 µL per sample. Using the new method, six kinds of OI added to urine and an unknown OI C12H23O11I in human milk were successfully identified. Moreover, the proposed identification scheme is also suitable for other ambient mass spectrometry (AMS) to determine unknown compounds with characteristic fragment ions.

[Thyroid hormones and their precursors. II. Species-specific properties]. / Pajzsmirigyhormonok és eloanyagaik. II. Részecske-specifikus tulajdonságok.

This paper surveys the species-specific physico-chemical parameters (basicity and lipophilicity) and related biological functions of thyroid hormones (thyroxine, liothyronine and reverse liothyronine) and their biological precursors (tyrosine, monoiodotyrosine and diiodotyrosine). The protonation macroconstants were determined by 1H NMR-pH titrations while the microconstants were determined by a multimodal spectroscopic-deductive methodology using auxiliary derivatives of reduced complexity. Our results show that the different number and/or position of iodine are the key factors to influence the phenolate basicity. The ionization state of the phenolate site is crucial in the biosynthesis and protein binding of thyroid hormones. The role of the protonation state in the receptor binding was investigated by an in silico docking method. Microspecies of thyroid hormones were docked to the thyroid hormone receptor isoforms. Our results quantitate at the molecular level how the ionization stage and the charge distribution influence the protein binding. The anionic form of the carboxyl group is essential for the protein binding, whereas the protonated form of the amino group loosens it. The protonation state of the phenolate plays a role of secondary importance in the receptor binding. The combined results of docking and microspeciation studies show that microspecies of the highest concentration at the pH of blood are not the strongest binding ones. The site-specific lipophilicity of our investigated molecules was determined with the measurement of distribution coefficients at different pH using carboxymethyl- and O-methyl-derivatives to mimic the partition of some of the individual microspecies. Correction factors were determined and introduced. Our data show that the iodinated aromatic ring system is the definitive structural element that fundamentally determines the lipophilicity of thyroid hormones, whereas the protonation state of the aliphatic part is essential in receptor binding. The membrane transport of thyroid hormones can be well interpreted in terms of the site-specific lipophilicity. At physiological pH these biomolecules are strongly amphipathic due to the lipophilic aromatic rings and hydrophilic amino acid side chains which can well be the reason why thyroid hormones cannot cross membranes by passive diffusion and they even become constituents of biological membranes. The site-specific physico-chemical characterization of the thyroid hormones is of fundamental importance to understand their (patho) physiological behavior and also, to influence the therapeutic properties of their drug candidate derivatives at the molecular level.

[Thyroid hormones and their precursors I. Biochemical properties]. / Pajzsmirigyhormonok és eloanyagaik I. Biokémiai tulajdonságok.

This paper and the following one (see the next issue of Acta Pharmaceutica Hungarica) survey the biological roles and the related site-specific physico-chemical parameters (basicity and lipophilicity) of the presently known thyroid hormones (thyroxine, liothyronine and reverse liothyronine) and their biological precursors (monoiodotyrosine and diiodotyrosine). Here the literature of the thyroid hormone biochemistry, biosynthesis, plasma- and membrane transport is summarized, focusing on the pH-dependent processes. Biosyntheses of the thyroid hormones take place by oxidative coupling of two iodotyrosine residues catalyzed by thyreoperoxidase in thyreoglobulin. The protonation state of the precursors, especially that of the phenolic OH is crucial for the biosynthesis, since anionic iodotyrosine residues can only be coupled in the thyroid hormone biosyntheses. In the blood more than 99% of the circulating thyroid hormone is bound to plasma proteins among which the thyroxine-binding globulin and transthyretin are crucial. The amphiphilic character of the hormones is assumed to be the reason why their membrane transport is an energy-dependent, transport-mediated process, in which the organic anion transporter family, mainly OATP1C1, and the amino acid transporters, such as MCT8 play important roles. Liothyronine is the biologically active hormone; it binds the thyroid hormone receptor, a type of nuclear receptor. There are two major thyroid hormone receptor (TR) isoforms, alfa (TRalpha) and beta (TRbeta). The activation of the TRalpha is associated with modifications in cardiac behavior, while activation of the TRbeta is associated with increasing metabolic rates, resulting in weight loss and reduction of blood plasma lipid levels. The affinity of the thyroid hormones for different proteins depends on the ionization state of the ligands. The site-specific physico-chemical characterization of the thyroid hormones is of fundamental importance to understand their (patho)physiological behavior and also, to influence their therapeutic properties at the molecular level.

Congenital Hypothyroidism in Two Sudanese Families Harboring a Novel Iodotyrosine Deiodinase Mutation (IYD R279C).

Background: Congenital hypothyroidism due to defects in iodotyrosine deiodinase has variable phenotypes and can present as hypothyroid or with normal thyroid testing. Methods: Whole exome sequencing was performed in individuals from two families originating from different regions of Sudan. Mass spectrometry of urine and serum iodotyrosines was performed on subjects from both families. Results: A novel iodotyrosine deiodinase (IYD) mutation (c.835C>T; R279C) was identified in individuals from two Sudanese families inherited as autosomal recessive. The mutation was identified by multiple in silica analyses to likely be detrimental. Serum and urine monoiodotyrosine (MIT) and diiodotyrosine (DIT) were markedly elevated in the homozygous subjects. Conclusion: Measurement of serum and urine DIT and MIT was more sensitive than that of urine iodine or serum thyroid function tests to determine the effect of the IYD mutation.

Differentiation of enantiomeric drugs by iodo-substituted L-amino acid references under electrospray ionization mass spectrometric conditions.

RATIONALE: In chiral differentiation by mass spectrometry, use of a single reference that differentiates various classes of compounds including drugs is ideal, but so far there are no such reports in the literature. We have successfully used iodo-substituted amino acids for the chiral differentiation of ten enantiomeric pairs of drugs. METHODS: To achieve the chiral differentiation, the trimeric Cu complex ion consisting of two chiral reference molecules and an analyte molecule was generated under positive ion electrospray ionization (ESI) conditions and subsequently subjected for collision- induced dissociation (CID) experiments using an LCQ ion trap mass spectrometer. The spectra were recorded under identical experimental conditions for both the enantiomers, and were averages of 30 scans. Cooks' kinetic method and chiral recognition ratio method (CR method) were used to arrive at the R(chiral) /CR values, respectively. RESULTS: The R(chiral) or CR values of the studied drugs are higher for 3,5-diiodo-L-tyrosine as the reference, than for 4-iodo-L-phenylalanine, except for isoproterenol and atenolol. Both the references show the same selectivity (R- or S-selectivity) towards all the studied drugs. With 3,5-diiodo-L-tyrosine as the reference, an R(chiral) value of 12.75 is obtained for DOPA and this is the highest reported value in the literature till now. The suitability of the current method in measuring enantiomeric excess is also demonstrated for DOPA. CONCLUSIONS: The use of 4-iodo-L-phenylalanine or 3,5-diiodo-L-tyrosine as a chiral reference for the chiral differentiation of ten enantiomeric pairs of pharmaceutically important drugs has been demonstrated. The chiral differentiation of pregabalin, tenofovir and pramipexole is reported for the first time. This study shows that it is possible to develop a single chiral reference compound for the differentiation of a group of chiral drugs having some similarities.

Identification of natural product 3, 5-diiodotyrosine as APOBEC3B inhibitor to prevent somatic mutation accumulation and cancer progression.

BACKGROUND: The development of cancer is largely dependent on the accumulation of somatic mutations, indicating the potential to develop cancer chemoprevention agents targeting mutation drivers. However, ideal cancer chemoprevention agents that can effectively inhibit the mutation drivers have not been identified yet. METHODS: The somatic mutation signatures and expression analyses of APOBEC3B were performed in patient with pan-cancer. The computer-aided screening and skeleton-based searching were performed to identify natural products that can inhibit the activity of APOBEC3B. 4-nitroquinoline-1-oxide (4-NQO)-induced spontaneous esophageal squamous cell carcinoma (ESCC) and azoxymethane/dextran sulfate sodium (AOM/DSS)-induced spontaneous colon cancer mouse models were conducted to investigate the influences of APOBEC3B inhibitor on the prevention of somatic mutation accumulation and cancer progression. RESULTS: Here, we discovered that the cytidine deaminase APOBEC3B correlated somatic mutations were widely observed in a variety of cancers, and its overexpression indicated poor survival. SMC247 (3, 5-diiodotyrosine), as a source of kelp iodine without side effects, could strongly bind APOBEC3B (KD=65 nM) and effectively inhibit its deaminase activity (IC50=1.69 µM). Interestingly, 3, 5-diiodotyrosine could significantly reduce the clusters of mutations, prevent the precancerous lesion progression, and prolong the survival in 4-NQO-induced spontaneous ESCC and AOM/DSS-induced spontaneous colon cancer mouse models. Furthermore, 3, 5-diiodotyrosine could reduce colitis, increase the proportion and function of T lymphocytes via IL-15 in tumor microenvironment. The synergistic cancer prevention effects were observed when 3, 5-diiodotyrosine combined with PD-1/PD-L1 blockade. CONCLUSIONS: This is the first prove-of-concept study to elucidate that the natural product 3, 5-diiodotyrosine could prevent somatic mutation accumulation and cancer progression through inhibiting the enzymatic activity of APOBEC3B. In addition, 3, 5-diiodotyrosine could reduce the colitis and increase the infiltration and function of T lymphocytes via IL-15 in tumor microenvironment. 3, 5-diiodotyrosine combined with PD-1/PD-L1 blockade could elicit synergistic cancer prevention effects, indicating a novel strategy for both prevent the somatic mutation accumulation and the immune-suppressive microenvironment exacerbation.

Serum diiodotyrosine - a biomarker to differentiate destructive thyroiditis from Graves' disease.

OBJECTIVE: Conventional diagnostic methods are limited in their ability to differentiate destructive thyroiditis from Graves' disease. We hypothesised that serum diiodotyrosine (DIT) and monoiodotyrosine (MIT) levels could be biomarkers for differentiating destructive thyroiditis from Graves' disease. DESIGN: Patients with destructive thyroiditis (n = 13) and Graves' disease (n = 22) were enrolled in this cross-sectional study. METHODS: We assayed the serum DIT and MIT levels using liquid chromatography-tandem mass spectrometry. A receiver operating characteristic (ROC) curve analysis was used to determine the sensitivity and specificity of the serum DIT and MIT levels as biomarkers for differentiating destructive thyroiditis from Graves' disease. RESULTS: The serum DIT and MIT levels were significantly higher in patients with destructive thyroiditis than in those with Graves' disease. The ROC curve analysis showed that the serum DIT levels (≥359.9 pg/mL) differentiated destructive thyroiditis from Graves' disease, significantly, with 100.0% sensitivity and 95.5% specificity (P < 0.001). The diagnostic accuracy of the serum MIT levels (≥119.4 pg/mL) was not as high as that of the serum DIT levels (sensitivity, 84.6%; specificity, 77.3%; P = 0.001). CONCLUSIONS: The serum DIT levels may serve as a novel diagnostic biomarker for differentiating destructive thyroiditis from Graves' disease.

Thyroid hormone binding motifs and iodination pattern of thyroglobulin.

A phylogenetically conserved 5-residue thyroid hormone (TH)- binding motif was originally found in a few TH plasma carriers and, more recently, in all known plasma and cell-associated proteins interacting with TH as well as in proteins involved in iodide uptake. Minor variations of the motif were found, depending on the particular class of those proteins. Since thyroglobulin (Tg) is the protein matrix for TH synthesis starting from iodination of a selected number of tyrosines (to form first monoiodotyrosine (MIT) and diiodotyrosine (DIT) and then T3 and T4), we hypothesized that by searching the presence of perfect or imperfect versions of that motif in two Tg species (human and murine) in which the iodinated tyrosines and pattern of iodotyrosine/iodothyronine formation are known, we could have found relevant explanations. Explanations, which are not furnished by the simple possession of tyrosine-iodination motifs and sequence of the iodination motif, concern why only some (but not other) tyrosine residues in one species are iodinated and why they have a particular iodination pattern. In this bioinformatics study, we provide such explanations.

Environmental photochemical fate and UVC degradation of sodium levothyroxine in aqueous medium.

The synthetic hormone sodium levothyroxine (LTX) is one of the most prescribed drugs in the world and the most effective in hypothyroidism treatment. The presence of LTX in the environment has become a matter of major concern due to the widespread use of this hormone and by the fact that it is only partially removed in conventional water and sewage treatment plants. However, information regarding the photochemical fate of this hormone in environmental or engineered systems is scarce in the literature. In this work, the sunlight-driven direct and indirect LTX degradation was investigated by determining the photolysis quantum yield, ΦLTX = 3.80 (± 0.02) × 10-5, as well as the second-order kinetic constants of the reactions with hydroxyl radicals, kLTX,•OH = 1.50 (± 0.01) × 1010 L mol-1 s-1 and singlet oxygen, kLTX,1O2 = 1.47 (± 0.66) × 108 L mol-1 s-1. Mathematical simulations indicate that LTX photodegradation is favored in shallow, nitrite-rich, and dissolved organic matter (DOM)-poor environments, with LTX half-life times varying from less than 10 days to about 80 days. LTX removals of 85 and 95% were achieved by UVC photolysis and UVC/H2O2 after 120 min, respectively. Three transformation products, triiodothyronine, diiodothyronine, and diiodotyrosine, were identified during LTX degradation by the UVC-based processes studied. The results herein regarding photo-induced kinetics coupled with environmental fate simulations may help evaluate LTX persistence and also the design of water and wastewater treatment processes.

Molecular characterization of iodotyrosine dehalogenase deficiency in patients with hypothyroidism.

CONTEXT: The recent cloning of the human iodotyrosine deiodinase (IYD) gene enables the investigation of iodotyrosine dehalogenase deficiency, a form a primary hypothyroidism resulting from iodine wasting, at the molecular level. OBJECTIVE: In the current study, we identify the genetic basis of dehalogenase deficiency in a consanguineous family. RESULTS: Using HPLC tandem mass spectrometry, we developed a rapid, selective, and sensitive assay to detect 3-monoiodo-l-tyrosine and 3,5-diodo-l-tyrosine in urine and cell culture medium. Two subjects from a presumed dehalogenase-deficient family showed elevated urinary 3-monoiodo-l-tyrosine and 3,5-diodo-l-tyrosine levels compared with 57 normal subjects without thyroid disease. Subsequent analysis of IYD revealed a homozygous missense mutation in exon 4 (c.658G>A p.Ala220Thr) that co-segregates with the clinical phenotype in the family. Functional characterization of the mutant iodotyrosine dehalogenase protein showed that the mutation completely abolishes dehalogenase enzymatic activity. One of the heterozygous carriers for the inactivating mutation recently presented with overt hypothyroidism indicating dominant inheritance with incomplete penetration. Screening of 100 control alleles identified one allele positive for this mutation, suggesting that the c.658G>A nucleotide substitution might be a functional single nucleotide polymorphism. CONCLUSIONS: This study describes a functional mutation within IYD, demonstrating the molecular basis of the iodine wasting form of congenital hypothyroidism. This familial genetic defect shows a dominant pattern of inheritance with incomplete penetration.

Reversibly Switchable, pH-Dependent Peptide Ligand Binding via 3,5-Diiodotyrosine Substitutions.

Cell type-specific targeting ligands utilized in drug delivery applications typically recognize receptors that are overexpressed on the cells of interest. Nonetheless, these receptors may also be expressed, to varying extents, on off-target cells, contributing to unintended side effects. For the selectivity profile of targeting ligands in cancer therapy to be improved, stimuli-responsive masking of these ligands with acid-, redox-, or enzyme-cleavable molecules has been reported, whereby the targeting ligands are exposed in specific environments, e.g., acidic tumor hypoxia. One possible drawback of these systems lies in their one-time, permanent trigger, which enables the "demasked" ligands to bind off-target cells if released back into the systemic circulation. A promising strategy to address the aforementioned problem is to design ligands that show selective binding based on ionization state, which may be microenvironment-dependent. In this study, we report a systematic strategy to engineer low pH-selective targeting peptides using an M2 macrophage-targeting peptide (M2pep) as an example. 3,5-Diiodotyrosine mutagenesis into native tyrosine residues of M2pep confers pH-dependent binding behavior specific to acidic environment (pH 6) when the amino acid is protonated into the native tyrosine-like state. At physiological pH of 7.4, the hydroxyl group of 3,5-diiodotyrosine on the peptide is deprotonated leading to interruption of the peptide native binding property. Our engineered pH-responsive M2pep (Ac-Y-Î-Î) binds target M2 macrophages more selectively at pH 6 than at pH 7.4. In addition, 3,5-diiodotyrosine substitutions also improve serum stability of the peptide. Finally, we demonstrate pH-dependent reversibility in target binding via a postbinding peptide elution study. The strategy presented here should be applicable for engineering pH-dependent functionality of other targeting peptides with potential applications in physiology-dependent in vivo targeting applications (e.g., targeting hypoxic tumor/inflammation) or in in vitro receptor identification.

Materials indistinguishable from iodotyrosines in normal human serum and human serum albumin.

Materials indistinguishable from authentic mono- and diiodotyrosines were identified in extracts of normal human serum as well as in extracts of purified human serum albumin. These materials were not found in association with the other serum proteins. Identification of MIT and DIT was made by a technique using rechromatography to constant specific activity, as well as by the Barker wet ash distillation method, which established the compounds in question as being iodinated ones. By two different extraction and chromatographic methods we estimated the amounts of both MIT and DIT present in normal human serum or albumin; the estimates were in good agreement. These compounds together constituted between 19% and 25% of the extractable serum iodine.

A multiple ligand-binding radioimmunoassay of diiodotyrosine.

A radioimmunoassay has been developed for the measurement of 3,5-diiodo-L-tyrosine (DIT) in serum. DIT was coupled to porcine thyroglobulin (PTg) with a molar ratio of 205:1. Rabbits were immunized with 1 mg of immunogen emulsified in complete Freund's adjuvant. Sera were screened for their ability to bind trace amounts of [125I]DIT. A serum that bound 40% of the tracer at a final dilution of 1:1,750 was used in the assay. Assay specificity was improved by the use of thyroxine (T4)-binding globulin as a second ligand-binding protein to decrease T4 and triiodothyronine (T3) cross-reactivity with the antibody. Double antibody and polyethylene glycol radioimmunoassays were compared. DIT present in the second antiserum shifted the double antibody assay standard curve and altered estimates of assay specificity and assay sensitivity. By using the polyethylene glycol system and butanol:ethanol extracts of serum, DIT was measured in human serum. In 35 apparently healthy young adult controls DIT levels averaged 156 ng/100 ml. Random DIT levels averaged 158 ng/100 ml in 11 untreated hyperthyroid patients and 84 ng/100 ml in 15 untreated primary hypothyroid patients. No diurnal pattern in DIT levels could be demonstrated. Thyroid-stimulating hormone administration led to a variable but small rise in DIT levels, but short term T3 suppression was not associated with a measurable fall in DIT concentrations. Paired serum samples from the carotid artery and thyroid vein of 10 euthyroid goiter patients and one patient with a toxic solitary adenoma all showed a positive transthyroidal gradient indicating the thyroidal release of DIT in each patient. Measurable DIT levels of 45, 47, 68, and 80 ng/100 ml, respectively, were found in four fasting athyrotic patients indicating that the thyroid is not the only source of serum DIT.

Effects of 3-nitro-L-tyrosine on thyroid function in the rat: an experimental model for the dehalogenase defect.

The effects on thyroid function of an inhibitor of tyrosine dehalogenase, 3-nitro-L-tyrosine (MNT) have been investigated in rats. In preliminary studies, marked inhibition of iodotyrosine deiodination was demonstrated in rats drinking 8 mM MNT. A series of experiments was then performed in which rats received Remington low iodine diet and 8 mM MNT as drinking fluid. This regimen had the following effects, compared to the effects of a low iodine diet alone: (a) a decrease in serum protein-bound iodine, elevation of serum thyrotropin level, goiter, and growth inhibition all prevented or reversed by iodine supplements: (b) on initiation of MNT, a 2- to 3-fold increase in the rate of release of radioiodine from the thyroid and concomitant urinary excretion of large amounts of organic iodine: and (c) after 2 wk of MNT, a greatly increased rate of thyroidal uptake and release of (131)I, an increase in the ratio of monoiodotyrosine-(131)I to diiodotyrosine-(131)I in thyroid proteolysates and the appearance of labeled iodotyrosines in serum. Acute administration of MNT intraperitoneally to rats on either an iodine-deficient or iodine-sufficient diet did not inhibit thyroidal uptake of (131)I or alter the distribution of (131)I among thyroidal iodoamino acids. It is concluded that MNT is an effective inhibitor of iodotyrosine deiodination in vivo, without other important actions on thyroid function. Thus, MNT treatment affords a model for the human dehalogenase defect. By provoking iodotyrosine secretion and consequent urinary loss of iodine, MNT can exaggerate the effects of a low iodine intake, producing goitrous hypothyroidism despite a rapid rate of iodine turnover in the thyroid.

Formation of diiodotyrosine from thyroxine. Ether-link cleavage, an alternate pathway of thyroxine metabolism.

Studies were performed to elucidate the nature of the pathway of hepatic thyroxine (T4) metabolism that is activated by inhibitors of liver catalase. For this purpose, the metabolism of T4 in homogenates of rat liver was monitored with T4 labeled with 125I either at the 5'-position of the outer-ring (125I-beta-T4) or uniformly in both the outer and inner rings (125I-U-T4). In homogenates incubated with 125I-beta-T4 in an atmosphere of O2, the catalase inhibitor aminotriazole greatly enhanced T4 degradation, promoting the formation of large proportions of 125I-labeled iodide (125I-I-) and chromatographically immobile origin material (125I-OM), but only a minute proportion of 125I-labeled 3,5,3'-triiodothyronine (125I-T3) (T3 neogenesis). In an atmosphere of N2, in contrast, homogenates produced much larger proportions of 125I-T3, and aminotriazole had no effect. In incubations with 125I-U-T4, under aerobic conditions, control homogenates degraded T4 slowly; formation of 125I-labeled 3,5-diiodotyrosine (125I-DIT) was seen only occasionally and in minute proportions. However, in homogenates incubated under O2, but not N2, aminotriazole consistently elicited the formation of large proportions of 125I-DIT, indicating that the ether link of T4 was being cleaved by an O2-dependent process. Formation of 125I-DIT in the presence of aminotriazole and O2 was markedly inhibited by the substrates of peroxidase, aminoantipyrine, and guaiacol. GSH greatly attenuated the increase in DIT formation induced by aminotriazole, whereas the sulfhydryl inhibitor N-ethylmaleimide (NEM) activated the DIT-generating pathway, even in the absence of aminotriazole. Activation of the in vitro formation of 125I-DIT from 125I-U-T4 was also produced by the in vivo administration of aminotriazole or bacterial endotoxin, an agent that reduces hepatic catalase activity. Studies with 125I-DIT as substrate revealed it to be rapidly deiodinated by liver homogenates under aerobic conditions. Recovery of 125I-DIT from 125I-U-T4 was increased by the addition of the inhibitor of iodotyrosine dehalogenase, 3,5-dinitrotyrosine. However, as judged from studies conducted in parallel with radioiodine-labeled DIT and 125I-U-T4 as substrates, none of the factors that altered the proportion of 125I-DIT found after incubations with 125I-U-T4 did so by altering the degradation of the 125I-DIT formed. The factors that influenced DIT formation from T4 in rat liver had opposite effects on T3 neogenesis. Thus, aminotriazole, endotoxin, NEM, and an aerobic atmosphere, all of which enhanced DIT formation, were inhibitory to T3 neogenesis. In contrast, anaerobiosis and GSH inhibited ether-link cleavage of T4, but facilitated T3 neogenesis. The foregoing results suggest that a pathway for the ether-link cleavage of T4 to yield DIT is present in rat liver. Activity of this pathway, which appears to be peroxidase mediated, is inversely related to activity of the pathway for the T3 neogenesis. It is further suggested that this reciprocity reflects a reciprocal relationship between hepatic GSH and H2O2, the former increasing T3 formation and inhibiting DIT formation, and the latter producing opposite effects.

Ether link cleavage is the major pathway of iodothyronine metabolism in the phagocytosing human leukocyte and also occurs in vivo in the rat.

These studies were performed to test the hypothesis that ether link cleavage (ELC) is an important pathway for the metabolism of thyroxine (T(4)) in the phagocytosing human leukocyte. When tyrosyl ring-labeled [(125)I]T(4)([Tyr(125)I]T(4)) was incubated with phagocytosing leukocytes, 50% of the degraded label was converted into [(125)I]3,5-diiodotyrosine ([(125)I]DIT). Of the remaining [Tyr(125)I]T(4) that was degraded, two-thirds was recovered as [(125)I]-nonextractable iodine ([(125)I]NEI), and one-third as [(125)I]iodide. The production of [(125)I]DIT was not observed when phenolic ring-labeled [(125)I]T(4) ([Phen(125)I]T(4)) was used, although [(125)I]NEI and [(125)I]iodide were produced. None of these iodinated compounds were formed in leukocytes that were not carrying out phagocytosis. The fraction of T(4) degraded by ELC was decreased by the addition of unlabeled T(4) and by preheating the leukocytes, findings which suggested that the process was enzymic in nature. ELC was enhanced by the catalase inhibitor aminotriazole, and was inhibited by the peroxidase inhibitor propylthiouracil, suggesting that the enzyme is a peroxidase and that hydrogen peroxide (H(2)O(2)) is a necessary cofactor in the reaction. To test this hypothesis, studies were performed in several inherited leukocytic disorders. ELC was not observed in the leukocytes of patients with chronic granulomatous disease, in which the respiratory burst that accompanies phagocytosis is absent. ELC was normal in the leukocytes of two subjects homozygous for Swiss-type acatalasemia, and aminotriazole enhanced ELC in these cells to an extent not significantly different from that observed in normal cells. ELC was normal in the leukocytes of a patient with myeloperoxidase deficiency, but could be induced by the incubation of [Tyr(125)I]T(4) with H(2)O(2) and horseradish peroxidase in the absence of leukocytes. The in vivo occurrence of ELC in the rat was confirmed by demonstrating the appearance of [(125)I]DIT in serum from parenterally injected [(125)I]3,5-diiodothyronine, but no [(125)I]DIT was produced when [(125)I]3',5'-diiodothyronine was administered. FROM THESE FINDINGS WE CONCLUDE THE FOLLOWING: (a) ELC is the major pathway for the degradation of T(4) during leukocyte phagocytosis, and accounts for 50% of the disposal of this iodothyronine; (b) the NEI and iodide formed by phagocytosing cells are derived from the degradation of the phenolic and tyrosyl rings of T(4), although ELC per se accounts for only a small fraction of these iodinated products; (c) the process by which ELC occurs is enzymic in nature, and its occurrence requires the presence of the respiratory burst that accompanies phagocytosis; (d) the enzyme responsible for ELC is likely to be a peroxidase, although a clear role for myeloperoxidase as the candidate enzyme remains to be established; (e) iodothyronines are also degraded by ELC in vivo, and the quantitative importance of this pathway in various pathophysiological states requires further investigation.

Demonstration of iodide transport defect but normal iodide organification in nonfunctioning nodules of human thyroid glands.

Benign and malignant nodules in human thyroid glands, which did not concentrate iodide in vivo, were also unable to accumulate iodide in vitro. The mean thyroid-to-medium ratio (T/M) in seven benign nodules was 0.8+/-0.2 compared with 7+/-2 in adjacent normal thyroid tissue. In four malignant thyroid nodules, the mean T/M was 0.5+/-0.1 compared with 11+/-4 in adjacent normal thyroid. Despite the inability of such nodules to concentrate iodide, iodide organification was present but was only one-half to one-third as active as in surrounding normal thyroid. Thyroid-stimulating hormone (TSH) increased iodide organification equally in both benign nodules and normal thyroid although it had no effect in three of the four malignant lesions. The reduction in organification is probably related to the absence of iodide transport, since incubation of normal thyroid slices with perchlorate caused similar diminution in iodide incorporation but no change in the response to TSH. Monoiodotyrosine (MIT) and di-iodotyrosine (DIT) accounted for most of the organic iodide in both the nodules and normal tissue. The MIT/DIT ratio was similar in normal and nodule tissue. The normal tissue contained much more inorganic iodide than the nodules, consistent with the absence of the iodide trap in the latter tissue. The thyroxine content of normal thyroid was 149+/-17 mug/g wet wt and 18+/-4 mug/g wet wt in the nodules. The transport defect in the nodules was not associated with any reduction in total, Na(+)-K(+)- or Mg(++)-activated ATPase activities or the concentration of ATP. Basal adenylate cyclase was higher in nodules than normal tissue. Although there was no difference between benign and malignant nodules, the response of adenylate cyclase to TSH was greater in the benign lesions. These studies demonstrate that nonfunctioning thyroid nodules, both benign and malignant, have a specific defect in iodide transport that accounts for their failure to accumulate radioactive iodide in vivo. In benign nodules, iodide organification was increased by TSH while no such effect was found in three of four malignant lesions, suggesting additional biochemical defects in thyroid carcinomas.

Protein radical formation on thyroid peroxidase during turnover as detected by immuno-spin trapping.

Thyroid peroxidase (TPO) is a 933 amino acid residue, heme-containing, integral membrane glycoprotein that catalyzes two steps in the maturation of the thyroid hormone precursor. As with other peroxidases, these reactions require hydrogen peroxide and initial enzyme oxidation. Previous researchers studied the oxidative state of the TPO heme moiety using spectrophotometric and catalytic analyses. We use a novel antiserum to 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to detect radical-derived DMPO spin-trapped TPO. Our work reveals that TPO generates radical adducts in the presence of H2O2, but that the generation of these adducts can be suppressed by the addition of substrates and inhibitors. Chemical alteration of the tyrosine residues of TPO greatly reduces the generation of TPO-DMPO adducts. Iodide strongly suppresses the H2O2-generated production of TPO radical adducts and protects the enzyme from loss of enzyme activity. Because the normal catalytic mechanism of TPO involves the production of radical species, TPO is potentially more susceptible to oxidative damage than most enzymes which do not require H2O2 as a substrate. We hypothesize that oxidatively damaged TPO may trigger the production of anti-TPO autoantibodies, resulting in the development of autoimmune thyroid disorders. Evidence that correlates iodine deficiencies with development of thyroid autoimmune disorders supports this conjecture.