Rat liver iodothyronine monodeiodinase. Evaluation of the iodothyronine ligand-binding site

Selenium-More than Just a Fortuitous Sulfur Substitute in Redox Biology

Living organisms use selenium mainly in the form of selenocysteine in the active site of oxidoreductases. Here, selenium's unique chemistry is believed to modulate the reaction mechanism and enhance the catalytic efficiency of specific enzymes in ways not achievable with a sulfur-containing cysteine. However, despite the fact that selenium/sulfur have different physicochemical properties, several selenoproteins have fully functional cysteine-containing homologues and some organisms do not use selenocysteine at all. In this review, selected selenocysteine-containing proteins will be discussed to showcase both situations: (i) selenium as an obligatory element for the protein's physiological function, and (ii) selenium presenting no clear advantage over sulfur (functional proteins with either selenium or sulfur). Selenium's physiological roles in antioxidant defence (to maintain cellular redox status/hinder oxidative stress), hormone metabolism, DNA synthesis, and repair (maintain genetic stability) will be also highlighted, as well as selenium's role in human health. Formate dehydrogenases, hydrogenases, glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases will be herein featured.

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.

Screening the ToxCast Phase 1, Phase 2, and e1k Chemical Libraries for Inhibitors of Iodothyronine Deiodinases

Deiodinase enzymes play an essential role in converting thyroid hormones between active and inactive forms by deiodinating the pro-hormone thyroxine (T4) to the active hormone triiodothyronine (T3) and modifying T4 and T3 to inactive forms. Chemical inhibition of deiodinase activity has been identified as an important endpoint to include in screening chemicals for thyroid hormone disruption. To address the lack of data regarding chemicals that inhibit the deiodinase enzymes, we developed robust in vitro assays that utilized human deiodinase types 1, 2, and 3 and screened over 1800 unique chemicals from the U.S. EPA's ToxCast phase 1_v2, phase 2, and e1k libraries. Initial testing at a single concentration identified 411 putative deiodinase inhibitors that produced inhibition of 20% or greater in at least 1 of the 3 deiodinase assays, including chemicals that have not previously been shown to inhibit deiodinases. Of these, 228 chemicals produced enzyme inhibition of 50% or greater; these chemicals were further tested in concentration-response to determine relative potency. Comparisons across these deiodinase assays identified 81 chemicals that produced selective inhibition, with 50% inhibition or greater of only 1 of the deiodinases. This set of 3 deiodinase inhibition assays provides a significant contribution toward expanding the limited number of in vitro assays used to identify chemicals with the potential to interfere with thyroid hormone homeostasis. In addition, these results set the groundwork for development and evaluation of structure-activity relationships for deiodinase inhibition, and inform targeted selection of chemicals for further testing to identify adverse outcomes of deiodinase inhibition.

3-Iodothyronamine-A Thyroid Hormone Metabolite With Distinct Target Profiles and Mode of Action

The rediscovery of the group of thyronamines (TAMs), especially the first detailed description of their most prominent congener 3-iodothyronamine (3T1AM) 14 years ago, boosted research on this thyroid hormone metabolite tremendously. TAMs exert actions partly opposite to and distinct from known functions of thyroid hormones. These fascinating metabolic, anapyrexic, cytoprotective, and brain effects quickly evoked the hope to use hormone-derived TAMs as a therapeutic option. The G protein-coupled receptor (GPCR) TAAR1, a member of the trace amine-associated receptor (TAAR) family, was identified as the first target and effector of TAM action. The initial enthusiasm on pharmacological actions of exogenous TAMs elicited many questions, such as sites of biosynthesis, analytics, modes of action, inactivation, and role of TAMs in (patho)physiology. Meanwhile, it became clear that TAMs not only interact with TAAR1 or other TAAR family members but also with several aminergic receptors and non-GPCR targets such as transient receptor potential channels, mitochondrial proteins, and the serum TAM-binding protein apolipoprotein B100, thus classifying 3T1AM as a multitarget ligand. The physiological mode of action of TAMs is still controversial because regulation of endogenous TAM production and the sites of its biosynthesis are not fully elucidated. Methods for 3T1AM analytics need further validation, as they revealed different blood and tissue concentrations depending on detection principles used such as monoclonal antibody-based immunoassay vs liquid chromatography- matrix-assisted laser desorption/ionization mass spectrometry or time-of-flight mass spectrometry. In this review, we comprehensively summarize and critically evaluate current basic, translational, and clinical knowledge on 3T1AM and its main metabolite 3-iodothyroacetic acid, focusing on endocrine-relevant aspects and open but highly challenging issues.

Comparison of succinimidyl [(125)I]iodobenzoate with iodogen iodination methods to study pharmacokinetics and ADME of biotherapeutics

PURPOSE

To assess the application of succinimidyl iodobenzoate (SIB) iodination method in labeling biotherapeutics to study their pharmacokinetics (PK) and biodistribution.

METHOD

An IgG molecule (protein-01) and a 40 KDa protein (protein-02) were evaluated. Pharmacokinetics (PK) and biodistribution of the radiolabeled IgG ((125)I-protein-01) in mice compared parameters from SIB and Iodogen protein iodination labeling methods. In addition, PK of radiolabeled 40 KDa protein ((125)I-protein-02) using SIB was compared with non-labeled protein-02 analyzed by ligand binding assay (LBA).

RESULTS

Up to 72 h following a single IP injection to mice, the percentage of "free-label" determined by the soluble counts after TCA precipitation to total radioactivity in serum samples was 2.8-49.4% vs. <1.0% for (125)I-protein-01 iodinated via Iodogen or SIB methods, respectively, suggesting a higher in vivo stability of (125)I-protein-01 labeled via the SIB method. The serum exposure of (125)I-protein-01 was two-fold higher, and correspondingly, the tissue exposure was also higher (averaging 3.6 fold at 168 h) when using SIB protein labeling method than when using the Iodogen method. In addition, following subcutaneous (SC) administration to mice, the serum exposure of (125)I-protein-02 labeled via SIB method was similar to protein-02 measure by LBA.

CONCLUSION

In this study, iodination of proteins using SIB methodology has overcome the dehalogenation problem in vivo which is inherent in Iodogen method, and PK parameters of a protein iodinated via SIB were comparable to the un-labeled protein measured by LBA. The SIB iodination method is an improved labeling approach for biotherapeutics used in studying PK and biodistribution characteristics.

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.

A nonselenoprotein from amphioxus deiodinates triac but not T3: is triac the primordial bioactive thyroid hormone?

Thyroid hormone (TH) is important for metamorphosis in many species, including the cephalochordate Branchiostoma floridae, a marine invertebrate (amphioxus) living in warmer coastal areas. Branchiostoma expresses a TH receptor, which is activated by 3,3',5-triiodothyroacetic acid (TA(3)) but not by T(3). The Branchiostoma genome also contains multiple genes coding for proteins homologous to iodothyronine deiodinases in vertebrates, selenoproteins catalyzing the activation or inactivation of TH. Three Branchiostoma deiodinases have been cloned: two have a catalytic Sec, and one, bfDy, has a Cys residue. We have studied the catalytic properties of bfDy in transfected COS1 cells by HPLC analysis of reactions with (125)I-labeled substrates and dithiothreitol as cofactor. We could not detect deiodination of T(4), T(3), or rT(3) by bfDy but observed rapid and selective inner ring deiodination (inactivation) of TA(3) and 3,3',5,5'-tetraiodothyroacetic acid (TA(4)). Deiodination of TA(3) by bfDy was optimal at 25 C and 10 mm dithiothreitol. bfDy was extremely labile at 37 C, showing a half-life of less than 2 min, in contrast with a half-life of more than 60 min at 25 C. Deiodination of labeled TA(3) was inhibited dose dependently by unlabeled TA(3)≈TA(4)>T(4)≈T(3). Michaelis-Menten analysis yielded Michaelis-Menten constant values of 6.8 and 68 nm and maximum velocity values of 1.4 and 5.4 pmol/min·mg protein for TA(3) and TA(4), respectively. bfDy was not inhibited by propylthiouracil and iodoacetate and only weakly by goldthioglucose and iopanoic acid. In conclusion, we demonstrate rapid inactivation of TA(3) and TA(4) but not of T(3) and T(4) by the first reported natural nonselenodeiodinase. Our findings support the hypothesis that TA(3) is a primordial bioactive TH.

Halogenated phenolic contaminants inhibit the in vitro activity of the thyroid-regulating deiodinases in human liver

Halogenated contaminants, particularly brominated flame retardants, disrupt circulating levels of thyroid hormones (THs), potentially affecting growth and development. Disruption may be mediated by impacts on deiodinase (DI) activity, which regulate the levels of active hormones available to bind to nuclear receptors. The goal of this study was to develop a mass spectrometry-based method for measuring the activity of DIs in human liver microsomes and to examine the effect of halogenated phenolic contaminants on DI activity. Thyroxine (T4) and reverse triiodothyronine (rT3) deiodination kinetics were measured by incubating pooled human liver microsomes with T4 or rT3 and monitoring the production of T3, rT3, 3,3'-diiodothyronine, and 3-monoiodothyronine by liquid chromatography tandem mass spectrometry. Using this method, we examined the effects of several halogenated contaminants, including 2,2',4,4',5-pentabromodiphenyl ether (BDE 99), several hydroxylated polybrominated diphenyl ethers (OH-BDEs), tribromophenol, tetrabromobisphenol A, and triclosan, on DI activity. The Michaelis constants (K(M)) of rT3 and T4 deiodination were determined to be 3.2 ± 0.7 and 17.3 ± 2.3μM. The V(max) was 160 ± 5.8 and 2.8 ± 0.10 pmol/min.mg protein, respectively. All studied contaminants inhibited DI activity in a dose-response manner, with the exception of BDE 99 and two OH-BDEs. 5'-Hydroxy 2,2',4,4',5-pentabromodiphenyl ether was found to be the most potent inhibitor of DI activity, and phenolic structures containing iodine were generally more potent inhibitors of DI activity relative to brominated, chlorinated, and fluorinated analogues. This study suggests that some halogenated phenolics, including current use compounds such as plastic monomers, flame retardants, and their metabolites, may disrupt TH homeostasis through the inhibition of DI activity in vivo.

Essential molecular determinants for thyroid hormone transport and first structural implications for monocarboxylate transporter 8

Monocarboxylate transporter 8 (MCT8, SLC16A2) is a thyroid hormone (TH) transmembrane transport protein mutated in Allan-Herndon-Dudley syndrome, a severe X-linked psychomotor retardation. The neurological and endocrine phenotypes of patients deficient in MCT8 function underscore the physiological significance of carrier-mediated TH transmembrane transport. MCT8 belongs to the major facilitator superfamily of 12 transmembrane-spanning proteins and mediates energy-independent bidirectional transport of iodothyronines across the plasma membrane. Structural information is lacking for all TH transmembrane transporters. To gain insight into structure-function relations in TH transport, we chose human MCT8 as a paradigm. We systematically performed conventional and liquid chromatography-tandem mass spectrometry-based uptake measurements into MCT8-transfected cells using a large number of compounds structurally related to iodothyronines. We found that human MCT8 is specific for L-iodothyronines and requires at least one iodine atom per aromatic ring. Neither thyronamines, decarboxylated metabolites of iodothyronines, nor triiodothyroacetic acid and tetraiodothyroacetic acid, TH derivatives lacking both chiral center and amino group, are substrates for MCT8. The polyphenolic flavonoids naringenin and F21388, potent competitors for TH binding at transthyretin, did not inhibit T(3) transport, suggesting that MCT8 can discriminate its ligand better than transthyretin. Bioinformatic studies and a first molecular homology model of MCT8 suggested amino acids potentially involved in substrate interaction. Indeed, alanine mutation of either Arg(445) (helix 8) or Asp(498) (helix 10) abrogated T(3) transport activity of MCT8, supporting their predicted role in substrate recognition. The MCT8 model allows us to rationalize potential interactions of amino acids including those mutated in patients with Allan-Herndon-Dudley syndrome.

Thyronamines are isozyme-specific substrates of deiodinases

3-Iodothyronamine (3-T 1 AM) and thyronamine (T AM) are novel endogenous signaling molecules that exhibit great structural similarity to thyroid hormones but apparently antagonize classical thyroid hormone (T(3)) actions. Their proposed biosynthesis from thyroid hormones would require decarboxylation and more or less extensive deiodination. Deiodinases (Dio1, Dio2, and Dio3) catalyze the removal of iodine from their substrates. Because a role of deiodinases in thyronamine biosynthesis requires their ability to accept thyronamines as substrates, we investigated whether thyronamines are converted by deiodinases. Thyronamines were incubated with isozyme-specific deiodinase preparations. Deiodination products were analyzed using a newly established method applying liquid chromatography and tandem mass spectrometry (LC-MS/MS). Phenolic ring deiodinations of 3,3',5'-triiodothyronamine (rT3AM), 3',5'-diiodothyronamine (3',5'-T2AM), and 3,3'-diiodothyronamine (3,3'-T2AM) as well as tyrosyl ring deiodinations of 3,5,3'-triiodothyronamine (T3AM) and 3,5-diiodothyronamine (3,5-T2AM) were observed with Dio1. These reactions were completely inhibited by the Dio1-specific inhibitor 6n-propyl-2-thiouracil (PTU). Dio2 containing preparations also deiodinated rT(3)AM and 3',5'-T2AM at the phenolic rings but in a PTU-insensitive fashion. All thyronamines with tyrosyl ring iodine atoms were 5(3)-deiodinated by Dio3-containing preparations. In functional competition assays, the newly identified thyronamine substrates inhibited an established iodothyronine deiodination reaction. By contrast, thyronamines that had been excluded as deiodinase substrates in LC-MS/MS experiments failed to show any effect in the competition assays, thus verifying the former results. These data support a role for deiodinases in thyronamine biosynthesis and contribute to confining the biosynthetic pathways for 3-T 1 AM and T 0 AM.

Thyroid receptor ligands. Part 7: Indirect antagonists of the thyroid hormone receptor with improved affinity

Based on the concept of 'indirect antagonism' of nuclear receptors, a series of thyroid hormone receptor (TR) antagonists were prepared with improved affinity compared with what was previously described. The results of a binding assay for the human TR and reporter cell assay revealed, within this series, that an m-bromobenzoyl substituent (11f) was optimal in terms of affinity and antagonist activity. Compared with already reported TR antagonists, their affinities are within the same range, thus potentially representing useful approach to novel and high-affinity TR-antagonists.

Thyroid Receptor Ligands. 6. A High Affinity “Direct Antagonist” Selective for the Thyroid Hormone Receptor

A new high-affinity thyroid hormone antagonist 6 with druglike properties was designed and synthesized. The compound behaved as an antagonist in a cell transactivation assay, and in a first in vivo experiment in rats.

Preparation of N-succinimidyl 3-[*I]iodobenzoate: an agent for the indirect radioiodination of proteins

A procedure for the synthesis of N-succinimidyl 3-iodobenzoate labeled with any iodine isotope ([*I]SIB), which is an agent used in the radioiodination of proteins and peptides, from its tin precursor N-succinimidyl 3-(tri-n-butylstannyl)benzoate (STB) is described. Also included are protocols for the synthesis of an unlabeled standard of SIB and the tin precursor. Radioiododestannylation of STB using tert-butylhydroperoxide as the oxidant gives [*I]SIB in 80% radiochemical yields. The total time for the synthesis of [*I]SIB from STB is approximately 95 min. Use of [*I]SIB yields radioiodinated proteins that are considerably more stable in vivo than those radioiodinated by the direct electrophilic method.

Thyroid Receptor Ligands. 3. Design and Synthesis of 3,5-Dihalo-4-alkoxyphenylalkanoic Acids as Indirect Antagonists of the Thyroid Hormone Receptor

Based on the recently described concept of "indirect antagonism" of nuclear receptors, a series of thyroid hormone receptor (TR) antagonists were prepared, in which the outer ring of a thyromimetic was replaced with alkyl chains of variable length and branch. The results of a binding assay for the human TR and reporter cell assay revealed, within this series, a positive correlation between increasing bulk of the alkyl group and affinity to TRs. Compared with already reported TR antagonists, their affinities are within the same range, thus potentially representing a useful approach to novel and high affinity TR-antagonists.

Endocrine active compounds affect thyrotropin and thyroid hormone levels in serum as well as endpoints of thyroid hormone action in liver, heart and kidney

To assess interference with endocrine regulation of the thyroid axis, rats (female, ovariectomised) were treated for 12 weeks with the suspected endocrine active compounds (EAC) or endocrine disrupters (ED) 4-nonylphenol (NP), octyl-methoxycinnamate (OMC) and 4-methylbenzylidene-camphor (4-MBC) as well as 17beta-estradiol (E2) and 5alpha-androstane-3beta,17beta-diol (Adiol) on the background of a soy-free or soy-containing diet, and endpoints relevant for regulation via the thyroid axis were measured. Thyrotropin (TSH) and thyroid hormone (T4, T3) serum levels were altered, but not in a way consistent with known mechanisms of feedback regulation of the thyroid axis. In the liver, malic enzyme (ME) activity was significantly increased by E2 and Adiol, slightly by OMC and MBC and decreased by soy, whereas type I 5'-deiodinase (5'DI) was decreased by all treatments. This may be due rather to the estrogenic effect of the ED, as there is no obvious correlation with T4 or T3 serum levels. None of the substances inhibited thyroid peroxidase (TPO) in vitro, except for NP. In general, several endocrine active compounds disrupt the endocrine feedback regulation of the thyroid axis. However, there was no uniform, obvious pattern in the effects of those ED tested, but each compound elicited its own spectrum of alterations, arguing for multiple targets of interference with the complex network of thyroid hormone action and metabolism.

Disposition of radioactivity after injection of liver-targeted proteins labeled with 111In or 125I. Effect of labeling on distribution and excretion of radioactivity in rats

The effect of radiolabeling liver-specific proteins on the in vivo disposition of radioactivity was investigated. The suitability of 111In and 125I as radiolabels for protein disposition studies in vivo was examined. Galactosylated and cationized bovine serum albumin were labeled with either 125I by the chloramine-T method or 111In, using 1-(4-isothiocyanatobenzyl)ethylenediaminetetraacetic acid (SCN-BZ-EDTA) or diethylenetriaminepentaacetic acid (DTPA) as bifunctional chelating agents (BCAs) and administered intravenously to rats. 125I radioactivity disappeared rapidly from the liver with subsequent excretion in the urine and bile, mainly in the TCA soluble fraction. 111In-associated radioactivity, on the other hand, remained in the hepatic tissue in considerably higher amounts during the experiment and was excreted in the bile and urine to a lower extent when compared with 125I. When the effect of BCA on excretion of 111In radioactivity was compared, no significant differences were observed in the urinary clearances. However, biliary excretion was significantly higher for 111In-SCN-BZ-EDTA-bound radioactivity. In conclusion, when compared with 125I, 111In labeling seems to more accurately characterize the in vivo distribution of liver-targeted proteins after their iv administration in rats and allows a more accurate pharmacokinetic evaluation to be performed.

Local activation and inactivation of thyroid hormones: the deiodinase family

Tissue-specific activation and inactivation of ligands of nuclear receptors which belong to the steroid retinoid-thyroid hormone superfamily of transcription factors represents an important principle of development- and tissue-specific local modulation of hormone action. Recently, several enzyme families have been identified which act as 'guardians of the gate' of ligand-activated transcription modulation. Three monodeiodinase isoenzymes which are involved in activation the 'prohormone' L-thyroxine (T4), the main secretory product of the thyroid gland, have been identified, characterized, and cloned. Both, type I and type II 5'-deiodinase generate the thyromimetically active hormone 3,3',5-triiodothyronine (T3) by reductive deiodination of the phenolic ring of T4. Inactivation of T4 and its product T3 occurs by deiodination of iodothyronines at the tyrosyl ring. This reaction is catalyzed both the type III 5-deiodinase and also by the type I enzyme, which has a broader substrate specificity. The three deiodinases appear to constitute a newly discovered family of selenocysteine-containing proteins and the presence of selenocysteine in the protein is critical for enzyme activity. Whereas the selenoenzyme characteristics of the type I and type III deiodinases are definitively established some controversy still exists for the type II 5'-deiodinase in mammals. The mRNA probably encoding the type II 5'-deiodinase subunit is markedly longer than those of the two other deiodinases and its selenocysteine-insertion element is located more than 5 kB downstream of the UGA-codon in the 3'-untranslated region. The three deiodinase isoenzymes show a distinct development- and tissue-specific pattern of expression, operate at individual optimal substrate levels, are differently regulated and modulated by hormones, cytokines, signaling pathways, natural factors, and pharmaceuticals. Whereas circulating T3 mainly originates from hepatic production via the type I 5'-deiodinase, the local cellular thyroid hormone concentration in various tissues including the central nervous system is controlled by complex para-, auto-, and intracrine interactions of all three deiodinases. Local thyroid hormone availability is further modulated by conjugation reactions of the phenolic 4'-OH-group of iodothyronines, which also inactivate the thyroid hormones.

Binding of thyroxine to pig transthyretin, its cDNA structure, and other properties

Thyroxine binding to proteins in pig plasma during electrophoresis was observed in the albumin, but not in the prealbumin and post-albumin regions. Transthyretin could be identified in medium from in vitro pig choroid plexus incubations by size and number of subunits and a very high rate of synthesis and secretion. Its electrophoretic mobility was intermediate between that of thyroxine-binding globulin and albumin. It bound thyroxine, retinol-binding protein, anti-(rat transthyretin) antibodies and behaved similarly to transthyretins from other vertebrate species when plasma was extracted with phenol. Inhibition experiments with the synthetic flavonoid F 21388, analysing the binding of thyroxine, suggested that transthyretin is not a major thyroxine carrier in the bloodstream of pigs. Cloning and sequencing of transthyretin cDNA from both choroid plexus and liver showed that the same transthyretin mRNA is expressed in pig choroid plexus and liver. The amino acid sequence derived from the nucleotide sequence revealed that pig transthyretin differs from the transthyretins of all other studied vertebrate species by an unusual C-terminal extension consisting of the amino acids glycine, alanine and leucine. This extension results from the mutation of a stop codon into a codon for glycine. The unusual C-terminal extensions do not seem to interfere with the access of thyroxine to its binding site in the central channel of transthyretin.

Internalization and degradation of monoclonal antibody chCE7 by human neuroblastoma cells

Internalization and cellular degradation of a chimeric monoclonal anti-neuroblastoma antibody (MAb chCE7) is described. Immunofluorescence localization showed temperature-dependent redistribution of MAb chCE7 from the cell surface at 0 degrees C to vesicular structures after heating to 37 degrees C. SKN-AS cells which were pulse-labelled at 0 degrees C with radioiodinated chCE7 released 50% of the initial cell-bound radioactivity into the medium after 20 hr at 37 degrees C. Low-molecular-weight radioactivity in the medium was identified as iodotyrosine and the major intracellular degradation product was found to be an antibody fragment of 43 kDa. Degradation was blocked by chloroquine, an inhibitor of lysosomal function. MAb chCE7 binds to a carbohydrate epitope, as treatment with tunicamycin abolishes cell binding. In adherent cells in culture, inhibition of protein synthesis by cycloheximide affected cell-surface binding sites for chCE7 in a biphasic manner, with an initial increase followed by long-term decrease. Expression of binding sites was also found to be inhibited by sodium azide, by the lysosomotropic drug chloroquine and by Brefeldin A, a drug which prevents export of newly synthetized proteins to the cell surface. When cells were treated with high doses of chCE7 up to 24 hr and cell-surface binding sites were then measured after an acidic buffer wash, no loss of surface binding sites was found, indicating that pretreatment with MAb chCE7 does not induce down-regulation of its binding sites.

N-succinimidyl 4-methyl-3-(tri-n-butylstannyl)benzoate: synthesis and potential utility for the radioiodination of monoclonal antibodies

N-succinimidyl 4-methyl-3-(tri-n-butylstannyl)benzoate (MATE) was synthesized in two steps from 4-methyl-3-iodobenzoic acid. Radioiododestannylation of MATE proceeded more slowly than N-succinimidyl 3-(tri-n-butylstannyl)benzoate (ATE), but for reaction periods of 10 min or more, identical yields were obtained. Paired-label biodistribution studies were performed in mice with an intact monoclonal antibody and an F(ab')2 fragment labeled using MATE, ATE and Iodogen. Thyroid uptake with MATE was low, comparable to that seen with ATE, and considerably lower than that observed when the Iodogen method was used. With the F(ab')2 fragment, kidney uptake using MATE was 8-fold higher than that observed when either the ATE or Iodogen methods were used.

Mechanism of deiodination of 125I-human growth hormone in vivo. Relevance to the study of protein disposition

Examination of the disposition of proteins employing 125I-labeled tracers can be complicated by the in vivo deiodination of the tracer. The purpose of this study was to characterize the mechanism by which 125I-labeled proteins are deiodinated in vivo using 125I-human growth hormone (hGH) as a model compound. Intravenous (i.v.) administration of 125I-hGH resulted in a biphasic plasma kinetic pattern, with the majority of radioactivity removed from the plasma during the first 15 min. The level of circulating radioactivity at 2 hr was similar to that 15 min after administration. Radioactivity was eliminated from the animals almost exclusively in the urine. The chemical form of radioactivity in the plasma and urine was analyzed by HPLC, and precipitation of radioactivity with silver nitrate or trichloroacetic acid. Fifteen minutes after administration of 125I-hGH, 30% of the circulating radioactivity was present in the form of iodide (125I-). By 2 hr, the majority of radioactivity in the plasma was in the form of 125I-. The radioactivity in the urine was present exclusively in the form of 125I-. In vivo deiodination of 125I-hGH was reflected by the accumulation of radioactivity in the thyroid glands. There was no evidence for the presence of 125I-peptide intermediates in the plasma or urine of treated animals. In vitro, 125I-hGH was degraded to 125I-peptide intermediates by thyroid gland but not liver or kidney homogenates. In the absence of cofactors, 125I- was not observed as an in vitro metabolic product. However, in the presence of dithiothreitol and NADPH as cofactors, the predominant metabolic product formed by thyroid gland homogenates was 125I-. The deiodination of 125I-hGH by thyroid gland homogenates was inhibited by the serine protease inhibitor phenylmethylsulfonyl fluoride (PMSF), indicating that proteolysis of 125I-hGH was required for deiodination to occur. This was supported by the observation that 125I-labeled proteolytic fragments of 125I-hGH, but not 125I-hGH, were deiodinated by liver or kidney homogenates in the presence of these cofactors. Deiodination by thyroid gland homogenates was inhibited by the sulfhydryl-group blocking reagent, iodoacetate, in a concentration-dependent manner. The characteristics of the in vitro deiodination reaction suggest that a form of thyronine 5'-monodeiodinase may be involved in the in vivo deiodination of 125I-hGH and possibly other 125I-proteins. These data suggest that the disposition of proteins may be determined more accurately with 3H-, 14C- or 35S-labeled molecules which better represent the characteristics of the native protein.

Binding characteristics of a major thyroid hormone metabolite, 3,3',5'-triiodo-L-thyronine, to bovine serum albumin as measured by fluorescence

The interaction between a major thyroid hormone metabolite, 3,3',5'-triiodo-L-thyronine and bovine serum albumin was investigated by fluorescence measurements. The apparent binding constants were obtained at various pHs assuming the equivalence and independence of the interaction sites on the protein from the fluorescence titration curves. The maximum binding was attained at pH 8.0, and the apparent binding constant was (5.28 +/- 0.13).10(5) M-1 with one binding site per albumin molecule. Thermodynamic parameters were also determined from the van't Hoff plot of the apparent binding constants at pH 7.5. The free energy change, enthalpy change and entropy change were -7.70 +/- 0.09 kcal.mol-1, -4.59 kcal.mol-1 and 10.2 e.u., respectively.

Thyroid hormone analog inhibition of hepatic 5'-iodothyronine deiodinase activity

Studies using thyroid hormone analogs have provided insight into the structural requirements for thyromimetic activity and for thyroid hormone binding to thyroxine-binding globulin, thyroxine-binding prealbumin, and nuclear T3 receptors. To determine the structural specifications for iodothyronine interaction with 5'-iodothyronine deiodinase (5'-ITD), we examined the ability of 35 thyroid hormone analogs to inhibit hepatic T4 5'-deiodination in vitro. The compounds were incubated in concentrations of 0.1-500 microM with rat liver homogenates, and concentrations producing 50% inhibition of T3 production were calculated. Those iodothyronine analogs which likely serve as substrate for 5'-ITD, e.g.rT3 and 3',5'-T2, and those which have one tyrosyl iodide were the most potent inhibitors of 5'-ITD activity. The presence of tyrosyl iodides enhanced inhibition by compounds with alkyl and halogen substitutions. Inhibition was likely due to direct interaction with the enzyme, since it was readily reversed by DTT. The terminal amino and phenolic hydroxyl groups, as well as the ether linkage, do not appear to be essential components of enzyme interaction.

Thyroid hormone structure-activity relationships: molecular structure of 3,5,3'-triiodothyropropionic acid

The crystal and molecular structures of 3,5,3'-triiodothyropropionic acid (T3P), determined as an N-diethanolamine salt, were carried out and the results are compared with those of other thyroid hormone structures. These data show that T3P has an unusual conformation with the diphenyl ether bridge outside the range normally observed for other thyroactive acid structures and has the largest deviations from the ideal skewed conformation predicted for 3,5-diiodothyroactive compounds. These conformational properties are not observed in the structures of thyroformic or acetic acid analogues. Biochemical data indicate that thyropropionic acid analogue activity differs from that other acid analogues which could imply that their metabolism and activity can be controlled differently from that of other hormone metabolites.

A method for the radiohalogenation of proteins resulting in decreased thyroid uptake of radioiodine

A procedure is described for the radioiodination of proteins using an iodinated derivative of N-succinimidyl 3-(tri-n-butylstannyl) benzoate (ATE). Adequate removal of unreacted ATE from [125I]ATE was necessary for optimal protein radioiodination. Labeling efficiencies of greater than 60% could be obtained after a 20 min incubation of goat IgG with [125I]ATE at 4 degrees C. Paired-label experiments with goat IgG labeled with 125I using ATE and 131I using Iodogen demonstrated that use of the ATE reagent for protein labeling significantly reduced (P less than 0.005) the thyroid uptake of radioiodine.