Affinity For T3 Research Articles

Increased Absorption of Thyroxine in a Murine Model of Hypothyroidism Using Water/CO2 Nanobubbles.

Thyroxine (T4) is a drug extensively utilized for the treatment of hypothyroidism. However, the oral absorption of T4 presents certain limitations. This research investigates the efficacy of CO2 nanobubbles in water as a potential oral carrier for T4 administration to C57BL/6 hypothyroid mice. Following 18 h of fasting, the formulation was administered to the mice, demonstrating that the combination of CO2 nanobubbles and T4 enhanced the drug's absorption in blood serum by approximately 40%. To comprehend this observation at a molecular level, we explored the interaction mechanism through which T4 engages with the CO2 nanobubbles, employing molecular simulations, semi-empirical quantum mechanics, and PMF calculations. Our simulations revealed a high affinity of T4 for the water-gas interface, driven by additive interactions between the hydrophobic region of T4 and the gas phase and electrostatic interactions of the polar groups of T4 with water at the water-gas interface. Concurrently, we observed that at the water-gas interface, the cluster of T4 formed in the water region disassembles, contributing to the drug's bioavailability. Furthermore, we examined how the gas within the nanobubbles aids in facilitating the drug's translocation through cell membranes. This research contributes to a deeper understanding of the role of CO2 nanobubbles in drug absorption and subsequent release into the bloodstream. The findings suggest that utilizing CO2 nanobubbles could enhance T4 bioavailability and cell permeability, leading to more efficient transport into cells. Additional research opens the possibility of employing lower concentrations of this class of drugs, thereby potentially reducing the associated side effects due to poor absorption.

Effect of phenotype on thyroidal parameters and metabolic sequelae in Wistar fatty rats

To determine the effects of phenotype on parameters of peripheral thyroid hormone and actions, groups of adult male lean (LN) and obese (OB) Wistar Fatty Rats were fed a nutritionally adequate semisynthetic diet containing 54% carbohydrate, 16% mixed fats, 20% protein plus essential vitamins, minerals, and cellulose fiber for 8 weeks. Measures of weight gain (WG), feed efficiency ratio (FER), Resting (RMR) and norepinephrine (NE) stimulated VO2, serum triiodothyronine (T3), and in vitro T4-5’ deiodinase (5’DI) activity in liver (LVR), kidney (KID), gastrocnemius muscle (GNM) and interscapular brown adipose tissue (IBAT) were determined at baseline and after dithiothreitol (DTT) stimulation to optimize in vitro deiodination activity. The WG and FER of OB >> LN phenotype and FER increased over time in both phenotypes, with the greatest increase in the OB phenotype. The RMR of LN > OB, and NE-stimulation (200 µg/kg BW, sc) increased VO2 by 180% in LN but only ~30% in OB phenotype. Serum T3 of LN >> OB. Baseline 5’DI of OB > LN in LIV and KID and were of similar magnitude in both phenotypes in GNM and IBAT. After DTT stimulation, LIV increased ~200% in LN, ~30% in OB; In LN rats, KID +DTT increased ~200%, but only modest increases in OB phenotype, while in LN GNM, DTT resulted in 264% increase vs. +70% in OB rats. In IBAT, DTT resulted in ~ 30-fold increase in 5’DI in LN, and a 16-fold increase in 5’DI in the OB phenotype. These results indicate that parameters of thyroidal actions including circulating plasma levels of T3 and maximum capacity to generate T3 in peripheral tissues via 5’DI, although increased in the OB phenotype, were associated with decreases in RMR and NE-stimulated VO2. These observations occurred in association with an improved efficiency of FER and weight gain and dysregulation of intracellular T3 actions including parameters of T3 receptor affinity kinetics are likely to be among key contributors to the epigenetic expression of the OB+NIDDM phenotype in this strain and are consistent with a reduced affinity of T3- mediated cellular components of intermediary metabolism.

Altered Intracellular Trafficking of Mutant Thyroid Hormone Receptors in Resistance to Thyroid Hormone Syndrome

Resistance to Thyroid Hormone α (RTHα), a reduced sensitivity to thyroid hormone (T3) in peripheral tissues, is caused by mutations in thyroid hormone receptor α (TRα), a nuclear receptor that mediates T3-responsive gene expression. All mutations characterized in RTHα, to date, are in the ligand binding domain (LBD), resulting in reduced affinity for T3. In addition, some mutations result in truncated proteins lacking all or part of helix 12. Previously, we have used fluorescence recovery after photobleaching (FRAP) to examine the effects of select RTHα mutations on the intracellular trafficking of TRα. After transfecting HeLa cells with expression plasmids for green fluorescent protein (GFP)-tagged wild-type TRα and each of the mutants, we first assessed their intracellular distribution and initial intranuclear mobility. Although wild-type TRα is known to shuttle between the nucleus and cytoplasm, it is primarily localized to the nucleus at a steady state. We showed that TR-E403X, Ala382ProfsX7, and F397fs406X are also primarily localized to the nucleus, and FRAP revealed that wild-type TRα and the RTHα mutants are highly dynamic within the nucleus, indicating that the receptors rapidly dissociate and reassociate with DNA binding sites and/or other nuclear binding sites, independent of T3 (1). Some studies have shown that Nuclear Receptor Corepressor 1 (NCoR1), which interacts with the hinge region of TRα (the region between the DNA-binding domain and LBD) has a higher affinity for RTHα mutants compared to wild-type TRα, supporting the hypothesis that helix 12 of the LBD also functions to disassociate NCoR1 from TRα when it is bound to T3 (2). We proposed that this increased affinity for NCoR1 alters the mobility of TRα in the nucleus, impacting its function. Here, we show that NCoR1 has slower FRAP recovery kinetics compared with TRα, and we evaluate the intranuclear dynamics of RTH TRα1 mutants ΤR-E403X, Ala382ProfsX7, and F397fs406X in response to increased levels of NCoR1. Investigation of the effects of overexpression of NCoR1 will provide further insight into the impact of altered binding affinity for NCoR1 on the intranuclear dynamics of RTHα mutants.

SAT-434 Phenotype and Genotype Analysis of Patients with Resistance to Thyroid Hormone β: A Single-Center Experience

Introduction Resistance to thyroid hormone β (RTHβ) is caused by mutations in THRB, the gene that encodes thyroid hormone receptor β. The clinical phenotype is variable and may include goiter, tachycardia, and learning disability with or without hyperactive behavior. The biochemical hallmark of RTHβ is elevated T4 and T3 with non-suppressed TSH concentrations. We here describe the phenotype and genotype of three Thai patients diagnosed with RTHβ in a pediatric referral center. Patients had previously been misdiagnosed and inappropriately treated with antithyroid drugs (ATDs). Methods Clinical features and thyroid function tests (TFTs) of three unrelated RTHβ patients were retrospectively reviewed. Genomic DNA of the RTHβ patients and affected family members was amplified for exon 7-10 of the THRB gene and sequenced to identify mutation by Sanger sequencing. The impact of the p.L341V novel mutation on the affinity for T3 and T3-induced transcriptional activity was previously determined in vitro. Results Three female patients were diagnosed with RTHβ. All of them had been misdiagnosed with hyperthyroidism and treated with ATDs prior to referral. The mean age at diagnosis was 8 years. The main presenting symptoms were diffuse goiter and tachycardia. The mean duration of ATD treatment was 3 years. During the treatment, patients had fluctuating thyroid hormone and increased TSH levels. An older sister and mother of one patient also had similar TFTs abnormalities, for which the mother had undergone a subtotal thyroidectomy. RTHβ was diagnosed based on the high FT3 and FT4 with normal (non-suppressed) TSH concentrations and confirmed by mutation analysis. Anti-thyroid peroxidase, anti-thyroglobulin, and TSH receptor antibody (TRAb) were negative, excluding autoimmune thyroid disease. Heterozygous missense mutations of the THRB gene were identified in all patients and affected family members. Two mutations had been previously reported (p.R243W and p.L456F), and one mutation was novel (p.L341V). In vitro studies confirmed an important role of Leu341 in T3 binding of the TRβ and functional impairment of the p.L341V novel mutation and were reported separately. According to available literature, only nine Thai RTHβ patients (in three families) carrying three different mutations (p.G251V, p.M313T, and p.A317T) had been previously reported. Goiter was the most common clinical finding, and almost all patients had a history of receiving unnecessary treatment with ATDs. Conclusion We report a series of RTHβ patients carrying THRB gene mutations, including one novel mutation (p.L341V). Clinicians should be alert that RTHβ can be found in patients with goiter and tachycardia. Elevated T4 and T3 with non-suppressed TSH concentration is the main diagnostic clue for this disease. Mutation analysis allows definitive diagnosis of RTHβ and may help to avoid potential misdiagnosis and improper treatment.

OR01-01 Human Type 1 Iodothyronine Deiodinase (DIO1) Mutations Cause Abnormal Thyroid Hormone Metabolism

Iodothyronine deiodinases (Ds) mediate thyroid hormone (TH) action. They catalyze triiodothyronine (T3) production and degradation via, respectively; outer (ORD) and inner (IRD) ring deiodination. Type 1 D (D1) has a relatively high Km for substrates thyroxine (T4) and reverse T3 (rT3), catalyzes both ORD and IRD and is inhibited by propylthiouracil (PTU). Although no mutations in DIO1 gene have been reported so far in humans, based on the D1-deficient C3H mouse and the heterozygous Dio1 knockout mice, the expected phenotype of D1 loss-of-function is higher serum rT3 and slightly elevated T4. Our objective was to identify new gene defects causing unexplained and discrepant thyroid function tests using whole-exome sequencing (WES). Exons and splice sites are captured with Agilent SureSelectXT and sequenced in Illumina HiSeq2500. Data are analyzed with BWA, GATK, and ANNOVAR applications for alignment, variant calling and annotation, respectively. Sanger sequencing is used to confirm and segregate WES variants in families. A heterozygous pathogenic missense variant in the DIO1 gene (c.282C>A:p.N94K; N-linker) was identified in four family members with relatively higher serum rT3, T4, and free T4 than unaffected relatives, and normal TSH. We identified a second heterozygous pathogenic mutation in the DIO1 (c.603G>A:p.M201I; thioredoxin-fold) in a second family. Two affected individuals presented slightly elevated TSH, higher serum rT3, normal T4 levels, while the T3/T4 ratio was lower compared to unaffected members. To assess the functional activity of the mutant D1 protein, human embryonic kidney epithelial cells (HEK293) were transiently co-transfected with pcDNA3 plasmids expressing pD1-WT, pD1-N94K or pD1-M201I, and pGFP as transfection control. In assays performed with 1μM 125I-T4, the catalytic activities of pD1-N94K and pD1-M201I were 44.7% and 54.1% lower as compared to pD1-WT, respectively. To study the enzyme kinetics, the D1 assay was repeated in the presence of increasing 125I-T4 concentrations (0.1-20μM). The enzymatic activity assays revealed similar Vmax for pD1-N94K and pD1-M201I mutants compared to pD1-WT (VmaxpD1-N94K=53.7 vs VmaxpD1-WT=40.9 and VmaxpD1-M201I=58.8 vs VmaxpD1-WT=42), but higher Michaelis constant (Km) than pD1-WT (KmpD1-N94K=16.4 vs KmpD1-WT=6 and KmpD1-M201I=21.4 vs KmpD1-WT=6.9), which demonstrates a reduced affinity for T4. In conclusion, we report the first DIO1 mutations in humans with serum thyroid tests suggestive of TH metabolism defects. These mutations affect the catalytic activity of the D1, demonstrating impaired functional activity of the mutant enzymes and consequently altering TH metabolism.

The In Vitro Functional Impairment of Thyroid Hormone Receptor Alpha 1 Isoform Mutants Is Mainly Dictated by Reduced Ligand Sensitivity.

Background: Thyroid hormone (TH) acts on TH receptors (TRs) and regulates gene transcription by binding of TRs to TH response elements (TREs) in target gene promoters. The transcriptional activity of TRs is modulated by interactions with TR-coregulatory proteins. Mutations in TRα cause resistance to thyroid hormone alpha (RTHα). In this study, we analyzed if, beyond reduced triiodothyronine (T3) affinity, altered interactions with cofactors or different TREs could account for the differential impaired transcriptional activity of different mutants. Methods: We evaluated four mutants derived from patients (D211G, M256T, A263S, and R384H) and three artificial mutants at equivalent positions in patients with RTHβ (T223A, L287V, and P398H). The in vitro transcriptional activity was evaluated on TRE-luciferase reporters (DR4, IR0, and ER6). The affinity for T3 and interaction with coregulatory proteins (nuclear receptor corepressor 1 [NCoR1] and steroid receptor coactivator 1 [SRC1]) were also determined. Results: We found that the affinity for T3 was significantly reduced for all mutants, except for TRα1-T223A. The reduction in the T3 sensitivity of the transcriptional activity on three TREs, the dissociation of the corepressor NCoR1, and the association of the coactivator SRC1 recruitment for each mutant correlated with the reduced affinity for T3. We did not observe mutation-specific alterations in interactions with cofactors or TREs. Conclusions: In summary, the degree of impaired transcriptional activity of mutants is mainly determined by their reduced affinity for T3.

Relationship of TSH with BMI in subclinical hypothyroid patients

Thyroid hormones (T3 and T4) synthesized by thyroid gland and play an essential role in normal body development and tissue metabolism. Iodine is an essential factor that is required for the synthesis of thyroid hormones. The secretion of thyroid hormones is regulated by Negative feedback mechanism. After reached in blood circulation the majority of T4 and T3 bound with carrier proteins called thyroxine binding proteins (TBG) which shows higher affinity for T4. The common diseases related to thyroid hormones include Hypothyroidism, hyperthyroidism, thyroiditis, goiter, nodes, and tumors of thyroid glands (benign and malignant). Subclinical hypothyroidism also called mild thyroid failure is defined as normal thyroid hormones but mildly elevated TSH (5 – 10 uIU/ml) with no or mild sign and symptoms. The thyroid disorders are more prevalent in females than males. Hypothyroidism is associated with obesity and various studies conducted to find out the relationship between TSH and BMI observed different finding in the particular study. In the present study, a poor positive correlation between TSH and BMI was observed in the total euthyroid subjects. A poor negative association between TSH and BMI in euthyroid males and poor positive (r value – 0.21) relationship was observed in euthyroid females. In Subclinical hypothyroid patients (total, male and female), Poor negative correlation was observed between TSH and BMI. The conclusion of this study is, as TSH increased the BMI will also increase in (mostly females) euthyroid subjects. The Inverse or poor negative correlation was observed within TSH and BMI among euthyroid males and an inverse correlation was noticed in patients with subclinical hypothyroidism.

Structure and specificity of several triclocarban-binding single domain camelid antibody fragments.

The variable VHH domains of camelid single chain antibodies have been useful in numerous biotechnology applications due to their simplicity, biophysical properties, and abilities to bind to their cognate antigens with high affinities and specificity. Their interactions with proteins have been well-studied, but considerably less work has been done to characterize their ability to bind haptens. A high-resolution structural study of three nanobodies (T4, T9, and T10) which have been shown to bind triclocarban (TCC, 3-(4-chlorophenyl)-1-(3,4-dichlorophenyl)urea) with near-nanomolar affinity shows that binding occurs in a tunnel largely formed by CDR1 rather than a surface or lateral binding mode seen in other nanobody-hapten interactions. Additional significant interactions are formed with a non-hypervariable loop, sometimes dubbed "CDR4". A comparison of apo and holo forms of T9 and T10 shows that the binding site undergoes little conformational change upon binding of TCC. Structures of three nanobody-TCC complexes demonstrated there was not a standard binding mode. T4 and T9 have a high degree of sequence identity and bind the hapten in a nearly identical manner, while the more divergent T10 binds TCC in a slightly displaced orientation with the urea moiety rotated approximately 180° along the long axis of the molecule. In addition to methotrexate, this is the second report of haptens binding in a tunnel formed by CDR1, suggesting that compounds with similar hydrophobicity and shape could be recognized by nanobodies in analogous fashion. Structure-guided mutations failed to improve binding affinity for T4 and T9 underscoring the high degree of natural optimization.

Substrate-Induced Formation of Ribosomal Decoding Center for Accurate and Rapid Genetic Code Translation.

Accurate translation of genetic information is crucial for synthesis of functional proteins in all organisms. We use recent experimental data to discuss how induced fit affects accuracy of initial codon selection on the ribosome by aminoacyl transfer RNA in ternary complex ( T3) with elongation factor Tu (EF-Tu) and guanosine-5'-triphosphate (GTP). We define actual accuracy ([Formula: see text]) of a particular protein synthesis system as its current accuracy and the effective selectivity ([Formula: see text]) as [Formula: see text] in the limit of zero ribosomal binding affinity for T3. Intrinsic selectivity ([Formula: see text]), defined as the upper thermodynamic limit of [Formula: see text], is determined by the free energy difference between near-cognate and cognate T3 in the pre-GTP hydrolysis state on the ribosome. [Formula: see text] is much larger than [Formula: see text], suggesting the possibility of a considerable increase in [Formula: see text] and [Formula: see text] at negligible kinetic cost. Induced fit increases [Formula: see text] and [Formula: see text] without affecting [Formula: see text], and aminoglycoside antibiotics reduce [Formula: see text] and [Formula: see text] at unaltered [Formula: see text].

Clinical, Genetic, and Protein Structural Aspects of Familial Dysalbuminemic Hyperthyroxinemia and Hypertriiodothyroninemia.

Familial dysalbuminemic hyperthyroxinemia (FDH-T4) and hypertriiodothyroninemia (FDH-T3) are dominantly inherited syndromes characterized by a high concentration of thyroid hormone in the blood stream. The syndromes do not cause disease, because the concentration of free hormone is normal, but affected individuals are at risk of erroneous treatment. FDH-T4 is the most common cause of euthyroid hyperthyroxinemia in Caucasian populations in which its prevalence is about 1 in 10,000 individuals, but the prevalence can be much higher in some ethnic groups. The condition is caused by a genetic variant of human serum albumin (HSA); Arg218 is mutated to histidine, proline, or serine or Arg222 is changed to isoleucine. The disorder is characterized by greater elevation in serum l-thyroxine (T4) than in serum triiodothyronine (T3); T4 can be increased by a factor 8–15. The high serum concentration of T4 is due to modification of a binding site located in the N-terminal half of HSA (in subdomain IIA). Thus, mutating Arg218 or Arg222 for a smaller amino acid reduces the steric restrictions in the site and creates a high-affinity binding site. The mutations can also affect binding of other ligands and can perhaps cause modified pharmacokinetics of albumin-binding drugs. In normal HSA, the high-affinity site has another location (in subdomain IIIB). Different locations of these sites imply that persons with and without FDH-T4 can have different types of interactions, and thereby complications, when given albumin-binding drugs. FDH-T3 is caused by a leucine to proline mutation in position 66 of HSA, which results in a large increment of the binding affinity for T3 but not for T4. For avoiding unwanted treatment of euthyroid persons with hyperthyroxinemia or hypertriiodothyroninemia, protein sequencing and/or sequencing of the albumin gene should be performed.

Synergistic Anticancer Effect of Tocotrienol Combined with Chemotherapeutic Agents or Dietary Components: A Review.

Tocotrienol (T3), unsaturated vitamin E, is gaining a lot of attention owing to its potent anticancer effect, since its efficacy is much greater than that of tocopherol (Toc). Various factors are known to be involved in such antitumor action, including cell cycle arrest, apoptosis induction, antiangiogenesis, anti-metastasis, nuclear factor-κB suppression, and telomerase inhibition. Owing to a difference in the affinity of T3 and Toc for the α-tocopherol transfer protein, the bioavailability of orally ingested T3 is lower than that of Toc. Furthermore, cellular uptake of T3 is interrupted by coadministration of α-Toc in vitro and in vivo. Based on this, several studies are in progress to screen for molecules that can synergize with T3 in order to augment its potency. Combinations of T3 with chemotherapeutic drugs (e.g., statins, celecoxib, and gefitinib) or dietary components (e.g., polyphenols, sesamin, and ferulic acid) exhibit synergistic actions on cancer cell growth and signaling pathways. In this review, we summarize the current status of synergistic effects of T3 and an array of agents on cancer cells, and discuss their molecular mechanisms of action. These combination strategies would encourage further investigation and application in cancer prevention and therapy.

3,5-diiodo-L-thyronine (t2) in dietary supplements: what are the physiological effects?

3,5-diiodo-L-thyronine (t2) in dietary supplements: what are the physiological effects?

A Novel Albumin Gene Mutation (R222I) in Familial Dysalbuminemic Hyperthyroxinemia

Context:Familial dysalbuminemic hyperthyroxinemia, characterized by abnormal circulating albumin with increased T4 affinity, causes artefactual elevation of free T4 concentrations in euthyroid individuals.Objective:Four unrelated index cases with discordant thyroid function tests in different assay platforms were investigated.Design and Results:Laboratory biochemical assessment, radiolabeled T4 binding studies, and ALB sequencing were undertaken. 125I-T4 binding to both serum and albumin in affected individuals was markedly increased, comparable with known familial dysalbuminemic hyperthyroxinemia cases. Sequencing showed heterozygosity for a novel ALB mutation (arginine to isoleucine at codon 222, R222I) in all four cases and segregation of the genetic defect with abnormal biochemical phenotype in one family. Molecular modeling indicates that arginine 222 is located within a high-affinity T4 binding site in albumin, with substitution by isoleucine, which has a smaller side chain predicted to reduce steric hindrance, thereby facilitating T4 and rT3 binding. When tested in current immunoassays, serum free T4 values from R222I heterozygotes were more measurably abnormal in one-step vs two-step assay architectures. Total rT3 measurements were also abnormally elevated.Conclusions:A novel mutation (R222I) in the ALB gene mediates dominantly inherited dysalbuminemic hyperthyroxinemia. Susceptibility of current free T4 immunoassays to interference by this mutant albumin suggests likely future identification of individuals with this variant binding protein.

The allosteric modulation of thyroxine-binding globulin affinity is entropy driven

BackgroundThyroxine-binding globulin (TBG) is a non-inhibitory member of the serpin family of proteins whose main structural element is the reactive center loop (RCL), that, upon cleavage by proteases, is inserted into the protein core adopting a β-strand conformation (stressed to relaxed transition, S-to-R). After S-to-R transition thyroxine (T4) affinity decreases. However, crystallographic studies in the presence or absence of the hormone in different states are unable to show significant differences in the structure and interactions of the binding site. Experimental results also suggest the existence of several S states (differing in the number of inserted RCL residues), associated with a differential affinity. MethodsTo shed light into the molecular basis that regulates T4 affinity according to the degree of RCL insertion in TBG, we performed extended molecular dynamics simulations combined with several thermodynamic analysis of the T4 binding to TBG in three different S states, and in the R state. ResultsOur results show that, despite T4 binding in the protein by similar interactions in all states, a good correlation between the degree of RCL insertion and the binding affinity, driven by a change in TBG conformational entropy, was observed. ConclusionTBG allosteric regulation is entropy driven. The presence of multiple S states may allow more efficient T4 release due to protease activity. General significanceThe presented results are clear examples of how computer simulation methods can reveal the thermodynamic basis of allosteric effects, and provide a general framework for understanding serpin allosteric affinity regulation.

Analysis of the Interactions between GABA(A) Receptors and T3 using Electrophysiology and Molecular Dynamics Simulations

Disorders of the thyroid cause a multitude of neurological dysfunctions including depression, anxiety, and psychosis. Thyroid hormones have been primarily thought to act via genomic mechanisms throughout the organism; however, recently another mechanism has been proposed for the adult brain. We hypothesize that the thyroid hormone triiodothyronine (T3) acts directly on GABA(A) receptors via a mechanism similar to that of neurosteroids. Previous electrophysiology experiments on expressed recombinant GABA(A) receptors demonstrated an inhibition of GABA responses in the presence of T3, and, at much higher concentrations of T3 alone, a direct stimulation of receptor activity. Current experiments are being conducted to investigate stereoselective effects and interactions with other compounds known to bind directly to the GABA(A) receptor. using a GABA(A) receptor homology model based on the crystal structure of the related Glutamate-gated chloride channel, atomic resolution molecular dynamics simulations were conducted to analyze the possible interaction of T3 and the GABA(A) receptor. In simulations, T3 is stable in binding sites in the transmembrane domain of the GABA(A)receptor in a region that is associated with the activation by neurosteroids. Alchemical free energy perturbation calculations are underway to test the affinity of T3 for the binding site in the GABA(A) receptor. Our results provide strong evidence supporting earlier experimental findings indicating a role of T3 in regulating the activity of GABA(A) receptors in brain, while bringing additional insight into both the molecular binding mode and the mechanism of modulation.

Reaction Mechanisms of the Multicopper Oxidase CueO from Escherichia coli Support Its Functional Role as a Cuprous Oxidase

CueO from Escherichia coli is a multicopper oxidase (MCO) involved in copper tolerance under aerobic conditions. It features four copper atoms that act as electron transfer (T1) and dioxygen reduction (T2, T3; trinuclear) sites. In addition, it displays a methionine-rich insert which includes a helix that blocks physical access to the T1 site and which provides an extra labile site T4 adjacent to the T1 center. This T4 site is required for CueO function. Like many MCOs, CueO exhibits phenol oxidase activity with broad substrate specificity. Maximal activity with model substrate 2,6-dimethoxyphenol required stoichiometric occupation of T4 by Cu(II) (notation: Cu(II)-CueO). This was achieved in Mops buffer which has little affinity for Cu(2+). However, pH buffers that bind or precipitate Cu(2+) (Tris, BisTris, and KPi) generated enzyme with a vacant T4 site (notation: square-CueO) which has no phenol oxidase activity. Addition of excess Cu(2+) effectively generated a Cu(2+) buffer and recovered the activity partially or completely, depending upon the specific pH buffer. This phenomenon allowed reliable estimation of the affinity of T4 for Cu(II): K(D) = 5.5 x 10(-9) M. CueO is involved in copper tolerance and has been suggested to be a cuprous oxidase. The anion [Cu(I)(Bca)(2)](3-) (Bca = bicinchoninate) acted as a novel chromophoric substrate. It is a robust reagent, being air-stable and having a Cu(I) affinity comparable to those of periplasmic Cu(I) binding proteins. The influences of pH buffer composition and of excess Cu(2+) on cuprous oxidation were diametrically opposite to those seen for phenol oxidation, suggesting that square-CueO, not Cu(II)-CueO, is the resting form of the cuprous oxidase. Steady-state kinetics demonstrated that the intact anion [Cu(I)(Bca)(2)](3-), not "free" Cu(+), is the substrate that donates Cu(I) directly to T4. The data did not follow classical Michaelis-Menten kinetics but could be fitted satisfactorily by an extension that considered the effect of free ligand Bca. The K(m) term consists of two components, allowing estimation of the transient affinity of T4 for Cu(I): K(D) = 1.3 x 10(-13) M. It may be concluded that Cu(I) carried by [Cu(I)(Bca)(2)](3-) is oxidized only upon complete transfer of Cu(I) to T4. The transfer is required to induce a negative shift in the copper reduction potential to allow oxidation and electron transfer to the T1 site. The results provide compelling evidence that CueO is a cuprous oxidase. The new approach will have significant application to the study of metallo-oxidase enzymes.

Knockdown of the Type 3 Iodothyronine Deiodinase (D3) Interacting Protein Peroxiredoxin 3 Decreases D3-Mediated Deiodination in Intact Cells

The type 3 iodothyronine deiodinase (D3) is the primary deiodinase that inactivates thyroid hormone. Immunoprecipitation of D3, followed by fluorescent two-dimensional difference gel electrophoresis and mass spectrometry, identified peroxiredoxin 3 (Prx3) as a D3-associated protein. This interaction was confirmed using reverse coimmunoprecipitation, in which pull-down of Prx3 resulted in D3 isolation, and by fluorescence resonance energy transfer between cyan fluorescent protein-D3 and yellow fluorescent protein-Prx3. Prx3 overexpression did not change D3 activity in transfected HEK 293 cells; however, Prx3 knockdown resulted in a 50% decrease in D3-mediated whole-cell deiodination. Notably, D3 activity of cell lysates with dithiothreitol as an exogenous reducing factor and D3 protein levels were not decreased with Prx3 knockdown, indicating that the observed reduction in whole-cell deiodination was not simply due to a decrease in D3 enzyme levels. Prx3 knockdown did not change D3's affinity for T3 because saturation of D3-mediated whole-cell deiodination occurred between 20 and 200 nm T3 both with and without Prx3. Furthermore, the decrease in D3 activity in whole cells was not attributable to nonspecific oxidative stress because pretreatment with the antioxidant N-acetyl cysteine did not reverse the effects of Prx3 knockdown. Thioredoxin, the cofactor needed for Prx3 regeneration, supported D3 microsomal activity; however, Prx3 knockdown did not change D3 activity in this system. In conclusion, knockdown of Prx3 decreases D3 activity in whole cells, whereas absolute levels of D3 are unchanged, consistent with Prx3 playing a rate-limiting role in the regeneration of the D3 enzyme.

Effect of methylxanthines on binding of L-triiodothyronine (T3) to cytosolic and nuclear extract receptors of human leukocytes.

The effect of 15 mM methylxanthines (theophylline, coffeine) and 15 mM papaverine on T3 binding in the cytosol or nuclear extract receptors of human leukocytes was investigated by charcoal-dextran assay. In the cytosol, the presence of all three compounds increases the apparent association constants (KA) without affecting the total number of binding sites (N). Binding capacity was practically the same, only coffeine slightly decreases the number of binding sites. In the nuclear extract receptor protein, only theophylline increases its apparent affinity for T3.

Hormone affinity and fibril formation of piscine transthyretin: The role of the N-terminal

Transthyretin (TTR) transports thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3) in the blood of vertebrates. TH-binding sites are highly conserved in vertebrate TTR, however, piscine TTR has a longer N-terminus which is thought to influence TH-binding affinity and may influence TTR stability. We produced recombinant wild type sea bream TTR (sbTTRWT) plus two mutants in which 6 (sbTTRM6) and 12 (sbTTRM12) N-terminal residues were removed. Ligand-binding studies revealed similar affinities for T3 (Kd=10.6+/-1.7nM) and T4 (Kd=9.8+/-0.97nM) binding to sbTTRWT. Affinity for THs was unaltered in sbTTRM12 but sbTTRM6 had poorer affinity for T4 (Kd=252.3+/-15.8nM) implying that some residues in the N-terminus can influence T4 binding. sbTTRM6 inhibited acid-mediated fibril formation in vitro as shown by fluorometric measurements using thioflavine T. In contrast, fibril formation by sbTTRM12 was significant, probably due to decreased stability of the tetramer. Such studies also suggested that sbTTRWT is more resistant to fibril formation than human TTR.

Change in structure of the N‐terminal region of transthyretin produces change in affinity of transthyretin to T4 and T3

The relationship between the structure of the N-terminal sequence of transthyretin (TTR) and the binding of thyroid hormone was studied. A recombinant human TTR and two derivatives of Crocodylus porosus TTRs, one with the N-terminal sequence replaced by that of human TTR (human/crocTTR), the other with the N-terminal segment removed (truncated crocTTR), were synthesized in Pichia pastoris. Subunit mass, native molecular weight, tetramer formation, cross-reactivity to TTR antibodies and binding to retinol-binding protein of these recombinant TTRs were similar to TTRs found in nature. Analysis of the binding affinity to thyroid hormones of recombinant human TTR showed a dissociation constant (Kd) for triiodothyronine (T3) of 53.26+/-3.97 nM and for thyroxine (T4) of 19.73+/-0.13 nM. These values are similar to those found for TTR purified from human serum, and gave a Kd T3/T4 ratio of 2.70. The affinity for T4 of human/crocTTR (Kd=22.75+/-1.89 nM) was higher than those of both human TTR and C. porosus TTR, but the affinity for T3 (Kd=5.40+/-0.25 nM) was similar to C. porosus TTR, giving a Kd T3/T4 ratio of 0.24. A similar affinity for both T3 (Kd=57.78+/-5.65 nM) and T4 (Kd=59.72+/-3.38 nM), with a Kd T3/T4 ratio of 0.97, was observed for truncated crocTTR. The obtained results strongly confirm the hypothesis that the unstructured N-terminal region of TTR critically influences the specificity and affinity of thyroid hormone binding to TTR.