Thyroid hormone binding by human serum prealbumin (TBPA). Electrophoretic studies of triiodothyronine-TBPA interaction

Transport of the Thyroid Hormone Carrier Protein Transthyretin into Human Epidermoid Cells

Purpose: Transthyretin (TTR) is a protein with a growing number of biological functions in addition to its well-established binding and circulatory transport of thyroxine, and indirect retinoid transport through interaction with retinol-binding protein. Misfolded and aggregated wild-type and mutant TTRs are involved in amyloid diseases. Several aspects of TTR pathology and physiology remain poorly understood. Receptor-mediated cellular transport of TTR has been described in a few cell types; and such studies suggest the possibility of different TTR receptors and endocytic pathways. Our main objective was to further understand the endocytic pathways for TTR.Methods: In the current study, analyses of TTR endocytic transport were performed in the human A431 cell line. The results of TTR uptake were compared with those of the iron-carrier protein transferrin (Tf, a common stardard for endocytosis studies) in the same cell types.Results: A comparison of TTR and Tf endocytosis suggested similar early, 5-10 min, accumulation kinetics. But at a later time, 30 min, TTR accumulation was 20-30% lower than that of Tf (p < .05), a result that suggests different post-endocytic fates for these two ligands. Through the use of multiple endocytosis inhibitors, biochemical evidence is provided for an internalization pathway that differs from the clathrin-mediated endocytosis of Tf.Conclusions: These results for A431 cells are compared with others reported for different cell types; and it is suggested that this same hormone carrier protein can transit into cells through multiple endocytic pathways.

Drivers of α-Sheet Formation in Transthyretin under Amyloidogenic Conditions

Amyloid diseases make up a set of fatal disorders in which proteins aggregate to form fibrils that deposit in tissues throughout the body. Amyloid-associated diseases are challenging to study because amyloid formation occurs on time scales that span several orders of magnitude and involve heterogeneous, interconverting protein conformations. The development of more effective technologies to diagnose and treat amyloid disease requires both a map of the conformations sampled during amyloidogenesis and an understanding of the molecular mechanisms that drive this process. In prior molecular dynamics simulations of amyloid proteins, we observed the formation of a nonstandard type of secondary structure, called α-sheet, that we proposed is associated with the pathogenic conformers in amyloid disease, the soluble oligomers. However, the detailed molecular interactions that drive the conversion to α-sheet remain elusive. Here we use molecular dynamics simulations to interrogate a critical event in transthyretin aggregation, the formation of aggregation-competent, monomeric species. We show that conformational changes in one of the two β-sheets in transthyretin enable solvent molecules and polar side chains to form electrostatic interactions with main-chain peptide groups to facilitate and modulate conversion to α-sheet secondary structure. Our results shed light on the early conformational changes that drive transthyretin toward the α-sheet structure associated with toxicity. Delineation of the molecular events that lead to aggregation at atomic resolution can aid strategies to target the early, critical toxic soluble oligomers.

Identification of autoantibodies against transthyretin for the screening and diagnosis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic, autoimmune, systemic and inflammatory rheumatic disease that leads to inflammation of the joints and surrounding tissues. Identification of novel protein(s) associated with severity of RA is a prerequisite for better understanding of pathogenesis of this disease that may also have potential to serve as novel biomarkers in the diagnosis of RA. Present study was undertaken to compare the amount of autoantigens and autoantibodies in the plasma of RA patients in comparison to healthy controls. Plasma samples were collected from the patients suffering from RA, Osteoarthritis (OA), Systemic lupus erythematosus (SLE) and healthy volunteers. The screening of plasma proteins were carried out using 2-dimensional gel electrophoresis followed by identification of differentially expressed protein by MALDI-TOF MS/MS. Among several differentially expressed proteins, transthyretin (TTR) has been identified as one of the protein that showed significantly up regulated expression in the plasma of RA patients. The results were further validated by Western blot analysis and ELISA. In comparison to OA synovium, an exclusive significantly high expression of TTR in RA has been validated through IHC, Western blotting and IEM studies. Most importantly, the increase in expression of TTR with the progression of severity of RA condition has been observed. The autoantibodies against TTR present in the RA plasma were identified using immunoprecipitation-Western methods. The significant production of autoantibodies was validated by ELISA and Western blot analysis using recombinant pure protein of TTR. Hence, these novel observations on increase in TTR expression with the increase in severity of RA conditions and significant production of autoantibodies against TTR clearly suggest that a systematic studies on the role TTR in the pathogenesis of RA is immediately required and TTR may be used as a serum diagnostic marker together with other biochemical parameters and clinical symptoms for RA screening and diagnosis.

Abnormal gamma globulin binding of thyroid hormones

Thyroid hormone levels were studied in a thyrotoxic patient, who was treated with propylthiouracil. He had heavily increased triiodothyronine concentrations, measured by radioimmunoassay, in spite of only mild clinical symptoms of thyrotoxicosis. A moderately increased serum triiodothyronine concentration was observed in another patient, who was euthyroid and who had recently recovered from subacute thyroiditis. By gel electrophoresis and precipitation tests with human anti-IgG and anti-IgA, a binding to the gamma globulins of both triiodothyronine and thyroxine was detected in patient 1, and of triiodothyronine in patient 2. Such abnormal binding may result in serious errors in the determination of thyroid hormone concentration by radioimmunoassay.

Retinoic acid inhibition of thyroxine binding to human transthyretin

All-trans retinoic acid is a potent inhibitor of [125I]-thyroxine (T4) binding to human erythrocyte membranes and can block the activation by thyroid hormone of erythrocyte Ca(2+)-ATPase [J. Biol. Chem. (1989) 264, 687-689]. In the present studies, retinoic acid was examined for its ability to displace thyroxine from binding sites on human transthyretin (TTR). Scatchard analysis of [125I]T4 binding to purified TTR, determined by equilibrium dialysis, revealed two classes of binding sites with association constants of 3.2 x 10(9) M-1 and 8.1 x 10(6) M-1. All-trans retinoic acid also displaced [125I]T4; 40% of the specifically bound [125I]T4 was displaced at a retinoic acid concentration of 2 x 10(-5) M. Analysis of the high affinity T4 binding site suggests that the Ka for retinoic acid to that site is approx. 10(7) M-1. 8-Anilinonaphthalene-1-sulfonate (ANS), a strongly fluorescing dye, binds to the thyroxine binding sites on TTR. T4 and 3,5,3'-L-triiodothyronine (T3) shifted the fluorescence emission maximum and intensity of an ANS-TTR solution toward the spectrum obtained from uncomplexed ANS. All-trans retinoic acid caused a similar shift in the emission spectrum of ANS, but was less potent than T4. Retinol failed to quench the emission intensity of the ANS-TTR complex, while 13-cis-retinoic acid was less effective than all-trans retinoic acid.

Thyroxine binding to human serum albumin immobilized on sepharose and effects of nonprotein albumin-binding plasma constituents

125I-thyroxine (125I-T4) binding to human serum albumin (HSA) covalently attached onto CNBr-activated Sepharose (HSA-Sepharose) was studied. 125I-T4 binding to HSA-Sepharose was rapid and saturable. Nonlinear curve-fitting analysis of binding isotherms revealed two classes of binding sites. The values of dissociation constants of high and low affinity sites were 2.19 +/- 0.53 x 10(-6) M and 2.69 +/- 0.78 x 10(-5) M, respectively. The number of binding sites of the high and the low affinity sites were 1.28 +/- 0.46 mol/mol and 23.5 +/- 9.7 mol/mol of HSA, respectively. Fatty acids and bilirubin competitively inhibited the high-affinity binding of 125I-T4 to HSA-Sepharose without affecting the low-affinity binding. 8-anilino-1-naphthalene sulfonic acid (ANS) inhibited the high affinity T4 binding via reduction of the binding capacity. Unlabeled T4 showed little inhibition of ANS binding to HSA, as measured by fluorescence intensity. These results suggest that ANS allosterically inhibits the high-affinity T4 binding to HSA-Sepharose.

Competition of milrinone, a non-iodinated cardiac inotropic agent, with thyroid hormone for binding sites on human serum prealbumin (TBPA)

Milrinone [2-methyl-5-cyano-(3,4'-bipyridin)-6(1H)-one] is a positive cardiac inotropic agent recently shown to have thyromimetic activity in vitro in a rabbit myocardial membrane Ca2+-ATPase system [K. M. Mylotte et al., Proc. natn. Acad. Sci. U.S.A. 82, 7974 (1985)]. In the present studies, milrinone was examined for activity as an inhibitor of iodothyronine binding by human serum thyroid hormone transport proteins, thyroxine-binding globulin (TBG), prealbumin (TBPA) and albumin. Polyacrylamide gel electrophoresis at pH 9.0 of sera equilibrated with [125I]thyroxine showed that milrinone competed with L-thyroxine (T4) for binding sites on TBPA (10 and 100 microM milrinone caused 61 and 73% reductions, respectively, in T4 binding to TBPA, P less than 0.01); T4 displaced from TBPA was bound by TBG and albumin. Comparable reductions in T4 binding to TBPA were observed in electrophoretic studies conducted at pH 7.4. Binding of triiodo-L-thyronine (T3) to TBPA was electrophoretically confirmed and shown to be decreased in the presence of milrinone. Electrophoresis of purified TBPA also demonstrated that [14C]milrinone co-migrated with this transport protein and that milrinone displaced tracer T4 from TBPA. Amrinone, the 2-H-5-NH2 analog of milrinone, had less than 5% of the activity of milrinone as an inhibitor of T4 binding in electrophoretic studies. Scatchard analysis of T4 and milrinone binding to purified TBPA, measured by equilibrium dialysis, showed two classes of binding sites, with association constants, respectively, of 6.1 X 10(7) M-1 and 1.6 X 10(6) M-1 for T4, and 1.7 X 10(6) M-1 and 8.9 X 10(2) M-1 for milrinone. Computer graphic modeling of the binding of milrinone to the T4 site in the crystal structure of TBPA showed that milrinone best occupied this site when the substituted bipyridine ring overlapped the phenolic ring of T4. In this orientation the 5-cyano group, which has an electronegativity similar to that of iodine, occupied the same volume as the 5'-iodine of T4. The 5-amino group of amrinone lacks these characteristics. In this orientation, the keto function of milrinone overlapped the T4 4'-hydroxyl and could participate in similar intermolecular interactions. Thus, milrinone, a non-iodinated bipyridine, and thyroid hormone share structural and biochemical homologies and compete for the same binding site on TBPA.

Milrinone and thyroid hormone stimulate myocardial membrane Ca2+-ATPase activity and share structural homologies

We have recently shown that thyroid hormone in physiological concentrations stimulates sarcolemma-enriched rabbit-myocardial-membrane Ca2+-ATPase in vitro. In this study, milrinone [2-methyl-5-cyano-(3,4'-bipyridin)-6(1H)-one], a cardiac inotropic agent, was thyromimetic in the same system. At clinically achievable concentrations (50-500 nM), milrinone significantly stimulated membrane Ca2+-ATPase in vitro. This action was antagonized by W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], an agent that also blocks thyroid hormone action on the Ca2+-ATPase, at concentrations as low as 5 microM. Progressive additions of milrinone to membranes incubated with a fixed concentration of thyroxine (0.10 nM) or triiodothyronine resulted in a progressive obliteration of the thyroid hormone effect on Ca2+-ATPase. Amrinone [5-amino-(3,4'-bipyridin)-6(1H)-one], the parent bipyridine of milrinone, had no effect on myocardial Ca2+-ATPase activity. X-ray crystallographic analysis of milrinone and amrinone revealed structural homologies between the phenolic ring of thyroxine and the substituted ring of milrinone, whereas amrinone did not share these homologies. The mechanism(s) of the inotropic actions of thyroxine and of milrinone is not clearly understood, but these observations implicate Ca2+-ATPase, a calcium pump-associated enzyme, as one mediator of the effects on the heart of these two compounds.

Inhibition by quercetin of thyroid hormone stimulation in vitro of human red blood cell Ca2+-ATPase activity

Human red blood cell membrane Ca2+-ATPase activity is stimulated in vitro by physiological concentrations of thyroid hormone. Quercetin, a flavonoid that inhibits several membrane-linked ATPases, suppressed thyroid hormone action on red cell Ca2+-ATPase activity and also interfered with binding of the hormone by red cell membranes. These effects of quercetin were dose-dependent over a range of concentrations (1-50 microM). In contrast, in the absence of thyroid hormone, quercetin at low concentrations stimulated Ca2+-ATPase activity and at 50 microM inhibited the enzyme. The effects of quercetin at low concentrations (1-10 microM), namely, stimulation of Ca2+-ATPase and inhibition of membrane-binding of thyroid hormone, mimic those of thyroid hormone and are consistent with the thyronine-like structure of quercetin. At high concentrations, quercetin is generally inhibitory of Ca2+-ATPase activity. Chalcone, fisetin, hesperetin and tangeretin are other flavonoids shown to reduce susceptibility of membrane Ca2+-ATPase to hormonal stimulation.

Identification and properties of myocardial myoglobin as a binder of iodothyronines

Gel filtration of dog myocardial cytosol previously incubated with [125I]T4 or [125I]T3 revealed hormone binding in three fractions, one of which, M-2, was presumptively identified as myoglobin by absorbance maximum, molecular weight and specific immunodiffusion. Gel chromatography of purified horse or dog myoglobins incubated with labeled T3 or T4 resulted in coelution of the myoglobin and iodothyronine peaks. Excess unlabeled thyroid hormone displaced no more than 25% of tracer bound to myoglobin. Acid-acetone fractionation of myoglobin into heme and globin, and subsequent precipitation of the heme, localized hormone binding to the heme moiety. Hematin (ferric state heme) in solution was also shown to bind thyroid hormone. Added to human sera which were then subjected to T3 or T4 radioimmunoassay, myoglobin reduced detectable, endogenous iodothyronine by 77 and 26%, respectively. The myoglobin effect was concentration dependent. Heart myoglobin, like hemoglobin in the erythrocyte, is a cytoplasmic heme protein responsible for a major fraction of binding of intracellular iodothyronine. The nature of the interaction between iodothyronines and the heme prosthetic group is unclear.

Studies on the mechanism of thyroid hormone stimulation in vitro of human red cell Ca2+-ATPase activity

The stimulation in vitro of human red blood cell Ca2+-ATPase activity by thyroxine (T4) and triiodothyronine (T3) in physiological concentrations is shown to depend upon binding of iodothyronines to red cell membranes. Calmodulin enhances the activity of thyroid hormone in this model system but there is no direct interaction of calmodulin and hormone.

Thyroid hormone autoantibodies in a case of Hashimoto's disease

A case of Hashimoto's disease is reported. The serum contained antibodies binding T3 and T4. The binding capacity and affinity constants for T3 were 0.03 mumol/l and 3.4 x 10(9) l/mol and for T4 0.3 mumol/l and 1.4 x 10(9) l/mol, respectively. After total thyroid lobectomy on the left and subtotal on the right the autoantibodies disappeared from the serum. Thyroid tissue from the patient proved to contain an antithyroglobulin. According to the findings, the T3- and T4-binding antibodies are directed against thyroglobulin. It is of clinical importance to bear these autoantibodies in mind as possible errors in measuring T3 and T4 by radioimmunoassay (RIA). If RIA determination gives unexpectedly high or low T3 and/or T4 values, testing of non-specific binding is recommended.

Binding of thyroid hormone by human erythrocyte cytosol proteins

Gel filtration (G-100, 0.01 M Tris, pH 7.4) of post-100,000 x g supernatant from lysate of washed human erythrocytes (RBC) revealed 3 fractions (R-1, R-2, R-3) which bound labeled T3 and T4. Major peak R-2 emerged with the mehoglobin fraction (A560 nm) and binding by this fraction was partially dissociable; the dissociable site bound D-T4, but not tetraidothyroacetic acid or reverse T3. Non-dissociable binding characterized peaks R-1 and R-3. R-1, R-2, and R-3 were pronase-digestible and R-1 binding was acid-unstable (pH 6.8 vs. 7.4). Evidence developed herein and elsewhere indicates that hemoglobin, itself, accounts for the binding within fraction R-2. Intact RBCs maintained for 72 hr at 4C in buffer enriched with T3 or T4 showed progressive incorporation with time of iodothyronines into the hemoglobin fraction.

A numerical comparison of the use of T3-uptake values and of TBG levels for the estimation of free thyroxine in serum

The correlation of the diagnostic indices FTI and T4/TBG with actual free T4 levels (FT4) was investigated by numerical analysis. It was shown that, FT4 being kept constant, both indices vary with altered TBG concentrations. Thus neither index is truly proportional to FT4. In this respect both indices appear doubtful tools for diagnosing patients with abnormal TBG levels. A "map" constructed by plotting T4 versus T3U appeared to offer a higher discriminatory potential with respect to pathologic FT4 values. Such an idealized map was shown to consist of hypo-, eu- and hyperthyroid domains separated by lines governed by the limit values of the FT4 normal range. In practice, of course, these domains are separated by border regions rather than border lines and they should be derived from clinical data.

A new theoretical description of the binding of thyroid hormones by serum proteins

A theoretical model is proposed which describes the binding of thyroid hormones to serum proteins in terms of easily determined parameters, Not only free hormone concentration but also the distribution of hormone among various binding sites may be computed. The mathematical approach is capable of dealing with models of differing complexity of binding from simple 'one binding site per protein molecule' systems to those involving 'negative co-operativity'. The approach gives predictions of thyroid function parameters which are in good agreement with those observed in practice.

The influence of thyroxine on the serum free triiodothyronine concentration

By means of measurement of the serum free triiodothyronine fraction (%FT3) by equilibrium dialysis, the influence of varying serum concentrations of thyroxine (T4) on %FT3 was studied in conditions associated with lack or excess of serum T4 and in serum enrichment with T4 in vitro. The enrichment of pooled serum with T4 caused significant increases of %FT3 with T4 was added to elevate its T4 content between 10 and 25 mug/dl. In 4 subjects with primary hypothyroidism treated with T3 (25 mug once daily for two weeks, twice daily thereafter), progressive increases of serum T3 were observed while their T4, free T4 and T3 fractions remained unchanged. Consequently, the serum free T3 and thyrotropin concentrations became normal after the serum T3 concentrations became hypernormal. When comparisons of the serum thyroid hormone levels were made between 6 subjects with T3 toxicosis and 8 with a usual variety of hyperthyroidism matched for serum T3 concentration, the former sub-group of hyperthyroidism showed significantly lower serum free T3 concentration (0-51 +/- 0.22 ng/dl versus 0.79 +/- 0-21 ng/dl, p less than 0.05). The amount of T4 in serum is considered to affect its free T3 concentration by virtue of sharing serum-binding proteins.

Discrepancy between the measurement of thyroxine-binding prealbumin plasma level and binding capacity in protein-calorie malnutrition

The measurement of thyroxine-bindung prealbumin (TBPA) in protein-calorie malnutrition by two different techniques leads to the recognition of an unexpected discrepancy. Whereas TBPA plasma levels as measured by immunodiffusion are markedly decreased to 29.3% of the normal, those recorded by maximal binding capacity (TBPAcap) are characterized by a wide dispersion. In the control group, the ratio between TBPAcap and TBPA plasma level is 9.1. In the malnourished group on admission, the same ratio is 30.0. The possibility of a qualitative effect in the tetrameric TBPA structure, with the binding of additional thyroxine (T4) molecules on the secondary binding sites, has been investigated. This hypothesis has been discarded by Scatchard plot studies. The normal 1:1 molar ratio between TBPA and T4 is unaffected in protein-deficient patients. Discrepant results obtained for TBPAcap and TBPA levels appear to be the consequence of low plasma protein levels, leading to an artifact in the electrophoretic method.

In vitro testing of thyroid function: a review

Triiodothyronine (T(3)) is the major thyroid hormone and thyroxine (T(4)) may be only a "prohormone". A normal serum T(3) concentration can compensate for a low serum T(4) to maintain euthyroidism and on the other hand hyperthyroidism can exist in spite of a normal T(4) if the T(3) concentration is increased ("T(3)-toxicosis"). A raised serum thyroid stimulating hormone (TSH) concentration is the present most sensitive indicator of thyroidal hypothyroidism and the level can be used to titrate replacement therapy to the individual's own requirements. TSH concentration is classically low in hypothyroidism secondary to pituitary or to hypothalamic disorder and synthetic thyrotrophin release hormone can then be used to identify which of these two sites is at fault. Thyroxine is the best form of thyroid replacement for hypothyroidism because it produces more consistently physiological concentrations of T(3). Full replacement is achieved with 0.1 - 0.2 mg of T(4)/day and doses above this, as formerly widely used, may cause over-replacement. New reliable kit tests are available which give in one quick procedure a measure of free-thyroxine even in the presence of abnormalities of protein-binding. These kit tests are suitable for the routine screening of the whole spectrum of thyroid dysfunction and when combined, in appropriate instances, with radioimmunoassay procedures for serum T(3) and TSH, provide a battery of tests which will help in the diagnosis of the great majority of causes of thyroid dysfunction.