Variant thyroxine-binding globulin in serum of Australian aborigines: its physical, chemical and biological properties

Spatially Dependent Tissue Distribution of Thyroid Hormones by Plasma Thyroid Hormone Binding Proteins

Plasma thyroid hormone (TH) binding proteins (THBPs), including thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin (ALB), carry THs to extrathyroidal sites, where THs are unloaded locally and then taken up via membrane transporters into the tissue proper. The respective roles of THBPs in supplying THs for tissue uptake are not completely understood. To investigate this, we developed a spatial human physiologically based kinetic (PBK) model of THs, which produces several novel findings. (1) Contrary to postulations that TTR and/or ALB are the major local T4 contributors, the three THBPs may unload comparable amounts of T4 in Liver, a rapidly perfused organ; however, their contributions in slowly perfused tissues follow the order of abundances of T4TBG, T4TTR, and T4ALB. The T3 amounts unloaded from or loaded onto THBPs in a tissue acting as a T3 sink or source respectively follow the order of abundance of T3TBG, T3ALB, and T3TTR regardless of perfusion rate. (2) Any THBP alone is sufficient to maintain spatially uniform TH tissue distributions. (3) The TH amounts unloaded by each THBP species are spatially dependent and nonlinear in a tissue, with ALB being the dominant contributor near the arterial end but conceding to TBG near the venous end. (4) Spatial gradients of TH transporters and metabolic enzymes may modulate these contributions, producing spatially invariant or heterogeneous TH tissue concentrations depending on whether the blood-tissue TH exchange operates in near-equilibrium mode. In summary, our modeling provides novel insights into the differential roles of THBPs in local TH tissue distribution.

A minimal human physiologically based kinetic model of thyroid hormones and chemical disruption of plasma thyroid hormone binding proteins

The thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3), are under homeostatic control by the hypothalamic-pituitary-thyroid axis and plasma TH binding proteins (THBPs), including thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin (ALB). THBPs buffer free THs against transient perturbations and distribute THs to tissues. TH binding to THBPs can be perturbed by structurally similar endocrine-disrupting chemicals (EDCs), yet their impact on circulating THs and health risks are unclear. In the present study, we constructed a human physiologically based kinetic (PBK) model of THs and explored the potential effects of THBP-binding EDCs. The model describes the production, distribution, and metabolism of T4 and T3 in the Body Blood, Thyroid, Liver, and Rest-of-Body (RB) compartments, with explicit consideration of the reversible binding between plasma THs and THBPs. Rigorously parameterized based on literature data, the model recapitulates key quantitative TH kinetic characteristics, including free, THBP-bound, and total T4 and T3 concentrations, TH productions, distributions, metabolisms, clearance, and half-lives. Moreover, the model produces several novel findings. (1) The blood-tissue TH exchanges are fast and nearly at equilibrium especially for T4, providing intrinsic robustness against local metabolic perturbations. (2) Tissue influx is limiting for transient tissue uptake of THs when THBPs are present. (3) Continuous exposure to THBP-binding EDCs does not alter the steady-state levels of THs, while intermittent daily exposure to rapidly metabolized TBG-binding EDCs can cause much greater disruptions to plasma and tissue THs. In summary, the PBK model provides novel insights into TH kinetics and the homeostatic roles of THBPs against thyroid disrupting chemicals.

Role of thyroid hormones in normal and abnormal central nervous system myelination in humans and rodents

Thyroid hormones (THs) are instrumental in promoting the molecular mechanisms which underlie the complex nature of neural development and function within the central nervous system (CNS) in vertebrates. The key neurodevelopmental process of myelination is conserved between humans and rodents, of which both experience peak fetal TH concentrations concomitant with onset of myelination. The importance of supplying adequate levels of THs to the myelin producing cells, the oligodendrocytes, for promoting their maturation is crucial for proper neural function. In this review we examine the key TH distributor and transport proteins, including transthyretin (TTR) and monocarboxylate transporter 8 (MCT8), essential for supporting proper oligodendrocyte and myelin health; and discuss disorders with impaired TH signalling in relation to abnormal CNS myelination in humans and rodents. Furthermore, we explore the importance of using novel TH analogues in the treatment of myelination disorders associated with abnormal TH signalling.

Clinical recognition and evaluation of patients with inherited serum thyroid hormone-binding protein mutations

There are three important thyroid hormone-binding proteins in human serum, thyroxine-binding globulin, transthyretin, and albumin. Genetic variation in these proteins can lead to altered thyroid hormone binding and abnormalities in serum tests of thyroid hormone. Importantly, patients harboring these mutations are euthyroid; thus, the recognition of these conditions is crucial to prevent unnecessary repeated testing and treatment. This article provides an updated overview of serum thyroid hormone transport biology and reviews the underlying genetic alterations, clinical presentation, and appropriate evaluation of patients with suspected mutations in serum thyroid hormone-binding proteins.

Thyroid Hormone Distributor Proteins During Development in Vertebrates

Thyroid hormones (THs) are ancient hormones that not only influence the growth, development and metabolism of vertebrates but also affect the metabolism of (at least some) bacteria. Synthesized in the thyroid gland (or follicular cells in fish not having a discrete thyroid gland), THs can act on target cells by genomic or non-genomic mechanisms. Either way, THs need to get from their site of synthesis to their target cells throughout the body. Despite being amphipathic in structure, THs are lipophilic and hence do not freely diffuse in the aqueous environments of blood or cerebrospinal fluid (in contrast to hydrophilic hormones). TH Distributor Proteins (THDPs) have evolved to enable the efficient distribution of THs in the blood and cerebrospinal fluid. In humans, the THDPs are albumin, transthyretin (TTR), and thyroxine-binding globulin (TBG). These three proteins have distinct patterns of regulation in both ontogeny and phylogeny. During development, an additional THDP with higher affinity than those in the adult, is present during the stage of peak TH concentrations in blood. Although TTR is the only THDP synthesized in the central nervous system (CNS), all THDPs from blood are present in the CSF (for each species). However, the ratio of albumin to TTR differs in the CSF compared to the blood. Humans lacking albumin or TBG have been reported and can be asymptomatic, however a human lacking TTR has not been documented. Conversely, there are many diseases either caused by TTR or that have altered levels of TTR in the blood or CSF associated with them. The first world-wide RNAi therapy has just been approved for TTR amyloidosis.

Temperature-responsive release of thyroxine and its environmental adaptation in Australians

The hormone thyroxine that regulates mammalian metabolism is carried and stored in the blood by thyroxine-binding globulin (TBG). We demonstrate here that the release of thyroxine from TBG occurs by a temperature-sensitive mechanism and show how this will provide a homoeostatic adjustment of the concentration of thyroxine to match metabolic needs, as with the hypothermia and torpor of small animals. In humans, a rise in temperature, as in infections, will trigger an accelerated release of thyroxine, resulting in a predictable 23% increase in the concentration of free thyroxine at 39°C. The in vivo relevance of this fever-response is affirmed in an environmental adaptation in aboriginal Australians. We show how two mutations incorporated in their TBG interact in a way that will halve the surge in thyroxine release, and hence the boost in metabolic rate that would otherwise occur as body temperatures exceed 37°C. The overall findings open insights into physiological changes that accompany variations in body temperature, as notably in fevers.

C-terminal amino acid alteration rather than late termination causes complete deficiency of thyroxine-binding globulin CD-NeuIsenburg

CONTEXT

T(4)-binding globulin (TBG) is the main transport protein for T(4) in blood and a member of the superfamily of serine proteinase inhibitors. So far, 14 mutations leading to familial complete TBG deficiency have been reported. Eleven of these are caused by mutations leading to truncation of the molecule, and three are caused by single amino acid substitutions.

OBJECTIVE

We report and study the complete deficiency TBG variant found in a patient from NeuIsenburg, Germany (TBG-CDNI).

METHODS

Direct DNA sequencing was used to identify the TBG-CDNI mutation in the propositus, which was confirmed by allele-specific amplification. Site-directed mutagenesis and expression in Xenopus oocytes was used to study the secretion defect of TBG-CDNI and several variants by Western blot and T(4)-binding assay.

RESULTS

The deletion of two nucleotides in codon 384 (1211_1212delTC) causes a frameshift altering the last 11 residues, introduces a new glycosylation site, and elongates the molecule by seven new amino acids. In contrast to normal TBG, TBG-CDNI was not secreted by Xenopus oocytes. Elongation of normal TBG by seven alanines did not affect its secretion or binding properties. On the other hand, neither disruption of its new glycosylation site nor termination of TBG-CDNI at the normal length repaired its secretion defect.

CONCLUSIONS

In this first late termination variant of complete TBG deficiency, alteration of beta-strand 5B, located in the core of the molecule, rather than elongation of the molecule or introduction of a new glycosylation site, suffices to disrupt secretion of TBG-CDNI.

A variant of porcine thyroxine-binding globulin has reduced affinity for thyroxine and is associated with testis size

The field of genomics applies the dissection of genetic differences toward an understanding of the biology of complex traits. Quantitative trait loci (QTL) for testis size, plasma FSH in boars, and body composition (backfat) have been identified near the centromere on the X chromosome in a Meishan-White Composite resource population. Since thyroid function affects Sertoli cell development and adult testis size in rodents, and thyroxine-binding globulin (TBG) maps to this region on the porcine X chromosome, TBG was a positional candidate gene for testis size. We discovered a polymorphism in exon 2 of the porcine TBG gene that results in an amino acid change of the consensus histidine to an asparagine. This single nucleotide polymorphism (SNP) resides in the ligand-binding domain of the mature polypeptide, and the Meishan allele is the conserved allele found in human, bovine, sheep, and rodent TBG. Binding studies indicate altered binding characteristics of the allelic variants of TBG with the asparagine (White Composite) isoform having significantly greater affinity for thyroxine than the histidine (Meishan) isoform. Alternate alleles in boars from the resource population are also significantly associated with testis weight. Therefore, this polymorphism in TBG is a candidate for the causative variation affecting testis size in boars.

Loop variants of the serpin thyroxine-binding globulin: implications for hormone release upon limited proteolysis

Thyroxine-binding globulin (TBG) and corticosteroid-binding globulin are unique among non-inhibitory members of the superfamily of serine-proteinase inhibitors (serpins) in undergoing a dramatic increase in stability [stressed-to-relaxed (S-->R) transition] after proteolytic cleavage within their exposed reactive-site-loop (RSL) equivalent. This structural rearrangement involves the insertion of the cleaved loop as a new strand into the beta-sheet A and is accompanied by a decrease in hormone binding. To define the mechanism that leads to disruption of hormone binding of TBG after proteolytic cleavage, the effect of partial loop deletions and replacements by the alpha(1)-proteinase inhibitor homologues of TBG were evaluated. Unexpectedly, deletion of the loop's C-terminus, thought to be important for thyroxine binding, improved the binding affinity over that of normal TBG. Proteolytic cleavage of this variant revealed an intact S-->R transition and reduced its binding activity to that of cleaved TBG. In contrast, a chimaera with C-terminal loop extension mimicked the decreased binding affinity of cleaved TBG and had a thermal stability intermediate between that of native and cleaved serpins. This variant was still susceptible to loop cleavage and underwent an S-->R transition, yet without changing its binding affinity. Our data exclude a direct involvement of loop residues in thyroxine binding of native TBG. Limited insertion of the RSL into beta-sheet A appears to trigger hormone release after proteolytic cleavage. In support of this concept, residues within the hinge region of the TBG loop are phylogenetically highly conserved, suggestive of their physiological role as a functional switch in vivo.

Characterization and primary structures of bovine and porcine thyroxine-binding globulin

Thyroxine-binding globulin (TBG) is the major serum transport protein for iodothyronines in most of the large, omni- or herbivorous mammals. Characterization of human TBG (hTBG), including its 20 known natural variants, allowed the identification of the ligand-binding site and a correlation of diminished synthesis or loss of function with mutations in the TBG gene. Further refinement of the structure-function correlation, especially the high binding affinity and heat stability, requires characterization of other mammalian TBGs, of which only rat and sheep TBG were available. We now present some of the chemical and physical properties of bovine TBG (bTBG) and porcine TBG (pTBG) and their primary structures deduced from their cDNA sequences. The serum concentrations of bTBG and pTBG estimated by Scatchard analysis of T(4)-binding were similar to hTBG. The T(4)-binding affinity of human, bovine and porcine TBGs were all similar, at 1.2x10(10) M(-1). However, heat stability of the animal TBGs was reduced, with a half life of denaturation of 7 min (bTBG) and 5 min (pTBG) at 55 degreeC, compared with 21 min for hTBG. Nucleotide alignment revealed identity with hTBG of 85.5% (bTBG) and 83.7% (pTBG) and amino acid identity of 82.8% (bTBG) and 82.6% (pTBG). As expected, the relevant parts of the ligand-binding domain (amino acids 215-291, and 363-395) were highly conserved at more than 95% similarity. Comparison of the five known mammalian TBGs allows focusing of future mutagenesis experiments to further characterize the properties of the molecule.

Identification of thyroxine-binding globulin-San Diego in a family from Houston and its characterization by in vitro expression using Xenopus oocytes

T4-binding globulin (TBG) is a liver glycoprotein that transports iodothyronines in serum. Several TBG variants with reduced T4 binding affinity have been described, all of which are also characterized by reduced serum TBG concentrations and reduced heat stability. Their loss of binding thus appears to be due to a general defect of the molecule. We now report the occurrence of a variant TBG, detected in a family from Houston, TX, with half the normal T4 binding affinity and heat stability but normal serum concentration and isoelectric focussing pattern. The propositus was identified by reduced total T4 and T3 serum levels. All family members were euthyroid, and inheritance followed an X-linked pattern. Sequence analysis of the TBG gene of the propositus and his heterozygous mother revealed two amino acid substitutions: serine 23 with threonine (S23T), and the known polymorphism leucine 283 with phenylalanine (L283F). These substitutions are identical to those of TBG-San Diego (TBG-SD), a variant with similar properties except for a reduced serum concentration. Expression of recombinant TBG-SD/H with the S23T substitution in Xenopus oocytes reproduced the binding defect and heat lability. The amount of TBG-SD/H synthesized and secreted by the oocytes was not different from that of normal TBG. The difference in serum TBG concentrations in affected members of the San Diego and Houston families thus does not appear to be due to an error in the measurement of TBG, but may be related to differences in the rates of degradation.

Characterization of the thyroxine-binding site of thyroxine-binding globulin by site-directed mutagenesis

The principal transport protein for T4 in human blood, thyroxine-binding globulin (TBG), binds T4 with an exceptionally high affinity (Ka = 10(10) M(-1)). Its homology to the superfamily of the serpins has recently been used in the design of chimeric proteins, providing experimental evidence that an eight-stranded beta-barrel domain encompasses the ligand-binding site. We have now characterized the T4 binding site by site-directed mutagenesis. Sequence alignment of TBG from several species revealed a phylogenetically highly conserved stretch of amino acids comprising strands 2B and 3B of the beta-barrel motif. Mutations within this region (Val228Glu, Cys234Trp, Thr235Trp, Thr235Gln, Lys253Ala, and Lys253Asp), designed to impose steric hindrance or restriction of its mobility, had no significant influence on T4 binding. However, binding affinity was 20-fold reduced by introduction of an N-linked glycosylation site at the turn between strands 2B and 3B (Leu246Thr) without compromising the proper folding of this mutant as assessed by immunological methods. In most other serpins, this glycosylation site is highly conserved and has been shown to be crucial for cortisol binding of corticosteroid-binding globulin, the only other member of the serpins with a transport function. The ligand-binding site could thus be located to a highly aromatic environment deep within the beta-barrel. The importance of the binding site's aromatic character was investigated by exchanging phenylalanines with alanines. Indeed, these experiments revealed that substitution of Phe249 in the middle of strand 3B completely abolished T4 binding, while the substitution of several other phenylalanines had no effect.

Modularity of serpins. A bifunctional chimera possessing alpha1-proteinase inhibitor and thyroxine-binding globulin properties

An exciting application of protein engineering is the creation of proteins with novel functions by the retrofitting of native proteins. Such attempts might be facilitated by the idea of a mosaic architecture of proteins out of structural units. Even though numerous theoretical concepts deal with the delineation of structural "modules," their potential in the design of proteins has not yet been sufficiently exploited. To address this question we used a gain of function approach by designing modular chimeric molecules out of two structurally homologous but functionally diverse members of the superfamily of serine-proteinase inhibitors, alpha1-proteinase inhibitor and thyroxine-binding globulin. Substitution of two of four alpha1-proteinase inhibitor modules (Lys222 to Leu288 and Pro362 to Lys394, respectively), identified by alpha-backbone distance analysis, with their thyroxine-binding globulin homologues resulted in a bifunctional chimera with inhibition of human leukocyte elastase and high affinity thyroxine binding. To our knowledge, this is the first report on a bifunctional chimera engineered from modules of homologous globular proteins. Our results demonstrate how a modular concept can facilitate the design of new functional proteins by swapping structural units chosen from members of a protein superfamily.

Autosomally transmitted low concentration of thyroxine-binding globulin

The gene coding for thyroxine-binding globulin (TBG) is located on the long arm of the X chromosome; thus anomalies of TBG are transmitted in an X-linked fashion. We report a family with decreased concentration of circulating TBG and an unusual pattern of inheritance. In this family, a number of members showed decreased levels of serum TBG. The unusual finding was that the low TBG was transmitted from the male proband to his son. Thyroxine (T4) binding affinity and heat lability of serum TBG of the proband was not different from values obtained from a normal control. All exons and the critical region in the upstream of the TBG gene were sequenced and no alterations were found. The mechanism that causes decreased concentration of TBG of the proband and family members is unknown, but an abnormality in transcription factors that are important for TBG gene expression might be considered.

Rational use of thyroid function tests

In this era of cost containment, it is necessary to efficiently select the most important laboratory investigations for diagnostic and management purposes, with patient benefit as the ultimate objective. Thyroid function tests collectively represent a very costly item for laboratory services and have tended to be unselectively overutilized virtually until the present. Tests available for thyroid function testing include both in vitro and in vivo tests. Virtually all physicians are familiar with the free thyroxine (fT4) or its equivalents (fT4E), total and free triiodothyronine (TT3 and fT3), and sensitive thyrotrophin (TSH) assays, to which may be added plasma thyroglobulin (Tg), and thyroid autoantibodies (TAb) (including thyroid stimulating antibody [TSAb]). In vitro tests include thyroidal uptakes and scans, as well as other imaging techniques (e.g., ultrasound). Other less commonly used or obsolete techniques are not discussed. For screening or case finding where there is little probability that the patient(s) has thyroid dysfunction, a sensitive TSH assay is all that is initially required. If, however, the TSH result is elevated, a fT4E and TAb should then be performed. If the TSH is subnormal, then a fT4E, TT3, and (if still necessary) TSH response to TRH would determine if that patient was truly hyperthyroid (in elderly patients, a low TSH is often not associated with hyperthyroidism!). Conversely, not all patients with elevated fT4 are truly hyperthyroid. Uptakes and scans are primarily of importance in the correct diagnosis of unusual cases of hyperthyroidism and for nodular disease. When patients already diagnosed are being followed, proper selection of testing is again important, sometimes emphasizing one test over others (e.g., TT3); for example, in patients with Graves' Disease on anti-thyroid drugs, the fT4E and TSH values may not reflect the true status of the patient. On the one hand, the TSH may remain low for months after the patient has become euthyroid, and on the other hand, the fT4E may drop even below normal, whereas the T3 remains elevated (and the patient still hyperthyroid). The many other vagaries of these tests are also mentioned.

Molecular and structural characterization of the heat-resistant thyroxine-binding globulin-Chicago

Thyroxine-binding globulin (TBG) is the main transport protein for thyroxine (T4) in blood. It shares considerable sequence homology with alpha 1-antitrypsin (AT) and other members of the serine proteinase inhibitor (serpin) superfamily of proteins. The crystallographic structure of AT has been determined and was found to represent the archetype of the serpins. This model has been used for structure-function correlations of TBG. Sequence analysis of the heat-resistant variant TBG-Chicago (TBG-CH) revealed a substitution of the normal tyrosine 309 with phenylalanine. For further analysis, vectors containing the coding regions of normal TBG (TBG-N) and TBG-CH were constructed, transcribed in vitro, and expressed in Xenopus oocytes. Both TBGs were secreted into the culture medium and could not be distinguished by gel electrophoresis. Scatchard analysis of T4 binding to TBG-N and -CH revealed no significant differences in binding affinity. The rate of heat denaturation of TBGs was determined by measurement of residual T4 binding capacity after incubation at 60 degrees C for various periods of time. The half-life values of denaturation of TBG-N and -CH were 7 and 132 min, respectively. The tyrosine 309 to phenylalanine substitution of TBG-CH involves a highly conserved phenylalanine residue of the serpins. The respective phenylalanine 312 of AT ties the alpha-helix hI1 to the molecule, thus stabilizing the tertiary structure. A substitution with tyrosine would disrupt this interaction. Accordingly, stabilization of the TBG molecule by replacement of tyrosine with phenylalanine in position 309 causes the increased heat stability of TBG-CH.

Sequencing of the variant thyroxine-binding globulin (TBG)-San Diego reveals two nucleotide substitutions

Thyroxine-binding globulin (TBG) is a liver glycoprotein that transports thyroid hormone in serum. In 1989, a variant TBG was reported with reduced binding affinity for thyroxine (T4) and triiodothyronine (T3) which results in low serum T4 and T3 levels. This variant, TBG-San Diego (TBG-SD), also displays reduced heat stability but has a normal isoelectric focusing pattern. We now report the sequence of the entire coding region of TBG-San Diego. It reveals two nucleotide substitutions: one located in exon 1 which results in the replacement of the normal Ser-23 (TCA) with threonine (ACA) and the other, located in exon 3, changes the normal codon 283 of TTG (leucine) with that of TTT, (phenylalanine). Allele specific amplification was used to search for both nucleotide substitutions in four affected members of the family. Results confirmed the co-segregation of these nucleotide substitutions with the TBG-SD phenotype. The substitution in codon 283 has been previously described and exists as a polymorphism in some ethnic groups or in combination with other TBG variants with different physical characteristics. Thus, it appears that the replacement of Ser-23 with threonine is responsible for the observed alterations in physical properties of TBG-San Diego.

Molecular basis of inherited thyroxine-binding globulin defects

Thyroxine-binding globulin (TBG) is a liver glycoprotein that transports thyroid hormones in serum. Inherited TBG defects appear as partial or complete deficiency and TBG excess. Sequencing of the TBG gene located on the X-chromosome has revealed nucleotide substitutions in partial TBG deficiency, and substitutions or deletions in complete deficiency variants. Whereas the deduced changes of the primary structure of the protein have been sufficient to explain the observed alterations of properties in some of the TBG variants, this has not been the case in other inherited TBG defects studied at the gene level. Further analysis of these and other variants may provide helpful information on glycoprotein synthesis and processing and on protein-hormone interaction.

Impaired binding properties of thyroxine-binding globulin in hepatocellular carcinoma and chronic liver disease

To determine the factors underlying the apparent reduction in binding ability of thyroxine-binding globulin in hepatocellular carcinoma, hormone-binding characteristics were further examined in patients with this disease and in control subjects. No differences in affinity constants with respect to triodothyronine or serum thyroxine-binding globulin from hepatocellular carcinoma, cirrhotic and normal subjects were found. The affinity for thyroxine was significantly reduced in hepatocellular carcinoma (0.41 +/- 0.13 x 10(10) mol-1) and cirrhotic (0.65 +/- 0.1 x 10(10) mol-1) patients compared with normal subjects (0.94 +/- 0.7 x 10(10) mol-1). Investigations carried out on liver tissue obtained from patients with hepatocellular carcinoma and chronic liver disease showed that thyroxine-binding globulin within tumor tissue was elevated and bound less exogenous tracer hormone compared with that obtained from nontumor tissue. Tumor-derived thyroxine-binding globulin with altered binding properties is, at least partly, responsible for the abnormal behavior of the serum protein in patients with hepatocellular carcinoma.

Molecular basis for the properties of the thyroxine-binding globulin-slow variant in American blacks

Thyroxine-binding globulin-slow (TBG-S), a variant found in 4-12% of Black and Pacific Island populations, is inherited as an X-chromosome linked trait. This variant is detected on isoelectric focusing by the characteristic cathodal shift of all its isoforms, suggesting that the difference resides in the core protein. In addition, TBG-S is slightly more thermolabile, which explains why subjects expressing TBG-S have on the average lower serum TBG, and thus reduced T4, concentrations. We now report the molecular basis for this TBG variant, deduced from sequencing the TBG-S gene of an American Black man. Sequencing of the four coding regions and all intron/exon junctions revealed a single nucleotide substitution in the codon for amino acid 171 of the mature protein. The resulting change of the codon GAC to AAC results in replacement of the normal aspartic acid by asparagine. Since the negative charge provided by the aspartic acid is lost when replaced by the neutral asparagine, this substitution seems responsible for the cathodal shift on isoelectric focusing and slower electrophoretic mobility of TBG-S. An identical nucleotide substitution was identified in an unrelated American Black man expressing TBG-S. Whether the TBG-S phenotype observed in populations from the Pacific Islands is caused by the same mutation remains to be determined.

Sequence of the variant thyroxine-binding globulin of Australian aborigines. Only one of two amino acid replacements is responsible for its altered properties

A form of thyroxine-binding globulin (TBG) with reduced affinity for hormone and increased susceptibility to heat and acid denaturation has been identified in Australian Aborigines (TBG-A). Results of heat denaturation of TBG established that the TBGA allele is X linked and has a frequency of 50.9% in Western Australian Aborigines. The sequence of an isolated TBGA allele differed at two positions from that of the normal TBG allele (TBGC). One substitution was in codon 191, ACA (threonine) rather than GCA (alanine), and the other was in codon 283, TTT (phenylalanine) instead of TTG (leucine). These nucleotide substitutions resulted in the loss of sites for the enzymes Bgl 1 and Tth 111 II, respectively. The nucleotide substitutions in the TBG-A allele was confirmed by digestion of genomic DNA segments amplified using the polymerase chain reaction. The Bgl 1 and Tth 111 II sites were absent in the genes of two Aboriginal men expressing TBG-A and were present in those of three Aboriginal and six Caucasian males expressing TBG-C. The TBG gene of a seventh Caucasian male possessed the Bgl 1 site but had lost the Tth 111 II site; sequencing of this allele revealed only the substitution in codon 283 identical to that in the TBGA allele. As the biochemical properties of TBGPhe-283 expressed by this individual were indistinguishable from normal TBGLeu-283, we believe that the abnormal properties of TBG-A are due to substitution of alanine for threonine at residue 191.

Thyroxine-binding globulin deficiency re-examined

Seven sera, previously categorised as completely deficient in thyroxine-binding globulin (TBG) by an immunoelectrophoretic technique, were re-examined with a sensitive ELISA method. Only one of the sera was confirmed completely deficient by ELISA. The remaining six contained 0.08-0.19 mg/l. The protein was immunologically identical with 'normal' TBG, could bind to thyroxine and had the correct mobility in reverse-flow electrophoresis. Complete TBG deficiency may therefore be rare.

Evaluation of T4 and T3 binding kinetics in the thyroxine binding globulin abnormality of Australian aborigines

Low serum total T4 associated with subnormal concentrations of thyroxine binding globulin (TBG) has been reported in up to 40% of euthyroid Australian aborigines. It has been suggested that these subjects show both diminished concentration of TBG and reduced TBG affinity for T4 (Sarne et al., 1985). We have compared 12 euthyroid aborigines with low T4 (total T4 44 +/- 5 nmol/l) and aborigines with normal T4 (T4 99 +/- 9 nmol/l, n = 12) using measurements of free T4 and T3 by equilibrium dialysis. TBG was measured both by RIA (Henning, Berlin, FRG) and a method dependent on T4 binding (Corning Immophase). Aborigines with low T4 showed lower levels of free T4 (12.6 +/- 0.6 cf. 18.7 +/- 1.0 pM), free T4 index (66 +/- 8 cf. 98 +/- 13), total T3 (1.1 +/- 0.2 cf. 1.6 +/- 0.3 nmol/l), TBG RIA (14.0 +/- 0.6 cf. 25.0 +/- 1.2 ng/l), and TBG Immophase (9.0 +/- 0.5 cf. 22.0 +/- 1.2 mg/l) (P less than 0.01), but free T3 (5.3 +/- 0.4 cf. 4.7 +/- 0.4 pM) and TSH (1.9 +/- 0.2 cf. 1.8 +/- 0.2 mU/l) were not significantly different from the values found in aborigines with normal T4. Scatchard analysis of T4 and T3 binding was performed using serum diluted 1 : 20,000 for T4 and 1 : 500 for T3 (barbitone buffer pH 8.6, 4 degrees C, dextran-coated charcoal separation). In euthyroid low T4 aborigines compared to those with normal T4, both T4 capacity (106 +/- 14 cf. 238 +/- 13 nM, P less than 0.01) and affinity (5.05 X 10(10) cf. 8.47 X 10(10) M-1, P less than 0.05) were significantly reduced. Similarly, both T3 capacity (62 +/- 10 cf. 154 +/- 16 nM, P less than 0.01) and affinity (1.67 X 10(9) cf. 2.28 X 10(9) M-1, P less than 0.02) were reduced. A substantial minority of euthyroid Australian aborigines have a TBG variant characterized by both reduced capacity and affinity of T4 and T3. These findings suggest that TBG may be both qualitatively and quantitatively abnormal in these subjects.

Anomalous binding characteristics of human thyroxine binding globulin due to a dilution-dependent separation artefact

Contrary to the accepted view, a recent study using Sephadex column separation suggested that thyroxine binding globulin (TBG) binds T4 and T3 with similar affinity, but with a much larger capacity for T4 than T3. We have evaluated this finding by comparing this separation method with equilibrium dialysis, taking account of the effect of serum dilution with each method. Estimates of free T4 fraction by equilibrium dialysis (with magnesium chloride precipitation) were valid over a wide range of serum dilutions. In contrast, Sephadex column separation gave a major overestimate of free hormone (underestimate of binding) in less diluted serum, indicating that this method cannot be used to establish a value for T4 affinity independent of serum dilution. Such a systematic error will result in a greater underestimate of affinity for the ligand with higher affinity when two ligands are compared at a single serum dilution. By equilibrium dialysis at 37 degrees C, the affinity of T4 for TBG was approximately 13-fold higher than that of T3, while the capacity of TBG for both T4 and T3 was close to the concentration of immunoreactive TBG. The previous report of similar T4 and T3 affinities appears to be due to a dilution-dependent underestimate of T4 affinity inherent in Sephadex column separation. Direct comparison of binding kinetics of various ligands requires a separation method that is valid over a wide range of binding protein concentrations.

Two new inherited defects of the thyroxine-binding globulin (TBG) molecule presenting as partial TBG deficiency

Serum-denatured TBG (dnTBG) measured in 32 families deficient in native TBG (nTBG) was undetectable in all subjects with complete nTBG deficiency and was high in 2 of 16 families with partial nTBG deficiency. nTBG (in mean micrograms per decaliter +/- SD) in members of the Quebec and Montreal families, respectively were: 258 +/- 54 and 230 in affected men, 747 +/- 190 and 927 +/- 90 in affected women, and 1568 +/- 151 and 1300 +/- 195 in unaffected relatives. Corresponding mean dnTBG levels were: 14.3 +/- 2.9 and 21.3 in affected men, 8.6 +/- 1.0 and 11.6 +/- 3.1 in affected women, and less than 2.1 and less than 2.6 in unaffected relatives. All were euthyroid with normal free thyroxine and thyrotropin levels. In comparison to common type TBG, TBG-Quebec was more heat labile by 10 degrees C and TBG-Montreal by 12 degrees C. The degree of dnTBG elevation and nTBG lability at 37 degrees C were correlated (r = 0.99). Isoelectric focusing showed cathodal shift of all TBG bands: TBG-Quebec by 0.06 isoelectric points (pI) and TBG-Montreal by 0.02 pI. These two TBG variants represent different mutations most likely affecting the polypeptide chain of the molecule. Their inheritance is X-chromosome linked. The instability of these TBGs at 37 degrees C may lead to more rapid degradation in vivo resulting in low nTBG and high dnTBG concentrations in serum.

Recent advances in routine thyroid function testing

Thyroid function tests are one of the most common of endocrine laboratory investigations requested by general clinicians. The tests used therefore have to be efficient at identifying thyroid disease, monitoring treatment, and handling large numbers of tests. Recent advances in methodology have expanded both the range of in vitro thyroid function tests available and the techniques by which the well-established tests may be performed. This article reviews the methods and analytical and clinical performance of the routine tests currently available, concentrating particularly on the relatively new ones, and speculating on their role in strategies for the laboratory investigation of thyroid function.