Thyroxine transport and distribution in Nagase analbuminemic rats

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.

Molecular and thyroid hormone binding properties of lamprey transthyretins: The role of an N-terminal histidine-rich segment in hormone binding with high affinity

Transthyretin (TTR) is a plasma thyroid hormone (TH) binder that emerged from an ancient hydroxyisourate hydrolase by gene duplication. To know how an ancient TTR had high affinity for THs, molecular and TH binding properties of lamprey TTRs were investigated. In adult serum, the lipoprotein LAL was a major T3 binder with low affinity. Lamprey TTRs had an N-terminal histidine-rich segment, and had two classes of binding sites for 3,3',5-triiodo-L-thyronine (T3): a high-affinity and a low-affinity site. Mutant TTRΔ_3-11, lacking the N-terminal histidine-rich segment, lost the high-affinity T3 binding site. [¹²⁵I]T3 binding to wild type TTR and mutant TTRΔ_3-11, was differentially modulated by Zn²⁺. Zn²⁺ contents of wild type TTR were 7-10/TTR (mol/mol). Our results demonstrate that lamprey TTR is a Zn²⁺-dependent T3 binder. The N-terminal histidine-rich segment may be essential for neo-functionalization (i.e., high-affinity T3 binding activity) of an ancient TTR after gene duplication.

Transport of thyroid hormones via the choroid plexus into the brain: the roles of transthyretin and thyroid hormone transmembrane transporters

Thyroid hormones are key players in regulating brain development. Thus, transfer of appropriate quantities of thyroid hormones from the blood into the brain at specific stages of development is critical. The choroid plexus forms the blood-cerebrospinal fluid barrier. In reptiles, birds and mammals, the main protein synthesized and secreted by the choroid plexus is a thyroid hormone distributor protein: transthyretin. This transthyretin is secreted into the cerebrospinal fluid and moves thyroid hormones from the blood into the cerebrospinal fluid. Maximal transthyretin synthesis in the choroid plexus occurs just prior to the period of rapid brain growth, suggesting that choroid plexus-derived transthyretin moves thyroid hormones from blood into cerebrospinal fluid just prior to when thyroid hormones are required for rapid brain growth. The structure of transthyretin has been highly conserved, implying strong selection pressure and an important function. In mammals, transthyretin binds T4 (precursor form of thyroid hormone) with higher affinity than T3 (active form of thyroid hormone). In all other vertebrates, transthyretin binds T3 with higher affinity than T4. As mammals are the exception, we should not base our thinking about the role of transthyretin in the choroid plexus solely on mammalian data. Thyroid hormone transmembrane transporters are involved in moving thyroid hormones into and out of cells and have been identified in many tissues, including the choroid plexus. Thyroid hormones enter the choroid plexus via thyroid hormone transmembrane transporters and leave the choroid plexus to enter the cerebrospinal fluid via either thyroid hormone transmembrane transporters or via choroid plexus-derived transthyretin secreted into the cerebrospinal fluid. The quantitative contribution of each route during development remains to be elucidated. This is part of a review series on ontogeny and phylogeny of brain barrier mechanisms.

Evolutionary changes to transthyretin: evolution of transthyretin biosynthesis

Thyroid hormones are involved in growth and development, particularly of the brain. Thus, it is imperative that these hormones get from their site of synthesis to their sites of action throughout the body and the brain. This role is fulfilled by thyroid hormone distributor proteins. Of particular interest is transthyretin, which in mammals is synthesized in the liver, choroid plexus, meninges, retinal and ciliary pigment epithelia, visceral yolk sac, placenta, pancreas and intestines, whereas the other thyroid hormone distributor proteins are synthesized only in the liver. Transthyretin is synthesized by all classes of vertebrates; however, the tissue specificity of transthyretin gene expression varies widely between classes. This review summarizes what is currently known about the evolution of transthyretin synthesis in vertebrates and presents hypotheses regarding tissue-specific synthesis of transthyretin in each vertebrate class.

Marsupial models for understanding evolution of thyroid hormone distributor proteins

Marsupials are a group of mammals that are under-exploited, in particular in developmental and evolutionary studies of biological systems. In this review, the roles that marsupials have played in elucidating the evolution of thyroid hormone distribution systems are summarised. Marsupials are born at very early developmental stages, and most development occurs during lactation rather than in utero. Studying thyroid hormone distribution systems during marsupial development, in addition to comparing the two Orders of marsupials, gave clues as to the selection pressures acting on the hepatic gene expression of transthyretin (TTR), one of the major thyroid hormone distributor proteins in blood. The structure of TTR in marsupials is intermediate between that of avian/reptilian TTRs and eutherian ("placental mammalian") TTRs. Consequently, the function of marsupial TTR is intermediate between those of avian/reptilian TTRs and eutherian TTRs. Thus, in some respects marsupials can be considered as "missing links" in vertebrate evolution.

Molecular cloning, tissue distribution, and developmental expression of lamprey transthyretins

We isolated and cloned full-length cDNAs of transthyretin (TTR) from 2 genera of lamprey, Petromyzon marinus and Lampetra appendix. These sequences represent the first report of TTR sequences in vertebrates basal to teleost fishes. The deduced amino acid sequence of lamprey TTR cDNAs showed 36-47% identity with those from other vertebrates; secondary structure predictions and homology-based modeling were both consistent with TTRs from other vertebrates, and these cDNAs lacked the signatures found in TTR-like sequences of non-vertebrates. Of evolutionary interest is the observation that the N-termini of the lamprey TTR subunits are nine amino acids longer than those of eutherian TTRs and four to six amino acids longer than those from all other vertebrates. Sequencing of intron 1 confirmed that this longer N-terminal region is a result of the position of the intron 1/exon 2 splice site, further supporting previous studies. TTR mRNA was detected in a variety of larval lamprey tissues, with the highest levels found in the liver. TTR mRNA was also readily detected by Northern blotting, in the livers of animals at all phases of the lifecycle and was significantly elevated during metamorphosis. The upregulation of lamprey TTR gene expression during a major developmental event is consistent with observations in other vertebrates. In all other vertebrates studied to date, the transient upregulation of TTR gene expression or some other thyroid hormone distributor protein coincides with, and is thought to facilitate, the surge in serum thyroid hormone concentrations required for normal development. However, in lampreys, the upregulation of TTR gene expression occurs when serum thyroid hormone concentrations are at their lowest.

Developmentally regulated thyroid hormone distributor proteins in marsupials, a reptile, and fish

Thyroid hormones are essential for vertebrate development. There is a characteristic rise in thyroid hormone levels in blood during critical periods of thyroid hormone-regulated development. Thyroid hormones are lipophilic compounds, which readily partition from an aqueous environment into a lipid environment. Thyroid hormone distributor proteins are required to ensure adequate distribution of thyroid hormones, throughout the aqueous environment of the blood, and to counteract the avid partitioning of thyroid hormones into the lipid environment of cell membranes. In human blood, these proteins are albumin, transthyretin and thyroxine-binding globulin. We analyzed the developmental profile of thyroid hormone distributor proteins in serum from a representative of each order of marsupials ( M. eugenii; S.crassicaudata), a reptile ( C. porosus), in two species of salmonoid fishes ( S. salar; O. tshawytsch), and throughout a calendar year for sea bream ( S. aurata). We demonstrated that during development, these animals have a thyroid hormone distributor protein present in their blood which is not present in the adult blood. At least in mammals, this additional protein has higher affinity for thyroid hormones than the thyroid hormone distributor proteins in the blood of the adult. In fish, reptile and polyprotodont marsupial, this protein was transthyretin. In a diprotodont marsupial, it was thyroxine-binding globulin. We propose an hypothesis that an augmented thyroid hormone distributor protein network contributes to the rise in total thyroid hormone levels in the blood during development.

The evolution of transthyretin synthesis in the choroid plexus

Choroid plexus has the highest concentration of transthyretin (TTR) mRNA in the body, 4.4 microg TTR mRNA/g wet weight tissue, compared with 0.39 microg in the liver. The proportion of TTR to total protein synthesis in choroid plexus is 12%. All newly synthesized TTR is secreted towards the ventricles. Net transfer of T4 occurs only towards the ventricle and depends on ongoing protein synthesis. Thyroxine-binding globulin (TBG), TTR and albumin form a "buffering" system for plasma [T4] because of their overlapping affinities and on/off rates for L-thyroxine (T4)-binding. The individual components of this network determining T4 distribution are functionally highly redundant. Absence of TBG (humans), or TTR (mice), or albumin (humans, rats) is not associated with hypothyroidism. Natural selection is based on small, inheritable alterations improving function. The study of these alterations can identify function. TTR genes were cloned and sequenced for a large number of vertebrate species. Systematic, stepwise changes during evolution occurred only in the N-terminal region, which became shorter and more hydrophilic. Simultaneously, a change in function occurred: TTR affinities for T4 are higher in mammals than in reptiles and birds. L-triiodothyronine (T3) affinities show the opposite trend. This favors site-specific regulation of thyroid hormones by tissue-specific deiodinases in the brain.

Transthyretin as a thyroid hormone carrier: function revisited

Thyroid hormones are essential for normal mammalian development and for normal metabolism. Thyroxine (T4) is the principal product synthesized by the thyroid follicles, and triiodothyronine (T3), the biologically active hormone, derives mainly from tissue T4 deiodination. More than 99% of the circulating hormone is bound to plasma proteins, mainly to thyroxine-binding globulin, transthyretin and albumin in man, and to transthyretin and albumin in rodents. The role of plasma proteins in the transport of hormones to target tissues has, for a long time, been controversial. The liver and the choroid plexus are the major sites of transthyretin synthesis, tissues from which transthyretin is secreted into the blood and the cerebrospinal fluid, respectively. Transthyretin has been proposed to mediate thyroid hormone transfer into the tissues, particularly into the brain across the choroid-plexus-cerebrospinal fluid barrier. Studies in a transthyretin-null mice strain have shown conclusively that transthyretin is not indespensable for thyroid hormones' entry into the brain and other tissues, nor for the maintenance of an euthyroid status. An euthyroid status is also observed in man totally deprived of thyroxine-binding globulin and in rats without albumin. Taken together, these results exclude dependence of thyroid hormone homeostasis on any major plasma carrier per se. This evidence agrees with the free hormone hypothesis which states that the biologically significant fraction, that is taken up by the tissues, is the free circulating hormone.

Crocodile transthyretin: structure, function, and evolution

Structure and function were studied for Crocodylus porosus transthyretin (crocTTR), an important intermediate in TTR evolution. The cDNA for crocTTR mRNA was cloned and sequenced and the amino acid sequence of crocTTR was deduced. In contrast to mammalian TTRs, but similar to avian and lizard TTRs, the subunit of crocTTR had a long and hydrophobic NH(2)-terminal region. Different from the situation in mammals, triiodothyronine (T(3)) was bound by crocTTR with higher affinity than thyroxine (T(4)). Recombinant crocTTR and a chimeric construct, with the NH(2)-terminal region of crocTTR being replaced by that of Xenopus laevis TTR, were synthesized in the yeast Pichia pastoris. Analysis of the affinity of the chimeric TTRs showed that the NH(2)-terminal region modulates T(4) and T(3) binding characteristics of TTR. The structural differences of the NH(2)-terminal regions of reptilian and amphibian TTRs were caused by a shift in splice sites at the 5' end of exon 2. The comparison of crocodile and other vertebrate TTRs shows that TTR evolution is an example for positive Darwinian evolution and identifies its molecular mechanism.

Developmental profile of thyroid hormone distributor proteins in a marsupial, the tammar wallaby Macropus eugenii

The ontogeny of thyroxine distributor proteins in serum of the marsupial Macropus eugenii (tammar wallaby) was investigated from day 3 after birth until adulthood. The thyroxine distributor proteins in the serum of adult M. eugenii are transthyretin and albumin. Northern analysis of RNA prepared from liver showed that transthyretin mRNA levels were initially high (about adult levels at the earliest ages tested), reduced to about 60% adult levels (between days 50 and 150), and then steadily increased to adult levels (by days 200 to 250). Albumin mRNA levels were initially about 50% of adult levels (day 3) and steadily rose to 90% of adult levels by days 175 to 220. A globulin, "wallaby thyroxine-binding protein" (W-TBP), bound [(125)I]thyroxine from day 3 until about day 200. Of the protein-bound thyroxine, the proportion bound by transthyretin had a similar pattern to the transthyretin mRNA levels. From day 26 onward, about half of the protein-bound thyroxine was bound to albumin. On day 3, less than 10% was bound to W-TBP and the proportion steadily increased to a maximum of about 46% by about day 120 and then reduced to undetectable levels by around day 250. The developmentally regulated W-TBP was present throughout pouch life, when the pouch young is dependent on obtaining thyroxine required for normal growth and development from the mother. After the young tammar wallaby leaves its mother's pouch, a time when it has reached a level of physiological development approximately equivalent to that at the time of birth in precocious eutherian mammals such as cattle and sheep, W-TBP was no longer detected as a thyroxine distributor protein in serum.

Synthesis of an Analog of the Thyroid Hormone-binding Protein Transthyretin via Regioselective Chemical Ligation

Transthyretin is an essential protein responsible for the transport of thyroid hormones and retinol in human serum and is also implicated in the amyloid diseases familial amyloidotic polyneuropathy and senile systemic amyloidosis. Its folding properties and stabilization by ligands are of current interest due to their importance in understanding and combating these diseases. Here we report the solid phase synthesis of the monomeric unit of a transthyretin analog (equivalent to 127 amino acids) using t-Boc chemistry and peptide ligation and its folding to form a functional 54-kDa tetramer. The monomeric unit of the protein was chemically synthesized in three parts (positions 1--51, 54--99, and 102--127) and ligated using a chemoselective thioether ligation chemistry. The synthetic protein was folded and assembled to a tetrameric structure in the presence of transthyretin's native ligand, thyroxine, as shown by gel filtration chromatography, native gel electrophoresis, transthyretin antibody recognition, and thyroid hormone binding. Other folding products included a high molecular weight aggregate as well as a transient dimeric species. This represents one of the largest macromolecules chemically synthesized to date and demonstrates the potential of protein chemical synthesis for investigations of protein-ligand interactions.

The evolution of the thyroid hormone distributor protein transthyretin in the order insectivora, class mammalia

Thyroid hormones are involved in the regulation of growth and metabolism in all vertebrates. Transthyretin is one of the extracellular proteins with high affinity for thyroid hormones which determine the partitioning of these hormones between extracellular compartments and intracellular lipids. During vertebrate evolution, both the tissue pattern of expression and the structure of the gene for transthyretin underwent characteristic changes. The purpose of this study was to characterize the position of Insectivora in the evolution of transthyretin in eutherians, a subclass of Mammalia. Transthyretin was identified by thyroxine binding and Western analysis in the blood of adult shrews, hedgehogs, and moles. Transthyretin is synthesized in the liver and secreted into the bloodstream, similar to the situation for other adult eutherians, birds, and diprotodont marsupials, but different from that for adult fish, amphibians, reptiles, monotremes, and Australian polyprotodont marsupials. For the characterization of the structure of the gene and the processing of mRNA for transthyretin, cDNA libraries were prepared from RNA from hedgehog and shrew livers, and full-length cDNA clones were isolated and sequenced. Sections of genomic DNA in the regions coding for the splice sites between exons 1 and 2 were synthesized by polymerase chain reaction and sequenced. The location of splicing was deduced from comparison of genomic with cDNA nucleotide sequences. Changes in the nucleotide sequence of the transthyretin gene during evolution are most pronounced in the region coding for the N-terminal region of the protein. Both the derived overall amino sequences and the N-terminal regions of the transthyretins in Insectivora were found to be very similar to those in other eutherians but differed from those found in marsupials, birds, reptiles, amphibians, and fish. Also, the pattern of transthyretin precursor mRNA splicing in Insectivora was more similar to that in other eutherians than to that in marsupials, reptiles, and birds. Thus, in contrast to the marsupials, with a different pattern of transthyretin gene expression in the evolutionarily "older" polyprotodonts compared with the evolutionarily "younger" diprotodonts, no separate lineages of transthyretin evolution could be identified in eutherians. We conclude that transthyretin gene expression in the liver of adult eutherians probably appeared before the branching of the lineages leading to modern eutherian species.

Evolution of thyroid hormone binding by transthyretins in birds and mammals

Transthyretin, a protein synthesized and secreted by the choroid plexus and liver, binds thyroid hormones in extracellular compartments. This binding prevents accumulation of thyroid hormones in the lipids of membranes, establishing extracellular thyroid hormone pools for the distribution of the hormones throughout the body and brain. The N-termini of the transthyretin subunits are longer and more hydrophobic in chicken than in eutherian transthyretins. Here, we show that this is a general structural feature of avian transthyretins. Systematic changes of protein structure during evolution result from selection pressure leading to changes in function. The evolution of transthyretin function, namely, the binding of thyroid hormones, was studied in nine vertebrate species. The affinity of thyroxine binding to transthyretin is lowest in avians (mean Kd of about 30 nm), intermediate in metatherians (mean Kd of about 17 nm) and highest in eutherians (mean Kd of about 11 nm). The affinity for 3,5,3'-triiodothyronine shows an opposite trend, being four times higher for avian transthyretins than for mammalian transthyretins.

Abundant synthesis of transthyretin in the brain, but not in the liver, of turtles

The binding of thyroxine to proteins in the blood plasma of the turtle, Trachemys scripta, was analyzed by incubation with radioactive thyroxine, electrophoresis and autoradiography. Albumin and an alpha-globulin were found to bind thyroxine; no thyroxine-binding transthyretin was detected in the prealbumin region. In contrast to blood plasma, a thyroxine-binding prealbumin was observed in medium from T. scripta choroid plexus incubated in vitro. RNA was extracted from brain tissue containing choroid plexus and from liver of T. scripta and Chelydra serpentina and analyzed by hybridization with transthyretin cDNA from the lizard Tiliqua rugosa. The brain RNAs contained substantial amounts of transthyretin mRNA, whereas only trace amounts of transthyretin mRNA were detected in RNA from liver. No transthyretin mRNA was observed in RNA from kidney. The results support the hypothesis that the expression of the transthyretin gene first evolved in the choroid plexus of the brain at the stage of the stem reptiles, whereas abundant transthyretin synthesis in liver evolved much later, and independently, in mammals and birds.

Transthyretin is not essential for thyroxine to reach the brain and other tissues in transthyretin-null mice

As part of a study on tissue uptake of thyroxine (T4) in a transthyretin (TTR)-null mouse strain, kinetic parameters of thyroxine metabolism in wild-type mice under normal physiological conditions are presented. Kinetic analysis of injected [(125)I]T4 showed that TTR-null mutants have markedly increased [(125)I]T4 transfer rate constants from plasma to the fast-exchange compartments of liver and kidney and from fast to slow kidney compartments. Transfer rates from plasma to brain, testes, and fat were little affected. The T4 tissue content in the mutants was greatly reduced in brain but relatively normal in liver and kidney. No major changes were observed in brain 3,3',5-triiodothyronine concentrations, suggesting that availability of this hormone is not markedly altered in the mutant mice. The low T4 brain content probably reflects the absence of T4-TTR complexes in the mutant choroid plexus and cerebrospinal fluid. This study indicates that TTR is not essential for T4 tissue uptake or for T4 to reach the brain across the choroid plexus-cerebrospinal fluid and/or blood-brain barriers.

[Role of substitution albumin therapy in drugs, hormones, electrolytes and miscellanous substances transport]

Human serum albumin (HSA) is quantitatively the most important non specific transport protein. HSA binds a wide variety of both endogenous and exogenous ligands. Hypoalbuminaemia may lead to a decreased plasma binding capability of some compounds. Biological/pharmacological consequences depend on the ligand and the target tissue. Many experimental studies suggest that hypoalbuminaemia may influence the metabolism and toxicity of endogenous ligands (bilirubin, metallic ions, oxygen radicals) and the pharmacological effect of some drugs (among others: furosemide, phenytoin, warfarin). The relevance of such information for human surgical situations remains unclear. Clinical studies are scarce and inconclusive. There is a lack of pertinent data supporting the necessity of HSA infusions in order to maintain a minimal plasma concentration and a convenient plasma transport. However, experimental data indicate that major hypoalbuminaemia should be considered with caution.

Effect of loop diuretics and nonsteroidal antiinflammatory drugs on thyrotropin release by rat anterior pituitary cells in vitro

The close inverse-feedback relationship between serum free thyroxine (T4) and thyrotropin (TSH) is altered in some patients receiving therapeutic doses of drugs such as furosemide, fenclofenac, and diphenylhydantoin. We therefore examined the effect of nonsteroidal antiinflammatory drugs (NSAID), diuretics, and diphenylhydantoin on TSH release in rat anterior pituitary cells in primary culture. TSH content of the culture medium was measured at 22 hours at 37 degrees C either with or without thyrotropin-releasing hormone ([TRH] 10 nmol/L) in medium containing 0.5% bovine serum albumin. The mean basal TSH release by pituitary cells was 6.2 +/- 1.2 ng/mL (n = 10) and was not influenced by unlabeled triiodothyronine ([T3] 100 nmol/L) or any of the drugs tested at < or = 400 mumol/L, except ethyacrynic acid. TRH 10 nmol/L increased mean TSH release by 346% +/- 95% (n = 10). T3 1 and 100 nmol/L inhibited TRH-stimulated TSH release by 24% and 31%, respectively (P < .001), whereas TRH-stimulated TSH release was inhibited by 100 mumol/L meclofenamic acid (29%), fenclofenac (28%), furosemide (24%), and diphenylhydantoin (48%) (P < .001 v TRH alone). Meclofenamic acid and furosemide (100 mumol/L) did not significantly alter the inhibitory effect of T3 1 nmol/L on TRH-stimulated TSH release. These in vitro studies suggest that meclofenamic acid, fenclofenac, furosemide, and diphenylhydantoin could influence TSH release by attenuating the TSH response to TRH. This effect may influence T4-TSH relationships when these agents are used in vivo.

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

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

Evolution of transthyretin in marsupials

The evolution of the expression and the structure of the gene for transthyretin, a thyroxine-binding plasma protein formerly called prealbumin, was studied in three marsupial species: the South American polyprotodont Monodelphis domestica, the Australian polyprotodont Sminthopsis macroura and the Australian diprotodont Petaurus breviceps. The transthyretin gene was found to be expressed in the choroid plexus of all three species. In liver it was expressed in P. breviceps and in M. domestica, but not in S. macroura. This, together with previous studies [Richardson, S. J., Bradley, A. J., Duan, W., Wettenhall, R. E. H., Harms, P. J., Babon, J. J., Southwell, B. R., Nicol, S., Donnellan, S. C. & Schreiber, G. (1994) Am. J. Physiol. 266, R1359-R1370], suggests the independent evolution of transthyretin synthesis in the liver of the American Polyprotodonta and the Australian Diprotodonta. The results obtained from cloning and sequencing of the cDNA for transthyretin from the three species suggested that, in the evolution of the structure of transthyretin in vertebrates, marsupial transthyretin structures are intermediate between bird/reptile and eutherian transthyretin structures. In marsupials, as in birds and reptiles, a hydrophobic tripeptide beginning with valine and ending with histidine was found in transthyretin at a position which has been identified in eutherians as the border between exon 1 and intron 1. In humans, rats and mice, the nine nucleotides, coding for this tripeptide in marsupials/reptiles/birds, are found at the 5' end of intron 1. They are no longer present in mature transthyretin mRNA. This results in a change in character of the N-termini of the subunits of transthyretin from hydrophobic to hydrophilic. This change might affect the accessibility of the thyroxine-binding site in the central channel of transthyretin, since, at least in humans, the N-termini of the subunits of transthyretin are located in the vicinity of the channel entrance [Hamilton, J. A., Steinrauf, L. K., Braden, B. C., Liepnieks, J., Benson, M. D., Holmgren, G., Sandgren, O. & Steen, L. (1993) J. Biol. Chem. 268, 2416-2424].

Thyroid hormones, procollagen III peptide, body composition and basal metabolic rate in euthyroid individuals

We examined 103 euthyroid men and women within a wide range of body weights and ages. Fat free mass (FFM) and body fat (BF) were determined with the total body potassium technique, basal metabolic rate (BMR) by indirect calorimetry and serum concentrations of thyroid hormones (free and total T3 and T4) and the aminoterminal propeptide of collagen III (pIIIp) by immunoassays. BMR was positively related to FFM, BF, total T3, the free T3/free T4 ratio and pIIIp, and negatively to free T4 (men) and to the ratios free T4/total T4 and free T3/total T3. pIIIp was as strongly related to BMR as to total T3. It is suggested that pIIIp may serve as an indicator of peripheral energy expenditure. The negative relationship between BMR and free T4 was unexpected and different to the situation in hypo- and hyperthyreosis where BMR and thyroid hormone are positively related. Our hypothesis is that euthyroid subjects with low serum free thyroid hormone concentrations and comparatively high BMR may have high intracellular thyroid hormone concentrations.

Sheep thyroxine-binding globulin: cDNA sequence and expression

A full-length thyroxine-binding globulin clone was isolated from a sheep liver cDNA library. The nucleotide sequence consisted of 1379 nucleotides and contained the coding region of 1236 nucleotides. The deduced sequence of the mature protein was 396 amino acids long. The similarity of the sheep thyroxine-binding globulin amino acid sequence with those of human and rat were 80% and 76%, respectively. Expression of the thyroxine-binding globulin gene was investigated by Northern analysis. The gene was not expressed in choroid plexus nor at other extrahepatic sites. The expression of the thyroxine-binding globulin gene was also studied during sheep foetal development. Thyroxine-binding globulin mRNA was already detected in livers of 1 cm long foetuses and levels similar to those found in liver from adult sheep were reached half way through pregnancy.

Transthyretin expression evolved more recently in liver than in brain

1. Transthyretin was found to be synthesized and secreted by choroid plexus from rats, echidnas, and lizards, but not toads. 2. Transthyretin was observed in blood from placental mammals, birds, and marsupials, but not reptiles and monotremes. 3. The obtained data suggest that transthyretin synthesis by the liver evolved independently in the lineage leading to the placental mammals and marsupials and in that leading to the birds. 4. It is proposed that transthyretin gene expression in mammalian liver appeared about 200 million years later than its first occurrence in the choroid plexus of the stem reptiles.

Immunohistochemical localization of transthyretin in glomerular peripolar cells of newborn sheep

Purified transthyretin has been isolated from sheep serum. Antiserum raised against this protein has been used with an indirect immunoperoxidase histochemical technique to identify transthyretin in newborn lamb kidney tissue. Transthyretin was found in proximal tubule cells and in glomerular peripolar cells. Preabsorption studies using purified transthyretin protein indicate that the immunoreactivity of the antiserum is specific to transthyretin.

Inability to detect an inhibitor of thyroxine-serum protein binding in sera from patients with nonthyroid illness

Sera from 111 patients hospitalized on acute-care wards (including 32 in the intensive care unit) were examined for the possible presence of inhibitors of thyroxine (T4)-serum protein binding in an assay employing equilibrium dialysis. In 38 of these sera, the unbound (free) T4 fraction was 50% or more higher than the free T4 fraction in a pool of normal sera. From the free T4 fraction in each of the 111 serum samples and the free T4 fraction in the pool of normal sera, the predicted free T4 fractions in mixtures (1:1) of each of these sera with the normal pool were calculated (assuming the absence of binding inhibitors) from the appropriate mass action equations. It was reasoned that a free T4 fraction in any mixture that exceeded this predicted value would indicate the possible presence of a binding inhibitor. (The normal pool was selected for having a low serum triglyceride concentration, to minimize in vitro generation of free fatty acids.) However, for the 111 serum samples studied, the free T4 fraction in the mixture exceeded the upper 95% confidence limit of this predicted value in only one case, and then just barely. Thus, evidence for an inhibitor of T4-serum protein binding in sera from patients with nonthyroid illness could not be found. Twenty-eight of the serum samples were also examined in a similar assay that employed ultrafiltration of undiluted serum instead of equilibrium dialysis. Evidence for an inhibitor of T4-serum protein binding similarly could not be found. Because part of the reason for postulating the existence of such a binding inhibitor has been the performance of the triiodothyronine (T3) resin uptake test in patients with nonthyroid illness, an alternative explanation for this phenomenon was sought. When thyroid hormone-binding globulin (TBG) was desialylated by treatment with neuraminidase, its avidity for T4 was markedly decreased, but its avidity for T3 was unchanged. Thus, if desialylated TBG circulates in patients with nonthyroid illness as previously reported, it could explain not only the low serum T4 concentrations despite near normal immunoreactive TBG concentrations, but also the poor performance of the T3 resin uptake test (where T4 binding capacity is overestimated) in these patients.

Thyroxine uptake by perfused rat liver. No evidence for facilitation by five different thyroxine-binding proteins

Rates of hepatic uptake of thyroxine (T4) from dilute solutions of five different plasma T4-binding proteins were measured in the isolated perfused rat liver using an indicator dilution method. For each protein, this rate was compared with the rate of spontaneous dissociation of the T4-protein complex measured in vitro. Proteins studied were human T4-binding globulin (TBG), human T4-binding prealbumin (TBPA), human albumin, rat TBPA, and human albumin isolated from subjects with familial dysalbuminemic hyperthyroxinemia. For each of the five protein-hormone complexes studied, the rate of hepatic uptake of T4 (measured under conditions expected to result in dissociation-limited uptake) closely approximated the rate of spontaneous dissociation of the protein-hormone complex within the hepatic sinusoids. These findings indicate an absence of special cellular mechanisms that facilitate the hepatic uptake of T4 from its plasma binding proteins, and support the view that uptake occurs from the free T4 pool after spontaneous dissociation of T4 from its binding proteins.

Rates of dissociation of steroid and thyroid hormones from human serum albumin

A rapid filtration assay employing dextran-coated charcoal as acceptor particles for free hormone was used to measure rates of dissociation of steroid and thyroid hormones from human serum albumin. Modification of a previously described assay allowed measurements at 1-s intervals. Nevertheless, this still permitted only minimum estimates of the dissociation rate constants. The hormones studied were thyroxine, 3,5,3'-triiodothyronine, cortisol, corticosterone, testosterone, dihydrotestosterone, estradiol, progesterone, and aldosterone. The apparent dissociation rate constant of the thyroxine-albumin complex at 37 degrees C was 1.3 +/- 0.2 s-1 (t 1/2, 0.5 s). The apparent dissociation rate constants of the other hormone-albumin complexes at 37 degrees C generally exceeded 2 s-1 (t 1/2 less than 0.35 s). Apparent dissociation rate constants at 4 degrees C were only slightly lower. These findings indicate that steroid and thyroid hormones dissociate from albumin rapidly compared with the 1-s capillary transit times that characterize many tissues.