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Event Title

Transthyretin in serum, Binding

Key Event Overview

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AOPs Including This Key Event

AOP Name Event Type Essentiality
Interference with thyroid serum binding protein transthyretin and subsequent adverse human neurodevelopmental toxicity MIE Strong

Chemical Initiators

The following are chemical initiators that operate directly through this Event:

  1. pentachlorophenol
  2. Polychlorinated biphenyl

Taxonomic Applicability

Name Scientific Name Evidence Links
Homo sapiens Homo sapiens Strong NCBI
Bubalus bubalis Bubalus bubalis Strong NCBI
Rattus norvegicus Rattus norvegicus Strong NCBI
Mus musculus Mus musculus Strong NCBI
Gallus gallus Gallus gallus Strong NCBI
Sus scrofa Sus scrofa Strong NCBI
Pan troglodytes Pan troglodytes Strong NCBI
Monodelphis domestica Monodelphis domestica Strong NCBI
Xenopus tropicalis Xenopus (Silurana) tropicalis Strong NCBI
Bos taurus Bos taurus Strong NCBI
Macaca mulatta Macaca mulatta Strong NCBI
Meleagris gallopavo Meleagris gallopavo Strong NCBI
Canis lupus familiaris Canis lupus familiaris Strong NCBI
Ovis aries Ovis aries Strong NCBI
Erinaceus europaeus Erinaceus europaeus Strong NCBI

Level of Biological Organization

Biological Organization

How this Key Event works

The key event that initiates this AOP (i.e. the MIE) is binding of a xenobiotic to the T4-binding site of TTR, displacing thyroxine (T4) from the binding site of TTR and adding this to the pool of free T4 in serum.

Transthyretin is a 55-kDa tetramer protein composed of two identical monomer subunits, each of which is formed by two four-stranded beta sheets, which are assembled around the central channel of the protein (Blake et al. 1978). This central channel contains two identical binding sites; however, there is a 100-fold difference in binding constants between the first and second T4 molecule bound and this "negative cooperativity" results in a 1:1 relationship between T4 and TTR in terms of protein transport in serum (Ferguson et al. 1975). The phenolic hydroxyl and iodine substitutions on the phenolic ring of T4 are important structural characteristics that differentiate ability to bind to TTR, as opposed to other serum binding proteins like TBG (Andrea et al. 1980). T4 binds to TTR in one of two ways: "forward" and "reverse" mode. In forward mode, the phenolic ring of T4 is buried deeply in the TTR binding site while this ring interacts with residues at the entrance of the binding channel in reverse mode (Lans et al 1993).

TTR has been found to bind with a number of ligands, including pharmacologic agents as well as flavonoids and halogenated aromatic compounds. Studies with other xenobiotics with similar structural characteristics have found that a higher degree of halogen substitution, as well as placement on the ring relative to the hydroxyl group, increases binding affinity (Chauhan et al. 2000, Lans et al. 1993, Lans et al. 1994, Ren and Guo 2012, van den Berg 1990, van den Berg et al. 1991, Weiss et al 2015). Weiss et al (2015) compiled a database of almost 150 compounds found to bind to TTR and 52 of these were found to have higher affinity than T4 and thus, could be considered competitive binders in serum.

Rickenbacher et al (1986) provided initial direct evidence of competition for the T4 binding site using molecular modeling and binding assays using radiolabeled TH. Brouwer and van den Berg (1986) reported preferential binding of a metabolite of radiolabeled tetrachlorobiphenyl to TTR in rats (15 mg/kg, ip), using gel electrophoresis followed by HPLC analysis. Van den Berg (1990) used a competitive binding assay to assess the ability of hydroxylated chlorinated aromatic compounds to bind to radiolabeled T4. Van den Berg et al (1991) extended this work to 65 compounds from 12 different chemical groups in rats treated via a single ip dose and competitive binding assay. Chlorophenols were found to have higher affinity relative to other chlorinated aromatics, particular at higher levels of chlorination, and the combination and position of hydroxyl & chlorine atoms. {insert Figure 2/Van den Berg 1990}

Lans et al (1993) described the ability of hydroxylated metabolites of PCBs, PCDDs and PCDFs to act as competetive ligands at the TTR-T4 binding site using an in vitro binding assay. Many of the hydroxylated PCBs examined were more potent ligands than T4, as opposed to those PCDFs and PCDDs that lacked chlorine atoms substituted adjacent to hydroxyl groups. When the hydroxyl group was in the meta or para positions, a 35- to 136-fold greater potency was found relative to ortho substitutions. These results were confirmed through later competitive binding work published by Cheek et al (1999) and Chauhan et al (2000).

How it is Measured or Detected

Methods that have been previously reviewed and approved by a recognized authority should be included in the Overview section above. All other methods, including those well established in the published literature, should be described here. Consider the following criteria when describing each method: 1. Is the assay fit for purpose? 2. Is the assay directly or indirectly (i.e. a surrogate) related to a key event relevant to the final adverse effect in question? 3. Is the assay repeatable? 4. Is the assay reproducible?

TTR can be measured by electrophoresis, hormone saturation and by immunoassays, once separated from other serum proteins. Detailed structural analysis of the protein is available from X-ray crystallography (Rerat and Schwick 1967; Blake et al 1977).

Binding capacity of serum TTR in female rats is lower relative to males (Emerson et al 1990 in Zoeller et al 2007)

Evidence Supporting Taxonomic Applicability

The transthyretin protein has been found to be highly conserved among vertebrates, as supported by X-ray crystallography studies in man, rat, chicken and fish as well as comparison of amino acid sequence (Richardson 2007). Over time, evolution has driven the nature of the N-terminal region of TTR from a more hydrophobic and longer region (as found in fish and amphibians) to a shorter and more hydrophilic region (as found in the placental mammals). The consequence of this biochemical change in the TTR protein was to change the primary function of TTR from binding and carrying T3 in serum to carrying T4 in serum (as it does almost exclusively in rats, as rats lack TBG during adult life)(Alshehri et al. 2015). Thus, in the placental mammals, TTR operates to carry T4 to specific tissues, where it is displaced, transported across the cell membrane by a different ligand-mediated process, and then converted to T3 via deiodinase enzymes within the cell (where it activates the nuclear receptor to cause downstream effects).

TTR is encoded by a single gene on human chromosome 18 (18q11.2-12.1) (LeBeau and Geurts van Kessel 1991).

TTR is synthesized in the adult liver only by eutherians (birds and placental mammals) and herbivorous marsupials; however, it is also synthesized in the choroid plexus of reptiles, birds and mammals. TTR is one of three thyroid hormone serum transport proteins, along with albumin (ALB) and thyroid-binding globulin (TBG). Over evolutionary time, the N-terminal section of TTR has become shortened and more hydrophilic such that T3 is more tightly bound in birds and reptiles but T4 is more tightly bound to TTR in mammals (Schrieber 2002). They all differ in binding affinity and dissociation rate for both T4 and T3 and together form a buffered system that seeks to keep circulating T4 within a certain range: between the normal concentration of free T4 in serum (30 pM) and solubility limit in serum (2 uM) (Schreiber 2002). Given these differences, more T4 available for cellular uptake likely comes from ALB-bound T4 in capillaries with fast moving blood but TTR-bound T4 is likely more important in slow moving fluid (like CSF)(Schrieber 2002).


Alshehri, B., D’Souza, D. G., Lee, J. Y., Petratos, S., & Richardson, S. J. (2015). The Diversity of Mechanisms Influenced by Transthyretin in Neurobiology: Development, Disease and Endocrine Disruption. Journal of Neuroendocrinology, 27(5), 303–323. http://doi.org/10.1111/jne.12271

Andrea et al. 1980

Blake et al 1978

Chauhan, K. R., Kodavanti, P. R. S., & McKinney, J. D. (2000). Assessing the Role of ortho-Substitution on Polychlorinated Biphenyl Binding to Transthyretin, a Thyroxine Transport Protein. Toxicology and Applied Pharmacology, 162(1), 10–21. http://doi.org/10.1006/taap.1999.8826

Ferguson et al. 1975

Lans, M. C., Klasson-Wehler, E., Willemsen, M., Meussen, E., Safe, S., & Brouwer, A. (1993). STRUCTURE-DEPENDENT, COMPETITIVE INTERACTION OF HYDROXY-POLYCHLOROBIPHENYLS, -DIBENZO-p-DIOXINS AND -DIBENZOFURANS WITH HUMAN TRANSTHYRETIN. Chemico-Biological Interactions, 88, 7–21.

Lans, M. C., Spiertz, C., Brouwer, a, & Koeman, J. H. (1994). Different competition of thyroxine binding to transthyretin and thyroxine-binding globulin by hydroxy-PCBs, PCDDs and PCDFs. European Journal of Pharmacology, 270(2-3), 129–136. http://doi.org/10.1016/0926-6917(94)90054-X

Ren, X. M., & Guo, L. H. (2012). Assessment of the binding of hydroxylated polybrominated diphenyl ethers to thyroid hormone transport proteins using a site-specific fluorescence probe. Environmental Science and Technology, 46(8), 4633–4640. http://doi.org/10.1021/es2046074

Richardson, S. J. (2007). Cell and molecular biology of transthyretin and thyroid hormones. International Review of Cytology, 258(January), 137–93. http://doi.org/10.1016/S0074-7696(07)58003-4

Schreiber, G. (2002). The evolutionary and integrative roles of transthyrein in thyroid hormone homeostasis. Journal of Endocrinology, 175(1), 61–73. http://doi.org/10.1677/joe.0.1750061

Van den Berg, K. J. (1990). Interaction of chlorinated phenols with thyroxine binding sites of human transthyretin, albumin and thyroid binding globulin. Chemico-Biological Interactions, 76(1), 63–75. http://doi.org/10.1016/0009-2797(90)90034-K

Van den Berg, K. J., Van Raaij, J. a G. M., Bragt, P. C., & Notten, W. R. F. (1991). Interactions of halogenated industrial chemicals with transthyretin and effects on thyroid hormone levels in vivo. Archives of Toxicology, 65(1), 15–19. http://doi.org/10.1007/BF01973497

Weiss, J. M., Andersson, P. L., Zhang, J., Simon, E., Leonards, P. E. G., Hamers, T., & Lamoree, M. H. (2015). Tracing thyroid hormone-disrupting compounds: database compilation and structure-activity evaluation for an effect-directed analysis of sediment. Analytical and Bioanalytical Chemistry, 5625–5634. http://doi.org/10.1007/s00216-015-8736-9