Thyroglobulins of cyclostomes and an elasmobranch

Relationship between the dimerization of thyroglobulin and its ability to form triiodothyronine

Thyroglobulin (TG) is the most abundant thyroid gland protein, a dimeric iodoglycoprotein (660 kDa). TG serves as the protein precursor in the synthesis of thyroid hormones tetraiodothyronine (T₄) and triiodothyronine (T₃). The primary site for T₃ synthesis in TG involves an iodotyrosine acceptor at the antepenultimate Tyr residue (at the extreme carboxyl terminus of the protein). The carboxyl-terminal region of TG comprises a cholinesterase-like (ChEL) domain followed by a short unique tail sequence. Despite many studies, the monoiodotyrosine donor residue needed for the coupling reaction to create T₃ at this evolutionarily conserved site remains unidentified. In this report, we have utilized a novel, convenient immunoblotting assay to detect T₃ formation after protein iodination in vitro, enabling the study of T₃ formation in recombinant TG secreted from thyrocytes or heterologous cells. With this assay, we confirm the antepenultimate residue of TG as a major T₃-forming site, but also demonstrate that the side chain of this residue intimately interacts with the same residue in the apposed monomer of the TG dimer. T₃ formation in TG, or the isolated carboxyl-terminal region, is inhibited by mutation of this antepenultimate residue, but we describe the first substitution mutation that actually increases T₃ hormonogenesis by engineering a novel cysteine, 10 residues upstream of the antepenultimate residue, allowing for covalent association of the unique tail sequences, and that helps to bring residues Tyr²⁷⁴⁴ from apposed monomers into closer proximity.

KClO(4) inhibits thyroidal activity in the larval lamprey endostyle in vitro

An in vitro experimental system was devised to assess the direct effects of the goitrogen, potassium perchlorate (KClO(4)), on radioiodide uptake and organification by the larval lamprey endostyle. Organification refers to the incorporation of iodide into lamprey thyroglobulin (Tg). Histological and biochemical evidence indicated that endostyles were viable at the termination of a 4h in vitro incubation. A single iodoprotein, designated as lamprey Tg, was identified in the endostylar homogenates by polyacrylamide gel electrophoresis and Western blotting. Lamprey Tg was immunoreactive with rabbit anti-human Tg serum and had an electrophoretic mobility similar to that of reduced porcine Tg. When KClO(4) was added to the incubation medium, both iodide uptake and organification by the endostyle were significantly reduced relative to controls as determined by gamma counting, and gel-autoradiography and densitometry, respectively. Western blotting showed that KClO(4) significantly lowered the total amount of lamprey Tg in the endostyle. Based on the results of this in vitro investigation, we conclude that KClO(4) acts directly on the larval lamprey endostyle to inhibit thyroidal activity. These data support a previous supposition from in vivo experimentation that KClO(4) acts directly on the endostyle to suppress the synthesis of thyroxine and triiodothyronine, resulting in a decrease in the serum levels of these two hormones.

A comparative review of the structure and biosynthesis of thyroglobulin

Thyroglobulin, the major iodoglycoprotein of the thyroid (Mr 669 kDa) has a sedimentation coefficient of 19 S and an isoelectric point (pI) of 4.4-4.7. The protein has been isolated and purified from saline extracts of the gland of several animal species, by methods such as ammonium sulfate fractionation, DEAE-cellulose chromatography and Sepharose 4B/6B gel-filtration. DEAE-cellulose chromatography of thyroglobulin from many species, by linear gradient, yielded a complex elution pattern, while camel thyroglobulin showed only a major and minor peak. As an iodoprotein, the protein has 0.1-2.0% iodine. The amino acid and iodoamino acid composition of thyroglobulins, in general, is similar. However, a high thyroxine content (15 mol/mol protein) has been noted for buffalo species. Asparagine or aspartic acid has been reported as the major N-terminal amino acid for thyroglobulins of several animal species whereas glutamic acid is the sole N-terminal amino acid for buffalo thyroglobulin. As a glycoprotein, thyroglobulin contains 8-10% total carbohydrate with galactose, mannose, fucose, N-acetyl glucosamine and sialic acid residues. The carbohydrate in the protein is distributed as two distinct units, A and B. In addition, human thyroglobulin has carbohydrate unit C. The occurrence of sulfate and phosphate as Gal-3-SO4 and Man-6-PO4, respectively, has been reported in few species. The quaternary structure of native thyroglobulin is comprised of two equal sized subunits of 330 kDa. However, the protein appears to contain 4-8 non-identical units in few species. The synthesis of thyroid hormones occurs in the matrix of the protein and is regulated by pituitary thyrotropin. The role of tyrosine residues 5 and 130 in thyroxine synthesis has been well documented.

Isolation and characterization of hagfish thyroid iodoprotein by its non-thyroglobulin nature, very high iodine and carbohydrate contents and low hormone/iodine ratio

We have characterized the thyroid iodoprotein of a hagfish, Eptatretus burgeri, one of the lowest marine vertebrates. The iodoprotein was not very homogeneous in its apparent molecular mass which decreased with the increase in hormone/iodotyrosine ratio. Four subfractions with an apparent molecular mass of about 400 kDa were purified from one major fraction by size-exclusion and Mono Q ion-exchange HPLC. The subfractions appeared to have the same peptide backbone, since they showed a single band with the same mobility as a 160-kDa protein in SDS/PAGE and the same amino acid composition. However they differed from each other in having increasing iodine contents (1.9% to 5.9% by mass of total amino acids) associated with the increase in hormonal iodine proportion (8.4% to 16.7% of total iodine) and carbohydrate content (35.6% to 53.5% by mass). These values are strikingly different from those of thyroglobulin with an iodine content of less than 1%, hormonal iodine of 20-40% and carbohydrate content of less than 10%. The amino acid composition of the hagfish iodoprotein, especially the cysteine content of less than 1%, was also entirely different from that of thyroglobulin. These results suggest that most, if not all, tyrosine residues of the hagfish thyroid glycoprotein with a less rigid structure are susceptible to an iodinating system, but hormone residues are formed by a much less efficient mechanism than those in thyroglobulin, when poorly iodinated.

Epochal trace elements and evolution

The use of some trace elements by plants and animals during the evolutionary process has resulted in epochal changes. Noteworthy is the fact that plants (but not animals) needed boron in order to grow stems and roots as they left the seas and became anchored on land. Iodine is plentiful in sea water but rare on land. Therefore, the iodination of tyrosine provided an iodine transport mechanism which allowed for the metamorphosis and the development of warm bloodedness--a great evolutionary advantage. Zinc from clay was needed for the formation of the first primitive nucleic acids and, later, the presence of zinc in the retina provide the enhanced night vision of the nocturnal predators--a natural advantage. Hence, boron, iodine and zinc can be termed epochal trace elements. Inquiry should be directed towards the possible roles of other trace elements, which may have been epochal in evolution.

Light and electron microscopic immunocytochemical localization of thyroglobulin in the thyroid gland of the anadromous sea lamprey, Petromyzon marinus L., during its upstream migration

Antibodies made against thyroglobulin (TG) were used in an immunocytochemical study for the light and electron microscopic localization of TG in the thyroid gland of the anadromous sea lamprey, Petromyzon marinus, during its upstream migration. TG was found in the follicular lumen and in some colloid droplets within the follicular cells. Except for an immunoreactive product observed in a small portion of the interstitial connective tissue, the location of TG in the lamprey was similar to that in the thyroid of the rat.

Thyroglobulin biosynthesis in a larval (ammocoete) and adult freshwater lamprey (Lampetra planeri Bl.)

1. The biosynthesis of 18-19S thyroglobulin has been studied in a larval and adult freshwater lamprey (Lampetra planeri Bl.). 2. In vivo and in vitro experiments have been performed by injecting into the coelomic cavity or by incubating branchial region labeled constituents of Tg of higher vertebrates (125I, [3H]leucine and various [3H]carbohydrates). 3. Larvae (ammocoetes) and adults incorporate all labels into thyroglobulin (18-19S Tg), containing a small proportion of labeled T3 and T4, as identified by paper chromatography, and very minute amounts of stable iodine. 4. In adults, the biosynthesis of 18-19S Tg proceeds much more rapidly and the labels are incorporated in higher percentage than in larvae. 5. The demonstration of the biosynthesis of the specific thyroid protein, 18-19S Tg, in larvae indicates that the biochemical mechanism of hormonogenesis is present in larval endostyle before the morphological differentiation of thyroid cells and follicles occurring during metamorphosis. 6. Some 18-19S Tg is apparently stored in the endostyle.