The thyroid hormones thyroxine and triiodothyronine are small molecules that have large biological effects. They regulate metabolism in almost all cells, and are important for the development of the central nervous system, musculoskeletal system and lungs. These are also the only hormones that contain iodine and are synthesized partly inside and partly outside of cells. A dimeric glycoprotein called thyroglobulin (each monomer has a mass of about 330,000 daltons) serves as the precursor, scaffold, and reservoir of thyroid hormones [1]. It is also a marker of recurrence of well-differentiated malignant neoplasms of the thyroid gland and is used to monitor the treatment of these diseases.
Due to the large size of the molecule, the possibility of obtaining recombinant thyroglobulin is limited. As a result, the only available source of purified human thyroglobulin is thyroid tissue. But the population of human thyroglobulin from tissue donors is heterogeneous in terms of glycosylation and iodination, which creates problems with its use in diagnostics. In addition, different variants of thyregoglobulin affect the results of ELISA, which are used to screen for diseases of the cytothyroid gland.
The existence of a universal standard for recombinant thyroglobulin would eliminate the current problems with the commercially purified product, but to date, a conventional expression system has not been shown to support recombinant production of this protein. E. coli is perhaps the simplest recombinant protein expression system and has produced commercially valuable proteins such as human insulin and other therapeutic enzymes [2], but it poses difficulties in expressing large molecules such as thyroglobulin. Yeast has been used as a recombinant protein expression system because it is capable of disulfide bond formation and post-translational modifications; however, variations in glycosylation have been a liability in some species [3] and have resulted in decreased recombinant protein yields [2]. Baculovirus and mammalian cell systems have been used to produce recombinant proteins because they overcome the limitations of E. coli and yeast systems, but both pose technical barriers (e.g., maintaining cell cultures) and do not provide a cost advantage to the ongoing purification of recombinant thyregolobulin from human tissues.