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Naturally occurring fluorescence has been observed in multiple species ranging from bacteria to birds. In macroscopic animals such as birds and fishes, fluorescence provides a visual communication signal. However, the functional significance of this phenomenon is not known in most cases. Though photoprotection is attributed to fluorescence under ultraviolet (UV) light in some organisms, it lacks direct experimental evidence. Here, we have identified a new species of eutardigrade belonging to the genus Paramacrobiotus, which exhibits fluorescence under UV light. Using a natural variant of the same species that lacks fluorescence, we show that the fluorescence confers tolerance to lethal UV radiation. Remarkably, we could transfer this property to UV-sensitive Hypsibius exemplaris, another eutardigrade, and also to C. elegans, a nematode. Using high performance liquid chromatography (HPLC) we isolated the fluorescent compound from Paramacrobiotus sp. This compound has excitation maxima (λex) at 370 nm and emission maxima (λem) at 420-430 nm. We propose that Paramacrobiotus sp. uses a fluorescent shield that absorbs harmful UV radiation, and emits harmless blue light, thereby protecting itself from the lethal effects of UV radiation.Summary statementTardigrades are well known for their tolerance to extreme environmental conditions. In this study, we have identified a new tardigrade species that employs a fluorescent shield to protect itself from the germicidal ultra violet radiation.

The regioselective deiodinations of L-thyroxine (T4) play key roles in the thyroid hormone homeostasis. These reactions are catalyzed by three isoforms of the selenoenzymes, iodothyronine deiodinases (Dio1, Dio2 and Dio3), which are highly homologous in nature. Dio1 mediates 5'-or 5-deiodinations of T4 to produce T3 and rT3, respectively. In contrast, Dio2 and Dio3 are selective to 5'-or 5-deiodination to produce T3 and rT3, respectively. Understanding of the regioselectivity of deiodination at the molecular level is important as abnormal levels of thyroid hormone have been implicated in various clinical conditions, such as hypoxia, myocardial infarction, neuronal ischemia and cancer. In this paper, we report that the electronic properties of the iodine atoms in thyroxine (T4) can be modulated through a simple substitution in the 4'-phenolic moiety. This leads to the change in the regioselectivity of deiodination by different small molecule mimics of Dio enzymes. By using this chemical approach, we also show that the substitution of a strong electron withdrawing group facilitates the removal of all four iodine atoms in the T4 derivative. Theoretical investigations on the hydrogen bonded adducts of T4 with imidazole indicate that the charge on the iodine atoms depend on the nature of hydrogen bond between the À OH group of T4 and the imidazole moiety. While the imidazole can act as either hydrogen bond acceptor (HBA) or hydrogen bond donor (HBD), the protonated imidazole acts exclusively as HBD in T4-imidazole complex. These studies support the earlier observations that the histidine residue at the active sites of the deiodinases play an important role not only in the substrate binding, but also in altering the regioselectivity of the deiodination reactions.

March 2023 | 137 thyroid gland to make thyroid hormones (THs). Upon reaching an optimum concentration in the blood plasma, the secretion of T4 from the thyroid gland is stopped by a negative feedback mechanism. [3,4] THs regulate different physiological functions of the human body like cardiovascular function, normal growth, and maturation of bones and overall metabolism of fat, protein, and carbohydrate. They also control the body temperature, heart rate and protein synthesis. This general article highlights the basic chemistry associated with THs synthesis, transport, and metabolism (Figure 1). Biosynthesis of thyroid hormonesThe biosynthesis of THs takes place in the colloidal lumen of thyroid follicular cells with the help of thyroglobulin (Tg, a highly crosslinked glycoprotein), thyroid peroxidase (TPO), iodide (I-), and hydrogen peroxide albumin (HSA). T4 then enters the target cells through membrane-associated transporters like monocarboxylate transporter 8 (MCT8),10 (MCT10) and organic anion transporter 1c1 (OATP1C1). Subsequently, T4 undergoes monodeiodination by iodothyronine deiodinase (DIOs) enzymes to produce the active form of thyroid hormone T3, which further binds with the thyroid nuclear receptors and recognizes different thyroid hormone-responsive elements (TREs) of target genes and regulates the gene translation. The secretion of THs by thyroid are controlled by hypothalamus and pituitary. Hypothalamus secretes thyrotropin-releasing hormone (TRH), a tripeptide (pyro glutamyl-histidyl-proline amide), which stimulates the pituitary to synthesize and release the thyroid-stimulating hormone (TSH) which signals the