This study pioneers the reporting of Se isotopes in marine top predators and represents the most extensive Se isotopic characterization in animals to date. A methodology based on hydride generation─multicollector inductively coupled plasma mass spectrometry─was established for such samples. The study was conducted on various internal organs of giant petrels (Macronectes spp.), encompassing bulk tissues (δ82/78Sebulk), distinct Se-specific fractions such as selenoneine (δ82/78SeSEN), and HgSe nanoparticles (δ82/78SeNPs). The δ82/78Sebulk results (2.0-5.6‰) offer preliminary insights into the fate of Se in key internal organs of seabirds, including the liver, the kidneys, the muscle, and the brain. Notably, the liver of all individuals was enriched in heavier Se isotopes compared to other examined tissues. In nanoparticle fraction, δ82/78Se varies significantly across individuals (δ82/78SeNPs from 0.6 to 5.7‰, n = 8), whereas it exhibits remarkable consistency among tissues and individuals for selenoneine (δ82/78SeSEN, 1.7 ± 0.3‰, n = 8). Significantly, there was a positive correlation between the shift from δ82/78Sebulk to δ82/78SeSEN and the proportion of Se present as selenoneine in the internal organs. This pilot study proves that Se species-specific isotopic composition is a promising tool for a better understanding of Se species fate, sources, and dynamics in animals.

Selenoneine, an ergothioneine analog, is important for antioxidation and detoxification. SenB and SenA are two crucial enzymes that form carbon-selenium bonds in the selenoneine biosynthetic pathway. To investigate their underlying catalytic mechanisms, we obtained complex structures of SenB with its substrate UDP-N-acetylglucosamine (UDP-GlcNAc) and SenA with N-α-trimethyl histidine (TMH). SenB adopts a type-B glycosyltransferase fold. Structural and functional analysis of the interaction network at the active center provide key information on substrate recognition and suggest a metal-ion-independent, inverting mechanism is utilized for SenB-mediated selenoglycoside formation. Moreover, the complex structure of SenA with TMH and enzymatic activity assays highlight vital residues that control substrate binding and specificity. Based on the conserved structure and substrate-binding pocket of the type I sulfoxide synthase EgtB in the ergothioneine biosynthetic pathway, a similar reaction mechanism was proposed for the formation of C-Se bonds by SenA. The structures provide knowledge on selenoneine synthesis and lay groundwork for further applications of this pathway.

While research has identified several inhibitors of the main protease (Mpro) of SARS-CoV-2, a significant portion of these compounds exhibit reduced activity in the presence of reducing agents, raising concerns about their effectiveness in vivo. Furthermore, the conventional biosafety level 3 (BSL-3) for cellular assays using viral particles poses a limitation for the widespread evaluation of Mpro inhibitor efficacy in a cell-based assay. Here, we established a BSL-1 compatible cellular assay to evaluate the in vivo potential of Mpro inhibitors. This assay utilizes mammalian cells expressing a tagged Mpro construct containing N-terminal glutathione S-transferase (GST) and C-terminal hemagglutinin (HA) tags and monitors Mpro autodigestion. Using this method, GC376 and boceprevir effectively inhibited Mpro autodigestion, suggesting their potential in vivo activity. Conversely, carmofur and ebselen did not exhibit significant inhibitory effects in this assay. We further investigated the inhibitory potential of selenoneine on Mpro using this approach. Computational analyses of binding energies suggest that noncovalent interactions play a critical role in facilitating the covalent modification of the C145 residue, leading to Mpro inhibition. Our method is straightforward, cost-effective, and readily applicable in standard laboratories, making it accessible to researchers with varying levels of expertise in infectious diseases.

This study enhances the current limited understanding of the interaction between mercury (Hg) and selenium (Se) species in fish. Rainbow trout (Oncorhynchus mykiss), a model aquaculture fish, was exposed to Hg and Se species through controlled dietary conditions. Over a 6-month feeding trial, the impact of dietary Se on Hg bioaccumulation in fish, including flesh, brain, and liver, was tracked. Twelve dietary conditions were tested, including plant-based diets (0.25 µgSe g-1) and tuna byproduct diets (0.25 µgHg g-1, 8.0 µgSe g-1) enriched with methylmercury and/or Se as selenite or selenomethionine. The tuna byproduct diet resulted in lower Hg levels than the plant-based diets, with muscle Hg content below the European Commission\'s safe threshold. This study highlights the significant impact of specific Se compounds in the diet, particularly from tuna-based aquafeed, on Hg bioaccumulation. These promising results provide a strong recommendation for future use of fisheries byproducts in sustainable aquafeeds.

Selenium is an essential mineral yet both deficiency and excess are associated with adverse health effects. Dietary intake of Se in humans varies greatly between populations due to food availability, dietary preferences, and local geological and ecosystem processes impacting Se accumulation into agricultural products and animal populations. We argue there is a need to evaluate and reconsider the relevance of public health recommendations on Se given recent evidence, including the metabolic pathways and health implications of Se. This argument is particularly pertinent for Inuit populations in Northern Canada, who often exceed dietary tolerable upper intake levels and exhibit very high whole blood Se concentrations due to their dependence on local country foods high in the newly discovered Se compound, selenoneine. Since selenoneine appears to have lower toxicity compared to other Se species and does not contribute to the circulating pools of Se for selenoprotein synthesis, we argue that total dietary Se or total Se in plasma or whole blood are poor indicators of Se adequacy for human health in these populations. Overall, this review provides an overview of the current evidence of Se speciation, deficiency, adequacy, and excess and implications for human health and dietary recommendations, with particular reference to Inuit populations in the Canadian Arctic and other coastal populations consuming marine foods.

Selenoneine (SEN) is a natural histidine derivative with radical-scavenging activity and shows higher antioxidant potential than its sulfur-containing isolog ergothioneine (EGT). Recently, the SEN biosynthetic pathway in Variovorax paradoxus was reported. Resembling EGT biosynthesis, the committed step of SEN synthesis is catalyzed by a nonheme Fe-dependent oxygenase termed SenA. This enzyme catalyzes oxidative carbon‑selenium (C-Se) bond formation to conjugate N-α-trimethyl histidine (TMH) and selenosugar to yield selenoxide; the process parallels the EGT biosynthetic route, in which sulfoxide synthases known as EgtB members catalyze the conjugation of TMH and cysteine or γ-glutamylcysteine to afford sulfoxides. Here, we report the crystal structures of SenA and its complex with TMH and thioglucose (SGlc), an analog of selenoglucose (SeGlc) at high resolution. The overall structure of SenA adopts the archetypical fold of EgtB, which comprises a DinB-like domain and an FGE-like domain. While the TMH-binding site is highly conserved to that of EgtB, a various substrate-enzyme interaction network in the selenosugar-binding site of SenA features a number of water-mediated hydrogen bonds. The obtained structural information is beneficial for understanding the mechanism of SenA-mediated C-Se bond formation.

Ergothioneine and selenoneine are structurally related dietary antioxidants and cytoprotectants that may help prevent several chronic diseases associated with inflammation and aging. Both compounds share pharmacokinetic characteristics such as cellular uptake through the ergothioneine transporter, accumulation in red blood cells, and biotransformation to methylated metabolites. A rapid, sensitive, specific, precise, and cost-effective analytical method is required to further investigate the potential health benefits of these compounds, individually or combined, in large epidemiological studies. We developed and validated an isotope-dilution liquid chromatography tandem mass spectrometry (ID-LC-MS/MS) method for the simultaneous specific quantification of these analytes in human blood following a simple sample preparation consisting of dilution in aqueous dithiothreitol followed by centrifugal filtration. Chromatographic separation of the analytes is achieved using a reversed-phase chromatography within an 8-min run. Analyte detection is performed using triple quadrupole mass spectrometry in multiple reaction monitoring mode. Each analyte is quantified against its corresponding isotopically labeled internal standard either commercially available or synthesized in-house (77Se-labeled selenoneine compounds). The validated method demonstrates excellent linearity and very good precision (all CV < 10%). Matrix effects are minimal, suggesting that this method could easily be adapted to other matrices. Freeze/thaw cycles have little effect on methylated metabolites but significantly reduced concentrations of the parent compounds. The method was successfully applied to a small set of volunteer blood samples containing low levels of the analytes. The developed ID-LC-MS/MS method opens new avenues for exploring the roles of these bioactive compounds and their metabolites in human health and disease.

Selenoneine, 2-selenyl-Nα, Nα, Nα-trimethyl-L-histidine, is the major organic selenium compound in marine fish. To characterize biological antioxidant function of selenoneine in fish, the accumulation of selenoneine and other selenium compounds, i. e., sodium selenite and selenomethionine, in the muscle and other tissues of red seabream. We reared red seabream by feeding of 1% dry pellet containing of sodium selenite, selenomethionine, or selenoneine of body weight twice a day for 4 weeks. After that, we replaced to 1% of normal commercial dry pellet of body weight twice a day for 1 week from the selenium supplementation, and tissue distribution of total selenium was determined. Selenium supplementation with selenoneine, selenomethionine, and sodium selenite enhanced selenium accumulation in the white muscle, kidney, and hepatopancreas in comparison with the control group. By the dietary intake of selenoneine, total selenium concentrations were increased in the white muscle, heart, kidney, spleen, hepatopancreas, brain, and blood cells in a dose-dependent manner during the trials after 2 weeks. Dietary intake of selenoneine as well as sodium selenite and selenomethionine reduced oxidation-reduction potential (ORP). Selenoneine concentrations in the white muscle and blood cells were accumulated for 4 weeks by the selenoneine intake, whereas selenoneine concentration was not elevated by the intake of selenomethionine and sodium selenite, suggesting that tissue selenoneine levels might be derived from only selenoneine-containing diet. The uptake factor of selenoneine from the artificial feed containing selenoneine was calculated to be 0.0062 in the white muscle and 4.0 in the blood. The half-life of total selenium in the blood cells and white muscle were estimated to be 60 days in the white muscle and 30 days in the blood.

Selenium (Se), a semi-metallic element, has chemical properties similar to sulfur; however, it has comparatively low electronegativity as well as a large atomic radius than sulfur. These features bestow selenium-containing compounds with extraordinary reactivity, sensitivity, and potential for several applications like chemical alteration, protein engineering, chemical (semi)synthesis, etc. Organoselenium chemistry is emerging fastly, however, examples of effective incorporation of Se into the peptides are relatively scarce. Providentially, there has been a drastic interest in synthesizing and applying selenoproteins and selenium-containing peptides over the last few decades. In this minireview, the synthetic methodologies of selenium-containing peptides and a brief description of their chemistry and biological activities are summarized. These methodologies enable access to various natural and unnatural selenium-containing peptides that have been used in a range of applications, from modulating protein characteristics to structure-activity relationship (SAR) studies for applications in nutraceuticals and drug development. This review aims at the audience interested in learning about the synthesis as well as will open new dimensions for their future research by aiding in the design of biologically interesting selenium-containing peptides.

The history, chemistry, biology, and biosynthesis of the globally occurring histidine-derived alkaloids ergothioneine (10), ovothiol A (11), and selenoneine (12) are reviewed comparatively and their significance to human well-being is discussed.