What This Page Covers
From the UGA Codon to a Finished Selenoprotein
Unlike iron or zinc, selenium does not attach itself to a finished protein as a loosely bound ion. It is built into the chain during protein synthesis itself, as part of an amino acid. This explains why selenium status has a direct bearing on how much of a given selenoprotein a cell is able to make.
The SECIS Element as a Reading Instruction
Ribosomes normally halt translation as soon as they reach the codon UGA. In selenoprotein genes, the untranslated tail of the messenger RNA holds a sequence folded into a loop, the SECIS element. A protein that binds to it (SBP2) and a specialized elongation factor bring in a transfer RNA loaded with selenocysteine. The ribosome then inserts this building block at the UGA position and keeps reading.
Selenocysteine itself is not picked up ready-made from food. The cell assembles it directly on its transfer RNA: serine is attached there first and is then converted in several steps with the help of selenophosphate.
An Order of Priority When Supply Runs Low
When the available selenium is not enough for every selenoprotein, their levels do not drop evenly. The messenger RNA of some members, glutathione peroxidase 1 for instance, is broken down faster during a deficiency. Others, among them glutathione peroxidase 4, hold up for comparatively long. Researchers call this the selenoprotein hierarchy.
| Protein | Location in the cell | Documented task |
|---|---|---|
| Selenoprotein K (SELENOK) | Membrane of the endoplasmic reticulum | Palmitoylation of the IP3 receptor together with DHHC6; calcium release |
| Glutathione peroxidase 4 (GPx4) | Cytosol, mitochondria, close to membranes | Conversion of lipid hydroperoxides in membranes; survival of activated T cells in animal models |
| Glutathione peroxidase 1 (GPx1) | Cytosol | Breakdown of hydrogen peroxide, which immune cells also produce as a signaling molecule |
| Thioredoxin reductases (TXNRD1, TXNRD2) | Cytosol and mitochondria, respectively | Return of thioredoxin to its reduced form; influence on redox-dependent signaling proteins |
| Selenoprotein P (SELENOP) | Blood plasma, released by the liver | Distribution of selenium to tissues, including cells of the immune system |
Compiled from review articles on selenoprotein biology, including Labunskyy, Hatfield & Gladyshev, Physiological Reviews 94 (2014); Avery & Hoffmann, Nutrients 10 (2018).
Calcium Signals and Selenoprotein K
When a T lymphocyte picks up a matching signal through its receptor, the calcium concentration inside it climbs within seconds. This signal has a say in which genes the cell goes on to read. One important step is the release of calcium from the endoplasmic reticulum through the IP3 receptor.
Palmitoylation of the IP3 Receptor
Selenoprotein K has no enzymatic activity of its own in the classic sense. It acts as a cofactor for the enzyme DHHC6, which links a palmitic acid chain to the IP3 receptor. Only with this fatty acid in place does the receptor sit firmly in the membrane and form working channel complexes. Fredericks and colleagues described this relationship (PNAS, 2014).
What Mouse Models Without Selenoprotein K Revealed
Verma and colleagues (Journal of Immunology, 2011) studied mice in which the gene for selenoprotein K had been knocked out. After stimulation, T cells, neutrophils and macrophages from these animals showed a smaller calcium influx; T cells multiplied less, and neutrophils migrated in a less directed way.
Findings of this kind come from genetically modified animals or from cell cultures. They describe what a selenoprotein is fundamentally needed for, and they cannot be read as a statement about what a food or a supplement does in humans.
Selenium contributes to the normal function of the immune system.
EU-authorized wording · Regulation (EU) No 432/2012Membrane Lipids of Dividing Lymphocytes
An activated T cell can divide many times over within a few days. Every daughter cell needs new membranes, and their polyunsaturated fatty acids react readily with oxygen. The result is lipid hydroperoxides, which can spread through the membrane unless they are converted.
What Sets Glutathione Peroxidase 4 Apart
Of the eight human glutathione peroxidases, five contain selenocysteine. GPx4 is the only one of them able to reduce hydroperoxides directly within complex membrane lipids, without those lipids first being pulled out of the membrane. Glutathione acts as the electron donor.
Ferroptosis in T Cells Lacking GPx4
Matsushita and colleagues (Journal of Experimental Medicine, 2015) deleted GPx4 specifically from mouse T cells. The cells developed normally at first but perished after activation. The cell death could be traced to the iron-dependent mechanism of ferroptosis. A paper by Yao and colleagues (Nature Immunology, 2021) described a similar pattern in follicular helper T cells, a subgroup that works alongside B cells in the lymph nodes.
Thioredoxin Reductases as a Second Selenoprotein System
Alongside glutathione, cells rely on the small protein thioredoxin to reopen disulfide bridges in other proteins. The thioredoxin reductases, which restore thioredoxin after each cycle, carry selenocysteine at the end of their chain. In lymphocytes they have been described, among other roles, as providing building blocks for DNA synthesis during cell division.
Keeping the Claim and the Research Apart
For selenium in connection with the immune system, the European Commission has authorized exactly one wording: “Selenium contributes to the normal function of the immune system.” EU-authorized wording · Regulation (EU) No 432/2012.
The accounts of selenoprotein K, GPx4 and the thioredoxin reductases lay out the biological background to this wording. They do not widen the authorized claim.
Selenoprotein P, Intake and Reference Values
The selenium that immune cells in lymph nodes, spleen or bone marrow build into their proteins reaches them mainly through blood plasma. The liver takes up absorbed selenium, incorporates it into selenoprotein P and releases that protein. With up to ten selenocysteine residues per molecule, it is unusually rich in selenium.
Uptake Through Receptors on Target Cells
Cells recognize selenoprotein P through receptors belonging to the lipoprotein receptor family; ApoER2 (LRP8) and megalin (LRP2) are the best described. Once taken up, the protein is broken down, and the freed selenium is available for the cell’s own selenoprotein synthesis. Researchers regard the plasma concentration of selenoprotein P as one marker of selenium status.
Reference Figures and the Upper Limit
For nutrition labeling in the EU, Regulation (EU) No 1169/2011 sets a reference intake of 55 micrograms of selenium per day. The European Food Safety Authority (EFSA) derived an adequate intake of 70 micrograms per day for adults in 2014, and in 2023 it set a tolerable upper intake level of 255 micrograms per day for adults. The gap between requirement and upper limit is therefore only about three- to fourfold.
| Dish or ingredient | Micrograms per 100 g, rough range |
|---|---|
| Tuna, cooked | about 60–110 |
| Chicken egg | about 25–35 |
| Lentils, dried | depends heavily on the soil, often below 10 |
| Para nut | highly variable, from under 100 to well over 1,000 |
Figures vary with origin, because plants draw selenium from the soil. For that reason, a single para nut can already contain a large share of the upper limit.
Misreadings Around the Claim
Does More Selenium Mean More Benefit?
No. The wording concerns normal functioning when supply is adequate. No extra effect above requirements can be read into it, and because the upper limit sits so close, a high selenium intake is expressly undesirable.
Why Is Selenocysteine Absent From Many Tables of the Twenty Amino Acids?
The classic list covers the amino acids for which the genetic code provides a dedicated, unambiguous codon. Selenocysteine relies on a reinterpreted stop codon and is inserted only with the help of the SECIS element. For that reason, biochemistry usually lists it separately as the twenty-first proteinogenic amino acid.
Can the Mouse Findings Be Applied Directly to People?
The mouse models described here work with genes that have been switched off entirely. A state like that never arises from an ordinary diet. The studies are valuable for understanding what a protein does inside the cell, but they do not answer how a particular intake plays out in people.
Who Should Check With a Professional Before Taking a Selenium Product?
As a rule, anyone who already uses a product containing selenium, takes medication, is pregnant or breastfeeding, or has a pre-existing condition. The pocket guide explains how things fit together; it cannot stand in for a personal assessment by a physician or pharmacist.