Selenocysteine Unit Converter
Selenocysteine Unit Converter
Convert selenocysteine between mg/dL and µmol/L at 168.065 Da. The point of the page: free selenocysteine is not what anybody measures for selenium status. That is plasma or serum selenium, or selenoprotein P, and treating the three as one number is a real and repeated error.
Selenocysteine converter
mg/dL ⇄ µmol/LSelenocysteine 1.000 mg/dL in a prepared standard
Formula, conversion factor, and the three things this is not
mg/dL = µmol/L ÷ 59.5008
- 59.5008
- derived from the formula mass of selenocysteine, 168.065 Da: one mg/dL is 0.01 g/L, and 0.01 ÷ 168.065 × 10⁶ = 59.5008 µmol/L
- 168.065
- C3H7NO2Se, summed from the IUPAC 2021 standard atomic weights: carbon 3 × 12.011 = 36.033, hydrogen 7 × 1.008 = 7.056, nitrogen 14.007, oxygen 2 × 15.999 = 31.998, selenium 78.971. PubChem CID 25076 prints 168.05
- not selenium
- elemental selenium has an atomic weight of 78.971 and is the analyte your laboratory actually reports for selenium status, usually in µg/L or µmol/L on plasma or serum. It converts at its own atomic weight, not at 168.065. One µmol of selenocysteine carries one µmol of selenium, so the molar amounts are equal, but the MASSES differ by a factor of 2.13 — 168.065 ÷ 78.971 — and a mass-unit result converted with the wrong one of the two is out by that much
- not selenoprotein P
- the second analyte used for selenium status, and a better index of the functional selenium pool than total selenium at intakes near sufficiency. It is a glycoprotein of roughly 43 kDa reported in mg/L, and it has no fixed grams per mole, so it has no mass-to-molar bridge at all
- not cysteine
- C3H7NO2S, 121.16 Da, factor 82.5355 — the cysteine converter is the clinical page. The two residues are chemically parallel and a selenol is a far stronger reducing agent than a thiol, which is the whole reason selenocysteine sits in the active site of the thioredoxin and glutathione peroxidase families
Worked example
Selenocysteine 1.000 mg/dL in a prepared standard
1.000 × 59.5008 = 59.50 µmol/L
The same 1.000 mg/dL put through cysteine's factor of 82.5355 would read 82.54 µmol/L — 39% too high, and a plausible-looking number, which is what makes the error survive
That 59.50 µmol/L of selenocysteine carries 59.50 µmol/L of selenium, which as a mass is 59.50 × 78.971 = 4,699 µg/L
For scale: a normal plasma SELENIUM is of the order of 70 to 150 µg/L, roughly 0.9 to 1.9 µmol/L. The 4,699 µg/L above is three orders of magnitude higher, which is the clearest possible sign that free selenocysteine at mg/dL concentrations is not a physiological plasma quantity
No reference interval is printed above, and that last step is why
Three different selenium analytes, three different numbers
| Analyte | What it is | Typical unit | Mass-to-molar bridge |
|---|---|---|---|
| Selenium | The element, total, in plasma or serum. The usual first-line test for selenium status | µg/L or µmol/L | Atomic weight 78.971 |
| Selenoprotein P | The main selenium transport protein; an index of the functional pool | mg/L | None — a glycoprotein, no fixed grams per mole |
| Selenocysteine | The 21st amino acid, the residue selenium is delivered as inside selenoproteins | Not clinically reported free | Formula mass 168.065 — this page |
Which selenium analyte is on the report?
Selenocysteine is the 21st proteinogenic amino acid and, unlike pyrrolysine, it is in your own proteins. Roughly twenty-five human selenoproteins carry it, including the glutathione peroxidases, the thioredoxin reductases and the iodothyronine deiodinases. It is inserted at a UGA codon that would otherwise stop translation, read through by a selenocysteine insertion sequence (SECIS) element in the 3′ untranslated region together with a dedicated tRNA and elongation factor. Notably, and unlike every other residue, it is not charged onto its tRNA ready-made: serine is loaded first and then converted to selenocysteine on the tRNA itself.
That biosynthetic detail is the reason the free amino acid is not a clinically useful measurement. There is no free selenocysteine pool being drawn on for protein synthesis, so there is no free concentration that reflects selenium supply. Free selenocysteine released when a selenoprotein is degraded is dismantled by selenocysteine lyase almost immediately, which liberates selenide for re-use. So a laboratory that wanted to assess selenium status by measuring free selenocysteine would be measuring a transient intermediate at the wrong end of the pathway, and none of them do.
What they measure instead is total plasma or serum selenium, by ICP-MS, which is the first-line test and is interpreted against an interval of roughly 70 to 150 µg/L in most laboratories; or selenoprotein P, which plateaus once selenium intake is sufficient and is therefore the better marker of whether the functional pool is full. Erythrocyte or whole-blood glutathione peroxidase activity is a third, functional, option. Any of those three can be called a "selenium level" in a clinic letter, and the numbers are not interchangeable.
The specific error this page exists to head off is converting a selenium result at 168.065 or a selenocysteine result at 78.971. Because one mole of selenocysteine contains exactly one mole of selenium, the two agree perfectly in molar units and disagree by 2.13-fold in mass units — which is the worst possible failure mode, since a molar-to-molar comparison will look correct and reassure the person who then makes the mass-unit mistake. The second error, using cysteine’s 82.5355 because the two residues look alike on paper, inflates a result by 39%. The cysteine converter and the cystine converter are the pages for the sulphur analogues, and methionine converter is next door if the question is really about the transsulphuration pathway.
Frequently asked questions
What is the molecular weight of selenocysteine?
168.065 Da for the free amino acid C3H7NO2Se, summed from the IUPAC 2021 standard atomic weights with selenium at 78.971. PubChem CID 25076 prints 168.05. Cysteine, the sulphur analogue, is 121.16, so substituting selenium for sulphur adds 46.9 Da.
Is selenocysteine the test for selenium deficiency?
No, and this is the commonest misunderstanding about it. Selenium status is assessed on total plasma or serum selenium, usually by ICP-MS and usually reported in µg/L, or on selenoprotein P, or on a functional glutathione peroxidase activity. Free selenocysteine is not offered as a clinical test by any routine or referral laboratory, because it is a transient intermediate rather than a pool that tracks supply.
How do I convert a selenium result to a selenocysteine concentration?
You generally should not, but the arithmetic is simple because the stoichiometry is one to one: one mole of selenocysteine contains one mole of selenium, so the molar concentrations are equal and only the mass differs. A plasma selenium of 100 µg/L is 100 ÷ 78.971 = 1.27 µmol/L of selenium; if every atom of it were in selenocysteine that would be 1.27 µmol/L of selenocysteine, which as a mass is 1.27 × 168.065 = 213 µg/L. The reason not to do this is that plasma selenium is not all in selenocysteine — much of it is in selenoprotein P and albumin-bound selenomethionine — so the converted figure describes no real quantity.
Why is selenocysteine called the 21st amino acid rather than a modified serine?
Because it has its own codon usage, its own tRNA, its own elongation factor and its own insertion machinery, so it is a genuine addition to the genetic code rather than a post-translational modification. The serine involvement is real but happens on the tRNA before the residue is delivered to the ribosome, not on the finished protein. A residue modified after translation, such as hydroxyproline, is not counted.
Does this page carry a reference interval?
No, deliberately. There is no published reference interval for free plasma or serum selenocysteine because it is not a clinical analyte. Printing one would be a fabrication. The intervals worth knowing are for the analytes in the table above, and the figure to look up on your own report is the selenium one.
Related calculators
References
- Labunskyy VM, Hatfield DL, Gladyshev VN. Selenoproteins: molecular pathways and physiological roles. Physiol Rev. 2014;94(3):739–777. doi:10.1152/physrev.00039.2013
- Burk RF, Hill KE. Regulation of selenium metabolism and transport. Annu Rev Nutr. 2015;35:109–134. doi:10.1146/annurev-nutr-071714-034250
- Combs GF Jr. Biomarkers of selenium status. Nutrients. 2015;7(4):2209–2236. doi:10.3390/nu7042209 — on why selenoprotein P plateaus at sufficiency and total selenium does not.
- Xu XM, Carlson BA, Mix H, et al. Biosynthesis of selenocysteine on its tRNA in eukaryotes. PLoS Biol. 2007;5(1):e4. doi:10.1371/journal.pbio.0050004
- PubChem Compound Summary CID 25076, Selenocysteine. National Center for Biotechnology Information. Molecular formula C3H7NO2Se, molecular weight 168.05. Accessed 2026.
- IUPAC Commission on Isotopic Abundances and Atomic Weights. Standard Atomic Weights 2021: Se 78.971, C 12.011, H 1.008, N 14.007, O 15.999, S 32.06.
- data/_factors.json, analyte
selenocysteine: mw 168.065; "selenium status is assessed as plasma or serum SELENIUM (78.971) or as selenoprotein P, and the two must not be conflated."
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.