TB-500 is named after a seven-amino-acid fragment of thymosin β4. The material supplied under that name, here and almost everywhere else, is the full 43-residue protein — and the molecular weight on the certificate is how you can tell.
The lots in this catalogue report 4964.2 Da. The fragment the name refers to weighs about 963 Da. That is not a rounding difference or an instrument artefact; it is a different molecule, five times the mass. This post explains where the name came from, why the gap exists, how to read the certificate line that settles it, and what the actin-sequestering role actually means.
Two different molecules share one name
| "TB-500" as named | What is supplied | |
|---|---|---|
| Identity | LKKTETQ, residues 17–23 of thymosin β4 | Full-length thymosin β4 |
| Length | 7 residues | 43 residues |
| Approximate mass | ~963 Da | ~4963 Da (acetylated) |
| Origin of the name | The actin-binding region identified in fragment studies | — |
| What the certificates here report | — | 4964.2 Da |
The seven-residue sequence is real and genuinely significant: work by Sosne and colleagues (2010) mapped several of thymosin β4's activities onto short peptide sequences, LKKTETQ among them. That is where the interest in a fragment came from.
But "TB-500" became the commercial name for synthetic thymosin β4, and the fragment largely stayed in the literature. The result is a persistent mismatch between what the name describes and what is in the vial — one that most product pages never mention, because the molecular weight is the only place it shows up.
This is not an allegation of substitution. Full-length thymosin β4 is what buyers of TB-500 generally expect and what the research literature is mostly about. The point is narrower: if you are reading a paper about LKKTETQ and handling a vial at 4964 Da, those are not the same subject.
How to check which one you have
The identity line on the certificate, nothing else. Purity cannot distinguish them — a vial of either would chromatograph cleanly.
| Lot | Size | Purity (RP-HPLC, 214 nm) | Identity method | MW reported |
|---|---|---|---|---|
| TB10-0803 | 10 mg | 99.68% | LC-MS/MS | 4964.2 |
| TB10-0318 | 10 mg | 99.8% | MALDI-MS | 4964.3 |
Both lots land within a dalton of each other and both correspond to the full-length acetylated protein. The 0.1 Da difference between them reflects the two instruments and their calibration conventions, not different material — what LC-MS confirms covers why mass spectrometry is the identity test and why purity is not.
If a certificate for something sold as TB-500 reported a mass near 963 Da, you would have the actual fragment. In practice that is rare enough to be worth a direct question to the supplier.
What thymosin β4 does in a cell
Its main role is structural, and this is the part most summaries get wrong by making it sound like a signalling peptide.
Thymosin β4 is the principal G-actin sequestering protein in mammalian cells. Actin exists in two states: free monomers (G-actin) and polymerised filaments (F-actin). The balance between them governs whether a cell can change shape, migrate, or remodel its cytoskeleton. Thymosin β4 binds monomeric actin one-to-one and holds it in reserve, maintaining a pool that is available but not polymerised.
It is abundant — one of the most plentiful proteins in many cell types — and that abundance is consistent with a buffering role rather than a regulatory one. Goldstein, Hannappel and Kleinman's 2005 review covers how a protein with that structural job came to be studied in tissue repair contexts.
Being 43 residues and highly charged, it has little stable secondary structure in solution. It is best described as intrinsically disordered, folding locally when it binds actin rather than holding a fixed shape on its own.
What the research covers, and what it does not
The animal literature is substantial — cardiac, corneal and dermal injury models predominantly — and the in-vitro work on actin binding is well established and not controversial.
Human clinical evidence is a different matter. Thymosin β4 has been taken into clinical trials, notably in ophthalmology for corneal wound healing, so this is not a compound with no human exposure at all. But there is no approved product in any major jurisdiction, and the trial record does not support general tissue-repair claims. Why trial data does not transfer covers the general problem of reading across from models to people.
Prohibited in sport. WADA lists TB-500 under S2 — peptide hormones, growth factors, related substances and mimetics — prohibited at all times, in and out of competition. That is a straightforward regulatory fact worth knowing if any research context touches sport.
Handling
At 43 residues and strongly acidic, thymosin β4 behaves differently from short peptides:
| Property | What follows |
|---|---|
| Solubility | Readily water-soluble; the high proportion of charged residues helps |
| Structure | Intrinsically disordered, so there is little folded structure to denature |
| Oxidation | One methionine at position 6 is the realistic oxidation site — see oxidation-prone residues |
| Adsorption | Highly charged and relatively large; why peptides stick to plastic applies |
| Freeze-thaw | Aliquot after reconstitution, per aliquoting and freeze-thaw cycles |
| Lyophilized storage | Standard; how to store peptide vials |
The N-terminal acetylation matters for one practical reason: it blocks Edman degradation, so N-terminal sequencing will not read this protein. Mass spectrometry is the identity method, which is what the certificates use. How a peptide sequence is verified covers the alternatives.
Why it is so often blended with BPC-157
The two are supplied together frequently enough that the combination has its own certificate. They are unrelated molecules — no shared sequence, different lengths, different proposed mechanisms — and the pairing is a commercial convention rather than a pharmacological one.
A blend certificate reports each component's mass separately, which is the only way to confirm both are present. What a blend certificate shows explains how to read a two-component report, and peptide blends vs single compounds covers what blending does and does not change analytically.
Frequently asked questions
Is TB-500 the same as thymosin beta-4?
As supplied, in practice, yes — the mass on the certificate is full-length thymosin β4. As named, no: the name refers to a seven-residue fragment. The vial and the name disagree, and the vial is what you have.
Why does my certificate say 4964 and not 963?
Because the material is the full 43-residue protein rather than the LKKTETQ fragment. That is the normal and expected result for anything sold as TB-500.
Does the acetylation matter?
For identity work, yes — it blocks N-terminal sequencing and adds 42 Da to the calculated mass, which is why the figure is near 4964 rather than 4921. For handling, it makes no practical difference.
Is thymosin beta-4 a hormone?
No. "Thymosin" is a historical name from a fraction originally isolated from thymus tissue, not a description of function. Thymosin β4 is an intracellular actin-binding protein. The thymosin α and β families are unrelated to each other despite the shared name — thymosin alpha-1 vs thymalin covers the naming tangle.
Can a purity figure tell me whether I have the fragment or the full protein?
No. Purity is an area percent from the chromatogram and would look the same either way. Only the mass on the identity line distinguishes them. HPLC purity as area percent explains what that figure does measure.
References
- Goldstein AL, Hannappel E, Kleinman HK. Thymosin beta4: actin-sequestering protein moonlights to repair injured tissues. Trends in Molecular Medicine 2005;11(9):421-9. doi.org/10.1016/j.molmed.2005.07.004
- Crockford D, Turjman N, Allan C, Angel J. Thymosin beta4: structure, function, and biological properties supporting current and future clinical applications. Annals of the New York Academy of Sciences 2010;1194:179-89. doi.org/10.1111/j.1749-6632.2010.05492.x
- Sosne G, Qiu P, Goldstein AL, Wheater M. Biological activities of thymosin beta4 defined by active sites in short peptide sequences. FASEB Journal 2010;24(7):2144-51. doi.org/10.1096/fj.09-142307
- World Anti-Doping Agency. The Prohibited List - S2 Peptide Hormones, Growth Factors, Related Substances and Mimetics. www.wada-ama.org/en/prohibited-list
Every product mentioned is sold for laboratory research use only and is not for human or animal use. Nothing on this page describes or recommends use of the material sold here in humans or animals.
