TB-500 and thymosin β4: a 7-residue fragment is not a 43-residue protein

BioLabs Research

By BioLabs Research · Last reviewed: October 10, 2026

Storefronts often use "TB-500" and "thymosin beta-4" as if they were the same thing. Analytically, they are not. Thymosin β4 (Tβ4) is a 43-residue, N-terminally acetylated peptide of about 4963 Da. In the analytical chemistry literature, the material sold as "TB-500" has been identified as Ac-LKKTETQ: the N-acetylated fragment matching residues 17–23 of Tβ4 (Esposito et al., 2012). This page explains the difference, gives high-level laboratory context, and lists what a lot certificate for the TB-500 catalog listing should show.

Identity at a glance

The mass difference is more than fivefold, so a mass spectrometry result on the lot certificate settles quickly which molecule is in the vial. If a "TB-500" certificate reports a mass near 4963 Da, it describes full-length Tβ4. If it reports a mass near 889 Da, it describes the fragment. A label that does not say which one is incomplete.

Where the fragment sits in the full sequence

UniProt lists the human Tβ4 precursor as 44 residues, starting with an initiator methionine that is removed in the mature 43-residue peptide. In mature numbering, the stretch LKKTETQ runs from residue 17 to residue 23. The fragment is therefore a short excerpt of the protein, not a shortened version of the whole: it keeps seven residues and drops the other thirty-six, along with their mass.

What thymosin β4 is studied for in the laboratory literature

Tβ4's best-characterized biochemical role is binding monomeric G-actin. In 1991 Safer and colleagues showed that Tβ4 is the same molecule as "Fx", a previously described actin-sequestering peptide (Safer et al., 1991). Since then, Tβ4 has been a standard subject in cytoskeleton research.

At a high level, laboratory literature uses Tβ4 and related sequences to study:

• actin monomer sequestration and filament assembly in purified-protein assays;
• cytoskeletal organization in cultured cells;
• structure–activity questions about which part of the 43-residue sequence carries the actin-binding motif.

These are research questions about molecular mechanism.

Why the fragment's analytical record matters

The fragment's identity was worked out by analytical laboratories. Esposito and colleagues synthesized the acetylated 17–23 fragment as a reference material and characterized it (Esposito et al., 2012). We cite that work only because it documents what the "TB-500" name refers to chemically. For a buyer, the consequence is simple: the name alone does not define the material, and the certificate has to. In practice that means a TB-500 certificate should print the seven-residue sequence with its acetyl group, the expected mass for that sequence, and the observed mass, so anyone reading it can see which molecule was tested without relying on the catalog name.

Quality markers for the fragment

A useful TB-500 lot certificate includes:

• lot number matching the vial;
• intact mass by mass spectrometry, compared with ≈889 Da for Ac-LKKTETQ;
• acetylation check: the N-acetyl group adds about 42 Da, so a peak about 42 Da lower points to non-acetylated fragment;
• HPLC chromatogram with method ID, column and detection wavelength;
• counter-ion and water statement. With two lysines, the fragment is basic and usually carries counter-ions. Area percent is not net peptide content (see HPLC area percent vs net peptide content).

If the COA prints a registry number, check that it belongs to the fragment and not to full-length Tβ4. Our guide on how to read lot documents explains where each of these fields sits on a certificate, and the COA library shows complete examples.

Handling and storage as laboratory material

• Store sealed lyophilized vials cold, dry and away from light, at the temperature the lot paper and the safety data sheet state.
• Limit humidity and repeated warming. Moisture uptake and temperature changes are general degradation risks for peptides.
• Log receipt date, storage location and lot number against the certificate.
• Follow institutional chemical-hygiene and disposal rules.

Fragment, full length, or blend?

This catalog also lists a two-component blend. A blend certificate must identify each component separately, with its own mass and its own registry entry. See BPC-157 / TB-500 blend: two sequences, two masses.

Receiving checklist for a TB-500 lot

Before a vial goes into stock, confirm:

1. The carton name, the order and the invoice all name the same catalog listing.
2. The lot number on the vial appears on the certificate.
3. The certificate says whether the material is the Ac-LKKTETQ fragment or full-length thymosin β4.
4. The observed mass matches that choice: about 889 Da or about 4963 Da.
5. The acetylation state is confirmed, and the HPLC method is identified.
6. The vial is sealed, dry and intact.

If the certificate leaves the fragment-or-protein question open, record the lot as "identity unresolved" and ask for a revised certificate.

Next step: Request the lot certificate for the TB-500 listing before you order.

Related reading

• TB-500 vs thymosin β4: which sequence is on the vial
• BPC-157 / TB-500 blend: two sequences, two masses
• What is TB-500?

References

1. Safer D, Elzinga M, Nachmias VT. Thymosin beta 4 and Fx, an actin-sequestering peptide, are indistinguishable. J Biol Chem. 1991;266(7):4029–4032. https://pubmed.ncbi.nlm.nih.gov/1999398/
2. Esposito S, Deventer K, Goeman J, et al. Drug Test Anal. 2012;4(9):733–738. https://pubmed.ncbi.nlm.nih.gov/22962027/
3. UniProtKB P62328 (TYB4_HUMAN), Thymosin beta-4. https://www.uniprot.org/uniprotkb/P62328/entry
4. PubChem Compound Summaries, CID 45382195 (Thymosin beta-4: C212H350N56O78S, MW 4963) and CID 62707662 (Ac-LKKTETQ: C38H68N10O14, MW 889.0). https://pubchem.ncbi.nlm.nih.gov/compound/45382195 · https://pubchem.ncbi.nlm.nih.gov/compound/62707662

For research use only. Not for human or veterinary use.