Thymosin β4 and TB-500: Why the Fragment Is Not the Protein
TB-500 and thymosin β4 are routinely used as if they were interchangeable names for one substance. They are not, and conflating them makes a large body of literature harder to read than it needs to be.
The protein
Thymosin β4 is a 43-residue protein and one of the most abundant intracellular proteins in many cell types. Its best-characterised function is sequestering monomeric G-actin, which places it at the centre of actin cytoskeletal dynamics. Because actin polymerisation underlies cell migration, the protein appears throughout the cell-motility literature. See PubMed.
The fragment
TB-500 is a synthetic peptide corresponding to the actin-binding region of thymosin β4 — a short sequence containing the motif responsible for G-actin binding, not the full protein. The rationale for using the fragment is straightforward: it is far cheaper to synthesise than a 43-residue protein and it retains the actin-binding determinant.
What it does not retain is everything else. Thymosin β4 has reported activities that are not attributed to the actin-binding motif alone, and a fragment cannot be assumed to reproduce them. When a paper reports an effect of thymosin β4, that finding does not automatically transfer to TB-500, and the reverse is equally true.
Why this matters when reading papers
Search results for either term will return studies on both. Before citing a result, check which molecule was actually used and at what concentration — molar equivalence between a 43-residue protein and a short fragment is not the same as mass equivalence, and studies sometimes report one where the other is meant.
Analytical note
Because TB-500 is a defined short sequence, mass spectrometry gives an unambiguous identity confirmation. That is worth checking on a certificate: several actin-binding-region fragments of differing length are in circulation under the same trade name, and they are not the same molecule. Our note on why certificates disagree covers what else to look for.
Products: TB-500 5mg, TB-500 10mg, TB-500 and BPC-157 blend.
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