TB-500 and Thymosin Beta-4: Actin Sequestration and Fragment Confusion

The naming problem

Thymosin beta-4 (Tβ4) is a defined 43-amino-acid peptide, molecular weight approximately 4963 g/mol, and the principal G-actin sequestering protein in mammalian cells.

Its actin-binding activity localizes largely to a short internal motif, LKKTETQ, spanning residues 17-23. Some suppliers sell a synthetic fragment based on this motif — molecular weight near 889 g/mol — and label it TB-500.

These are different molecules with different molecular weights, different pharmacokinetics, and different published literature. A COA is the only reliable way to determine which one is in the vial. Check the stated molecular weight before comparing results against published work.

Actin sequestration

Tβ4 binds monomeric G-actin in a 1:1 complex, holding it in a polymerization-incompetent state. This buffers the cellular pool of free actin monomer and regulates the equilibrium between monomeric and filamentous actin.

Because actin polymerization dynamics underlie cell motility, the downstream research relevance extends to endothelial cell migration, tube formation assays, and wound-closure models in culture.

Additional reported activities include modulation of inflammatory cytokine expression and, in corneal models, effects on epithelial migration. Corneal work represents one of the better-characterized independent lines of research.

Laboratory handling

Store lyophilized at -20°C protected from light. Reconstituted solution at 2-8°C for approximately 30 days.

Reconstitute with bacteriostatic water directed down the vial wall, swirling gently. Tβ4 is a large, highly charged, intrinsically disordered peptide and dissolves readily; persistent particulate suggests a problem with the material.