BPC-157, CAS 137525-51-0, is a synthetic pentadecapeptide whose sequence is a partial fragment of body protection compound, a protein isolated from gastric juice. It is also catalogued as PL 14736 and as bepecin. TB-500, CAS 885340-08-9, is the N-terminally acetylated heptapeptide Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH, corresponding to residues 17 to 23 of thymosin beta-4, which is the actin binding site of that parent protein. The doping control literature is the clearest published statement of this identity (Ho, 2012). One naming trap is worth stating plainly. Full length thymosin beta-4, CAS 77591-33-4, average mass 4963.44, is a different material sometimes also labelled TB-500. The vial compared here is the heptapeptide.
On size the ordering runs opposite to the usual assumption. BPC-157 is recorded as C62H98N16O22 with an average mass of 1419.53 and the sequence H-Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val-OH, fifteen residues with three consecutive prolines that stiffen the backbone. TB-500 is recorded as C38H68N10O14 with an average mass of 889.02, seven residues, an acetyl cap on the amino terminus and a free acid at the carboxy terminus. BPC-157 is therefore the larger of the two by roughly 530 daltons. Neither formula contains sulfur, so neither sequence carries a methionine or a cysteine.
The pathways under study are different. Published work on BPC-157 in rodent and cell models examines fibroblast migration, focal adhesion signaling, nitric oxide related pathways and angiogenic markers such as VEGFR2 expression. The actin literature attached to TB-500 centres on G-actin sequestration, the regulation of actin polymerisation, and cell motility in culture, and almost all of it was generated with the 43 residue parent protein rather than with the heptapeptide. A repair model that responds to both is not evidence of a shared mechanism.
Manufacture and handling are closer than the reputation suggests. Fourteen couplings for BPC-157 and six for TB-500 are both modest syntheses, both resolve well on reverse phase columns, and high purity at 214 or 220 nanometres is the expectation for each. What differs is the identity check. Mass spectrometry is run against 1419.53 for BPC-157 and against 889.02 for TB-500, and that second figure separates the heptapeptide from the 4963.44 parent protein at a glance. With no methionine or cysteine in either sequence, oxidation is not the handling concern for either material, and moisture is the variable to control. Both are lyophilized powders held at minus 20 degrees Celsius protected from light and moisture, refrigerated at 2 to 8 degrees Celsius once reconstituted, and aliquoted so that freeze thaw is not repeated.
Laboratories choose between them by mechanism rather than by model. Work on signaling cascades and vascular markers uses BPC-157. Work on actin dynamics and cell motility uses the thymosin beta-4 fragment. Work that wants both readouts from one preparation uses the combined vial, labelled as 10 mg of each component. Note that equal masses are not equal moles there. At 1419.53 and 889.02, 10 mg of each is about 7.0 and 11.2 micromoles respectively, so the vial is 1 to 1 by mass and closer to 1 to 1.6 by molarity.
What is unsettled is most of the mechanism. No receptor has been definitively assigned to BPC-157, which makes concentration response interpretation awkward. For TB-500 the open question is the gap between fragment and parent. The heptapeptide carries the binding motif, but affinity equivalent to the intact 43 residue protein has not been demonstrated, and the published endpoints belong to the protein.