TB-500 vs Ac-SDKP: Two Different Pieces of Thymosin Beta-4

Q: Does the anti-fibrotic research on Ac-SDKP apply to TB-500?

Not directly. The Ac-SDKP fibrosis findings come mostly from rodent studies of Ac-SDKP itself. They may be relevant to full-length thymosin beta-4, which can release Ac-SDKP, but they are not evidence about the TB-500 heptapeptide.

> Research disclaimer: This article summarizes peer-reviewed literature for informational and research purposes only. It is not medical advice. TB-500, thymosin β4 and Ac-SDKP sold online are research chemicals, not FDA-approved drugs. Nothing here is a dosing protocol, an efficacy claim, or a recommendation for human or veterinary use.

Quick answer: Ac-SDKP and TB-500 both come from the same 43-residue protein, thymosin β4 (Tβ4), but they come from opposite ends of the story. Ac-SDKP (N-acetyl-seryl-aspartyl-lysyl-proline) is the first four residues of mature Tβ4. The body cuts it out of the full-length protein in two enzymatic steps, meprin-α first and then prolyl oligopeptidase (Kumar et al., Am J Physiol Renal Physiol, 2016), and angiotensin-converting enzyme (ACE) breaks it down. It was first purified in 1989 as an inhibitor of hematopoietic stem cell proliferation, and most of the later work on it is about fibrosis and inflammation in the heart and kidney. TB-500, as sold, is usually the acetylated heptapeptide Ac-LKKTETQ, which matches residues 17–23. That's the actin-binding region, and it contains no part of the SDKP sequence. A TB-500 fragment therefore cannot be converted into Ac-SDKP, and the Ac-SDKP literature tells you nothing direct about what a TB-500 vial will do.

Why This Comparison Keeps Coming Up

Most Tβ4 review papers mention Ac-SDKP in the same breath as wound healing, angiogenesis and anti-fibrotic effects. That leaves an easy wrong impression: that every Tβ4-derived product, TB-500 included, brings the Ac-SDKP biology along with it. Our pulmonary fibrosis review already flags the issue, because several lung studies tested Ac-SDKP rather than TB-500. This page treats the two molecules side by side, so you can tell which literature applies to which material.

Where Each Fragment Sits in Thymosin β4

UniProt entry P62328 gives the human Tβ4 sequence. Once the initiator methionine is removed, the mature chain starts S-D-K-P, and the serine at position 1 is N-acetylated. The 17–23 stretch, L-K-K-T-E-T-Q, contains the LKKTET motif that binds G-actin. Our amino acid sequence and structure guide walks through the full chain.

| Property | Ac-SDKP | TB-500 (typical research SKU) | Full-length Tβ4 |
| --- | --- | --- | --- |
| Sequence | Ac-Ser-Asp-Lys-Pro | Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln | 43 residues, Ac-SDKP…ES |
| Position in mature Tβ4 | Residues 1–4 (N-terminus) | Residues 17–23 (actin-binding region) | Whole chain |
| Approximate mass | Mr 487 (Lenfant et al., 1989) | ≈ 889 Da (PubChem CID 62707662) | ≈ 4,963 Da (PubChem CID 16132341) |
| How it arises in the body | Released from Tβ4 by meprin-α, then prolyl oligopeptidase | Not described as a natural cleavage product. It's a synthetic design | Expressed directly (gene TMSB4X) |
| Main breakdown enzyme | ACE, chiefly its N-terminal domain | Not well characterized | Not well characterized |
| Older names | Goralatide, seraspenide | "TB-500" is a trade label, not an INN | Timbetasin (INN) |
| Best-known research themes | Stem cell quiescence; cardiac and renal fibrosis; inflammation | Actin sequestration; cell migration (by inference from Tβ4) | Wound repair, angiogenesis, cardiac repair models |

There are two practical points in that table. First, the "TB-500" label covers different things depending on the supplier. Some sell the heptapeptide and some sell full-length Tβ4, and the fragment vs full-length guide explains how to tell them apart on a COA. Second, only full-length Tβ4 contains the SDKP sequence, so only full-length material can even in principle feed the Ac-SDKP pathway.

How Ac-SDKP Is Made: Meprin-α, Then Prolyl Oligopeptidase

For years researchers knew Ac-SDKP matched the start of Tβ4 but couldn't explain how it was released. Prolyl oligopeptidase (POP) cuts after proline, which fits the SDKP-P bond, but POP only works on peptides shorter than about 30 residues, and Tβ4 has 43. Kumar and colleagues (American Journal of Physiology – Renal Physiology, 2016; PubMed 26962108) found the missing first step:

Incubating Tβ4 with both meprin-α and POP released Ac-SDKP. Neither enzyme did it alone.

Rat kidney homogenates released Ac-SDKP from Tβ4, and the meprin-α inhibitor actinonin blocked that release.

Kidneys from meprin-α knockout mice had lower baseline Ac-SDKP and couldn't release it from added Tβ4.

In rats, the ACE inhibitor captopril raised plasma Ac-SDKP from 3.1 to 15.1 nmol/L. Adding actinonin cut that to 6.1 nmol/L.

So the sequence is meprin-α trimming full-length Tβ4 into shorter N-terminal intermediates, then POP releasing the tetrapeptide. For anyone comparing products this settles the TB-500 question. The heptapeptide doesn't start with SDKP, so neither enzyme can make Ac-SDKP from it.

How Ac-SDKP Is Broken Down: The ACE Connection

The other half of the pathway is degradation, and that's where Ac-SDKP links up with mainstream cardiovascular pharmacology. Reviews from the Rhaleb and Carretero group describe Ac-SDKP as degraded almost entirely by the N-terminal catalytic domain of ACE. ACE inhibitors therefore raise circulating Ac-SDKP, and human studies measured this directly:

Azizi and colleagues reported that acute captopril in healthy volunteers raised plasma Ac-SDKP about 5.5-fold for a short time (summarized in Hypertension 1997;30(5):1015).

In the same 1997 paper, hypertensive patients who had been on an ACE inhibitor for more than a month had a geometric mean plasma Ac-SDKP of 3.78 pmol/mL, against 0.75 pmol/mL in untreated controls, with no overlap between groups. The authors proposed Ac-SDKP as a marker of ACE-inhibitor adherence and said the long-term consequences of the build-up were unknown.

A 1999 Hypertension study (33(3):879) found plasma Ac-SDKP about 4-fold higher after a single captopril dose and about 4.8-fold higher after a week of treatment. In patients with chronic renal failure on ACE inhibitors, levels were 22 times higher than in renal-failure controls not taking them.

Why this matters for TB-500 research: in animal and cell work on full-length Tβ4, ACE inhibition is a confounder that's easy to miss. If a model uses captopril or another ACE inhibitor, or a comparison group happens to be on one, endogenous Ac-SDKP will be raised regardless of the peptide being tested. Several groups have used that deliberately. Others may have done it by accident. It's also why one sentence in our medications interaction overview needs a qualifier. ACE inhibitors have no known direct interaction with the TB-500 fragment, but they do change the metabolism of a peptide derived from full-length Tβ4. That is a pharmacology note for study design. It isn't a clinical recommendation.

Ac-SDKP's Original Job: Keeping Stem Cells Quiet

Ac-SDKP was not found by people studying wound healing. Lenfant and colleagues (PNAS, 1989;86(3):779–782; PubMed 2915977) purified it from fetal calf bone marrow as an inhibitor of pluripotent hematopoietic stem cell (CFU-S) proliferation. They identified its structure by amino acid analysis, fast-atom-bombardment mass spectrometry and NMR, and confirmed it with a synthetic copy. In mice, the synthetic peptide stopped CFU-S from entering DNA synthesis after a cytosine arabinoside challenge. Later work described the effect as blocking stimulated stem cells from being recruited into S phase rather than suppressing committed progenitors.

That property led to a clinical idea. If stem cells could be held out of the cycle during chemotherapy, cycle-specific drugs might spare them. Under the names seraspenide and goralatide, Ac-SDKP went into early human testing. A French phase I–II study published in 1992 (PubMed 1300237) reported, in its title and abstract, that the peptide protected against the blood toxicity of cytarabine and ifosfamide monotherapy in a small randomized crossover design. It was never approved, and the human evidence stayed at that early stage.

None of that work involved TB-500, and none of it supports any claim about TB-500. It does explain why some peptide catalogues describe Ac-SDKP as "hemoregulatory" while describing TB-500 as a "recovery" peptide. They really are different research stories.

The Anti-Fibrotic Literature

Most of the modern Ac-SDKP research is about fibrosis, and it's mostly from animal models:

Cardiac fibroblasts in culture. Rhaleb et al. (Hypertension, 2001;37(3):827) reported that Ac-SDKP inhibited DNA and collagen synthesis in rat cardiac fibroblasts.

Renovascular hypertension. In two-kidney, one-clip hypertensive rats, long-term Ac-SDKP prevented left-ventricular interstitial collagen deposition and reduced monocyte/macrophage infiltration, without lowering blood pressure or reversing hypertrophy. A follow-up study was titled "Ac-SDKP reverses cardiac fibrosis in rats with renovascular hypertension" (Hypertension, 2004; PubMed 14581293).

Angiotensin II hypertension. The same group later reported reduced cardiac collagen cross-linking and inflammation in angiotensin II–induced hypertensive rats.

Diabetic kidney disease. A 2014 review in Frontiers in Pharmacology argued that Ac-SDKP is an endogenous anti-fibrotic peptide worth studying in diabetic kidney fibrosis, and it discussed links to the ACE-inhibitor literature.

Lung. Bleomycin-model work is summarized in our pulmonary fibrosis article.

A review in the Canadian Journal of Physiology and Pharmacology (2019) ("Tβ4–Ac-SDKP pathway: Any relevance for the cardiovascular system?") raises a point that matters here. Tβ4's effects in these models may be direct, through a proposed receptor (Ku80) or actin biology, or indirect, through Ac-SDKP released from it. The authors note that Ac-SDKP receptors haven't been fully identified or cloned, and that Ac-SDKP's role in human patients isn't established. That direct-versus-indirect question applies to full-length Tβ4. It doesn't apply to the TB-500 heptapeptide, because nothing indirect can happen with it. Its only possible mechanisms are direct ones.

For the TB-500 and Tβ4 side of the cardiac and renal picture, see our cardiac research review and kidney research review.

> Sourcing note (mid-article): If your protocol depends on which part of Tβ4 you're studying, the identity data on the certificate of analysis matters more than the product name. Check the reported mass against the table above, about 889 Da for Ac-LKKTETQ or about 4,963 Da for full-length Tβ4, before you assume which literature applies. Our TB-500 buying guide and third-party testing guide explain how to read that paperwork.

Side-by-Side: Which Literature Applies to Your Material?

| If your vial contains… | Ac-SDKP literature applies? | Notes |
| --- | --- | --- |
| Ac-LKKTETQ ("TB-500" heptapeptide) | No | No SDKP sequence, so no meprin-α/POP pathway. Evidence on the fragment itself is thin. Most "TB-500" papers actually used full-length Tβ4. |
| Full-length Tβ4 (43 aa) | Possibly, indirectly | Ac-SDKP can be released in vivo. Separating direct Tβ4 effects from Ac-SDKP effects takes deliberate controls such as actinonin, POP inhibition or anti-Ac-SDKP antibodies, as used in the cited animal studies. |
| Synthetic Ac-SDKP | Yes | A separate research chemical with its own short half-life, set by ACE degradation. Not a TB-500 product. |

Research-Design Implications

1. Name the molecule, not the brand. Methods sections should say "Ac-LKKTETQ," "recombinant Tβ4" or "Ac-SDKP," not "TB-500."
2. Account for ACE inhibition. In any in vivo Tβ4 model, record whether animals or comparison groups received ACE inhibitors. They raise endogenous Ac-SDKP on their own.
3. Don't borrow Ac-SDKP findings for the fragment. Anti-fibrotic or stem-cell findings for Ac-SDKP aren't evidence for Ac-LKKTETQ.
4. Remember the stem-cell biology. Ac-SDKP's first described role was keeping hematopoietic stem cells out of the cycle. Any Tβ4 study measuring hematologic endpoints should keep that in mind.
5. Expect conflicting secondary sources. Vendor pages and forums regularly merge the two peptides. The primary papers cited here are the reliable reference.

Frequently Asked Questions

Is Ac-SDKP the same thing as TB-500?

No. Ac-SDKP is the N-terminal tetrapeptide (residues 1–4) of thymosin β4. The TB-500 heptapeptide Ac-LKKTETQ corresponds to residues 17–23, the actin-binding region. They share a parent protein and nothing else in sequence.

Can TB-500 turn into Ac-SDKP in the body?

Not the heptapeptide form. Ac-SDKP is released from full-length thymosin β4 by meprin-α followed by prolyl oligopeptidase, and that requires the SDKP sequence at the N-terminus, which the 17–23 fragment doesn't contain. Full-length Tβ4 can be processed this way.

Why do ACE inhibitors raise Ac-SDKP levels?

ACE, mainly its N-terminal domain, is the main enzyme that breaks Ac-SDKP down. Human studies found plasma Ac-SDKP about 4- to 5.5-fold higher after captopril and much higher in renal-failure patients on ACE inhibitors. That makes ACE inhibition a confounder in Tβ4 research.

What was Ac-SDKP originally studied for?

It was purified in 1989 from fetal calf bone marrow as an inhibitor of hematopoietic stem cell proliferation. As seraspenide and goralatide it went into small early-phase studies as a possible way to protect bone marrow during chemotherapy, and it was never approved.

Does the anti-fibrotic research on Ac-SDKP apply to TB-500?

Not directly. The Ac-SDKP findings in heart, kidney and lung fibrosis come mostly from rodent studies of Ac-SDKP itself. They may be relevant to full-length Tβ4, which can release Ac-SDKP, but they aren't evidence about the TB-500 heptapeptide.

Does this site recommend Ac-SDKP or TB-500 for any use?

No. Both are research chemicals. This page compares their chemistry and published literature and gives no dosing, efficacy or human-use guidance.

Before You Source a Lot

If your work is about the actin-binding fragment, buy the fragment and check its identity data. If it's about Tβ4 biology, including anything that touches the Ac-SDKP pathway, you need full-length material. Apollo Peptide Sciences publishes third-party COA documentation for its TB-500 lots. Compare the reported mass and sequence against the table above so you know which molecule you have before you choose your controls. The peptide buying guide has the broader sourcing checklist.

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Sources: Lenfant M. et al., "Inhibitor of hematopoietic pluripotent stem cell proliferation: purification and determination of its structure," PNAS 1989;86(3):779–782 (PMID 2915977); Kumar N. et al., "The anti-inflammatory peptide Ac-SDKP is released from thymosin-β4 by renal meprin-α and prolyl oligopeptidase," Am J Physiol Renal Physiol 2016 (PMID 26962108); Azizi M. et al., "High plasma level of N-acetyl-seryl-aspartyl-lysyl-proline," Hypertension 1997;30(5):1015 (doi:10.1161/01.HYP.30.5.1015); Azizi M. et al., "Renal and metabolic clearance of AcSDKP during ACE inhibition in humans," Hypertension 1999;33(3):879 (doi:10.1161/01.hyp.33.3.879); Rhaleb N.-E. et al., "Effect of N-acetyl-seryl-aspartyl-lysyl-proline on DNA and collagen synthesis in rat cardiac fibroblasts," Hypertension 2001;37(3):827; Rhaleb N.-E. et al., long-term Ac-SDKP and LV collagen in 2K-1C hypertensive rats (Europe PMC PMC4679287); Yang F. et al., "Ac-SDKP reverses cardiac fibrosis in rats with renovascular hypertension," Hypertension 2004 (PMID 14581293); Ac-SDKP cardiac collagen cross-linking in angiotensin II hypertension (PMC4240015); seraspenide phase I–II trial (PMID 1300237); "N-acetyl-seryl-aspartyl-lysyl-proline: a valuable endogenous anti-fibrotic peptide for combating kidney fibrosis in diabetes," Front Pharmacol 2014 (doi:10.3389/fphar.2014.00070); "Tβ4–Ac-SDKP pathway: Any relevance for the cardiovascular system?" Can J Physiol Pharmacol 2019 (doi:10.1139/cjpp-2018-0570); UniProt P62328; PubChem CIDs 16132341, 62707662.