TB-500 Fragment 1-4

Ac-SDKP circulates in human blood. It is produced by the body's own enzymatic processing of Thymosin Beta-4. Its plasma levels are regulated by a major cardiovascular enzyme. This is not an obscure fragment — it is a physiologically active tetrapeptide with an established role in fibrosis prevention.

Thymosin Beta-4 (Tβ4) is ubiquitously expressed in nearly all mammalian cells and circulates in blood. When the enzyme prolyl oligopeptidase (POP; also called prolyl endopeptidase, PEP) cleaves Tβ4 — specifically between the 4th (Pro) and 5th (Leu) amino acids — it releases the Ac-SDKP tetrapeptide from the N-terminus. A second enzyme, meprin-α, produces an intermediate that POP then processes. The N-terminal acetylation (Ac-) is inherited from the parent Tβ4 and is essential for Ac-SDKP's biological activity. Ac-SDKP has been measured in normal human plasma at approximately 2.4-4.5 nM — it is genuinely circulating at physiologically relevant concentrations.

Once in circulation, Ac-SDKP is selectively degraded by ACE's N-domain active site (the N-domain and C-domain of ACE have different substrate specificities; angiotensin I cleavage is primarily C-domain; Ac-SDKP cleavage is N-domain). This precise regulatory mechanism means Ac-SDKP plasma levels rise 4-5-fold when ACE is inhibited by any ACE inhibitor drug class. The physiological consequence: ACE inhibitors simultaneously lower blood pressure (via angiotensin II reduction) AND raise Ac-SDKP levels. The elevated Ac-SDKP is now recognized as potentially contributing to the organ-protective effects of ACE inhibitor therapy that go beyond blood pressure control.

THE CENTRAL TENSION

Ac-SDKP may be part of the reason ACE inhibitors protect the heart and kidneys in ways that exceed their blood pressure lowering effects. This hypothesis has been building in the cardiovascular research literature for two decades. The direct experimental support: in animal models, the anti-fibrotic effects of ACE inhibitors are attenuated when Ac-SDKP levels are normalized (i.e., when the Ac-SDKP elevation from ACE inhibition is blocked). This implicates Ac-SDKP as a functional mediator of ACE inhibitor benefit. The community is injecting Ac-SDKP to directly deliver the fragment without requiring ACE inhibitor co-administration. The hypothesis is compelling. The human clinical trial evidence for direct Ac-SDKP supplementation is zero.

Peng H, et al. (2010). Ac-SDKP inhibits transforming growth factor-β 1-induced differentiation of human cardiac fibroblasts into myofibroblasts. Am J Physiol Heart Circ Physiol. 298(5):H1357-64. [TGF-β1-driven cardiac fibroblast differentiation inhibited; α-SMA and collagen I reduced; primary human cell culture evidence for cardiac anti-fibrotic mechanism.]

Cavasin MA, et al. (2004). Decreased Ac-SDKP levels caused physiological fibrosis at baseline; ACE inhibitor fibrosis protection partially Ac-SDKP mediated. Am J Physiol Heart Circ Physiol. [Foundational paper establishing Ac-SDKP physiological fibrosis prevention role and its contribution to ACE inhibitor benefit.]

Kumar N, et al. (2016). The anti-inflammatory peptide Ac-SDKP is released from thymosin-β4 by renal meprin-α and prolyl oligopeptidase. Am J Physiol Renal Physiol. 310(10):F1026-F1034. [Biosynthetic pathway: meprin-α and POP cleave Tβ4 to release Ac-SDKP; renal regulation; confirms endogenous production mechanism.]

Rhaleb NE, Yang XP, Carretero OA. (2011). The kallikrein-kinin system as a regulator of cardiovascular and renal function. Compr Physiol. [ACE N-domain substrate specificity; Ac-SDKP as ACE N-domain substrate; physiological regulation; comprehensive review.]

The ACE-Ac-SDKP axis works as follows: ACE has two active sites — N-domain and C-domain — with different substrate specificities. The C-domain primarily cleaves angiotensin I to angiotensin II (the vasoconstriction pathway targeted by ACE inhibitors for hypertension). The N-domain primarily cleaves and inactivates Ac-SDKP. Standard ACE inhibitors (enalapril, lisinopril, ramipril, perindopril) inhibit both domains, raising both angiotensin II reduction (blood pressure effect) AND Ac-SDKP accumulation (anti-fibrotic effect). Domain-selective ACE inhibitors that spared the N-domain would lower blood pressure without raising Ac-SDKP — and have been studied specifically to test whether losing the Ac-SDKP elevation reduces the anti-fibrotic benefit.

The experimental result: N-domain-selective ACE inhibition (preserving C-domain activity for angiotensin I cleavage; preserving N-domain to degrade Ac-SDKP) produces blood pressure lowering but less cardiac and renal fibrosis protection than full ACE inhibition. The anti-fibrotic effect is partially, not fully, explained by Ac-SDKP elevation — the full ACE inhibitor benefit involves multiple mechanisms. But the Ac-SDKP contribution is real and has been quantified in these domain-selective inhibitor experiments.

Community protocol for Ac-SDKP is poorly developed compared to TB-500 Fragment 17-23 (LKKTETQ). Polaris and Limitless carry it. Community use context: cardiac and organ health maintenance; anti-fibrotic support; used occasionally alongside other healing peptides. No established community dose has emerged because the compound is less popularized than LKKTETQ. The anti-fibrotic animal models used doses in the range of 100-500 mcg/kg/day (subcutaneous osmotic pump in some studies), which for a 70 kg human would represent 7-35 mg/day — substantially higher than typical community peptide doses. The relationship between subcutaneous bolus injection (community method) and the continuous infusion osmotic pump delivery used in many animal models is not established.

Ac-SDKP (TB-500 Fragment 1-4) is the anti-fibrotic fragment of Thymosin Beta-4 — a physiologically circulating tetrapeptide that prevents cardiac and renal fibrosis in animal models and may explain part of ACE inhibitors' organ-protective effects.

Evidence grade C (animal) for anti-fibrotic effects; zero human RCT for direct supplementation. The mechanism is the most pharmacologically coherent anti-fibrotic story in the Tβ4 fragment literature. The community use is developing slowly; the compound warrants more attention than it receives relative to its more popular sibling Fragment 17-23.

• Anti-fibrotic application: cardiac and renal fibrosis prevention; mechanistically supported by animal data and ACE inhibitor pharmacology; Grade C evidence.
• ACE inhibitor interaction: ACE inhibitors raise endogenous Ac-SDKP 4-5x; co-administration of exogenous Ac-SDKP with ACE inhibitors may produce additive anti-fibrotic effects but no human data exists.
• vs Fragment 17-23 (LKKTETQ): completely different mechanism; LKKTETQ = actin binding/cell migration; Ac-SDKP = anti-fibrotic/TGF-β inhibition. Different indications, different mechanisms.
• Evidence grade: C (animal; multiple labs; anti-fibrotic); D (in vitro; human cardiac fibroblasts); E (community).

— End of TB-500 Fragment 1-4 (Ac-SDKP) —

THE PEPTIDE BIBLE | TB-500 Fragment 1-4 (Ac-SDKP) | For Research & Educational Purposes Only