Humanin vs MOTS-c vs SS-31: Three Mitochondria-Focused Peptides Compared

Research-use-only context. This article summarizes published third-party scientific literature and laboratory practice. It is not medical advice, not a therapeutic or performance claim, and not a usage guide. US Prime Peptides products are supplied strictly for in vitro laboratory research and are not for human or veterinary use.

Humanin, MOTS-c and SS-31 are all filed under "mitochondrial peptides," but two of them are encoded by the mitochondrial genome and one is a synthetic tetrapeptide that merely targets the mitochondrion. This article compares the three by origin, sequence length, molecular target class, chemical modification, regulatory status and literature maturity, and explains why one of the three is now the name of an approved drug.

The short answer: two mitochondrial-derived peptides and one cardiolipin binder

Humanin is a 24-residue peptide encoded by a short open reading frame inside the mitochondrial 16S ribosomal RNA gene. MOTS-c is a 16-residue peptide encoded by a short open reading frame inside the mitochondrial 12S ribosomal RNA gene. Both are natural products of the human mitochondrial genome, discovered in 2001 and 2015 respectively. SS-31 is D-Arg-Dmt-Lys-Phe-NH₂, a four-residue synthetic peptide designed in the Szeto and Schiller laboratories that concentrates in the inner mitochondrial membrane and binds cardiolipin. It has no genomic origin.

Neither humanin nor MOTS-c is an approved drug. SS-31, under the name elamipretide, is approved by the FDA as Forzinity (accelerated approval, September 2025). A research vial of SS-31 is not that product. The compound-level articles on humanin, MOTS-c and SS-31 cover each in depth.

Humanin: a 24-mer from the 16S rRNA gene

Humanin's sequence is Met-Ala-Pro-Arg-Gly-Phe-Ser-Cys-Leu-Leu-Leu-Leu-Thr-Ser-Glu-Ile-Asp-Leu-Pro-Val-Lys-Arg-Arg-Ala, average mass about 2,687 Da. It was found by functional expression screening: a 2001 Proceedings of the National Academy of Sciences paper by Hashimoto, Nishimoto and colleagues identified a cDNA from surviving tissue of an Alzheimer's brain that, when transfected into neuronal cells, abolished death caused by familial Alzheimer's disease genes and amyloid beta, and showed the secreted product was a short polypeptide whose rescue activity depended on primary structure. The sequence was then matched to the mitochondrial 16S rRNA gene. A 2003 Nature paper by Guo, Reed and colleagues reported that humanin binds Bax and prevents its translocation from cytosol to mitochondria in cell and isolated-mitochondria assays, and that reducing humanin expression by siRNA sensitized cells to Bax.

Humanin is unusual in having candidate receptors. A 2004 Journal of Immunology paper by Ying and colleagues reported that synthetic humanin induced chemotaxis of mononuclear phagocytes through the G-protein-coupled formyl peptide receptor-like 1 (FPRL1, now FPR2), and a 2004 Biochemical and Biophysical Research Communications paper by Harada and colleagues reported that humanin and its N-formylated form activated both FPRL1 and FPRL2 in transfected CHO cells, with an EC₅₀ of 3.5 nM for the unformylated peptide at FPRL1. A separate trimeric receptor built from gp130, the IL-27 receptor subunit WSX-1 and the CNTF receptor alpha subunit has been proposed from neuronal-cell work. The 2022 Frontiers in Cell and Developmental Biology review by Zhu and colleagues summarizes both receptor hypotheses and notes that the nuclear genome contains thirteen humanin-like paralogs (MTRNR2L1 to MTRNR2L13) of 24 to 28 residues, which complicates attribution of any measured humanin to the mitochondrial gene.

MOTS-c: a 16-mer from the 12S rRNA gene

MOTS-c is Met-Arg-Trp-Gln-Glu-Met-Gly-Tyr-Ile-Phe-Tyr-Pro-Arg-Lys-Leu-Arg, average mass about 2,174 Da. The 2015 Cell Metabolism paper by Lee, Cohen and colleagues that reported it found the open reading frame within the mitochondrial 12S rRNA, showed the peptide was present in cells and in circulation, and traced its cellular action to inhibition of the folate cycle and de novo purine synthesis with consequent AMP-activated protein kinase activation, with skeletal muscle as the apparent primary target tissue in mice. A 2018 Cell Metabolism paper by Kim, Lee and colleagues then showed that under glucose restriction MOTS-c translocates to the nucleus in an AMPK-dependent manner and regulates nuclear genes carrying antioxidant response elements, interacting with the transcription factor NRF2. The 2020 review by Merry and colleagues in the American Journal of Physiology lists MOTS-c alongside humanin and the six small humanin-like peptides as the eight mitochondrial-derived peptides identified to date.

MOTS-c has no identified cell-surface receptor. Its proposed mechanism is intracellular and metabolic. Human data consist of circulating-level measurements and a small number of early clinical studies of analogs; nothing has reached approval.

SS-31: a synthetic tetrapeptide that binds cardiolipin

SS-31 is D-Arg-Dmt-Lys-Phe-NH₂, where Dmt is 2',6'-dimethyltyrosine, a non-proteinogenic residue; average mass about 640 Da. The design principle, set out in a 2004 Journal of Biological Chemistry paper by Zhao, Szeto and colleagues, is alternating aromatic and basic residues, which the authors reported made the peptides cell-permeable and concentrated them roughly 1,000-fold in the inner mitochondrial membrane, with the dimethyltyrosine providing radical-scavenging capacity in neuronal N₂A cells and isolated mitochondria. A 2014 British Journal of Pharmacology paper by Birk, Szeto and colleagues showed by fluorescence, turbidity and NMR that SS-31 interacts only with liposomes and bicelles containing cardiolipin, and that it inhibits the peroxidase activity that cytochrome c acquires when bound to cardiolipin.

Two later papers refined the target picture. A 2020 Proceedings of the National Academy of Sciences paper by Chavez, Bruce and colleagues used chemical cross-linking mass spectrometry in isolated mitochondria to identify SS-31-interacting proteins, all of them known cardiolipin binders, in the oxidative phosphorylation and 2-oxoglutarate pathways. A 2022 eLife paper by Mitchell, Alder and colleagues produced the first NMR and molecular-dynamics structural models for this class, finding that SS-31 binds cardiolipin-containing membranes and alters membrane surface charge, and, unlike its analogs, does not adopt a compact reverse-turn conformation when bound. The target is therefore a lipid, not a protein, which places SS-31 in a different mechanism class from either mitochondrial-derived peptide.

Side-by-side comparison

The table shows that "mitochondrial peptide" covers three different kinds of molecule: a genome-encoded 24-mer with cell-surface receptors, a genome-encoded 16-mer with an intracellular metabolic mechanism, and a designed 4-mer whose target is a membrane lipid.

Regulatory status and the research-vial distinction

SS-31 was developed by Stealth BioTherapeutics as elamipretide (also MTP-131 and Bendavia). The 2024 Genetics in Medicine paper by Thompson and colleagues reports the 168-week open-label extension of the TAZPOWER trial in Barth syndrome, including cardiolipin and monolysocardiolipin as biomarkers. Elamipretide is approved by the FDA as Forzinity under the accelerated-approval pathway as of September 2025. A lyophilized research vial labeled SS-31 shares the tetrapeptide sequence but is not Forzinity, is not made to that product's specification, and carries only its own certificate; it is supplied for in vitro work only. Humanin and MOTS-c have never been approved anywhere. Both appear in research catalogs as synthetic peptides matching the mitochondrial-encoded sequences, and neither research vial has any regulatory status beyond research use.

What the certificate must show for each

The three present different analytical problems. Humanin has a single cysteine at position 8 and a run of four leucines, so a certificate should show the 2,687 Da monomer rather than a 5,372 Da disulfide dimer, and the chromatogram should be read for the +16 Da methionine-oxidation shoulder; its hydrophobicity also makes it the one most prone to adsorption loss and poor reconstitution. MOTS-c has two methionines and a tryptophan, so oxidation at +16, +32 and +48 Da is the main thing to look for. SS-31 is small and robust; the certificate should confirm a mass near 640 Da, which also confirms that dimethyltyrosine (28 Da heavier than tyrosine) is present, but mass spectrometry cannot confirm the D-configuration of Arg1. US Prime Peptides posts lot-matched certificates for Humanin 10 mg, MOTS-c 10 mg and SS-31 10 mg before purchase, each reporting identity, HPLC purity, net peptide content, endotoxin, sterility and heavy metals from an independent US laboratory. The article on what a certificate does not test for covers stereochemistry and aggregation, the two blind spots most relevant here.

Frequently Asked Questions

What is the difference between humanin and MOTS-c?

Both are encoded by the mitochondrial genome, but in different genes: humanin (24 residues, about 2,687 Da) comes from an open reading frame in the 16S rRNA gene, MOTS-c (16 residues, about 2,174 Da) from one in the 12S rRNA gene. Humanin has reported cell-surface receptors (FPRL1/FPR2) and binds Bax; MOTS-c has no receptor and acts through the folate cycle, AMPK and, under stress, nuclear gene regulation.

Is SS-31 a mitochondrial-derived peptide?

No. SS-31 is a synthetic tetrapeptide, D-Arg-Dmt-Lys-Phe-NH₂, with no genomic origin. It is called a mitochondrial peptide because it concentrates in the inner mitochondrial membrane and binds cardiolipin.

Is SS-31 an approved drug?

Yes. SS-31, under the name elamipretide, is approved by the FDA as Forzinity through the accelerated-approval pathway (September 2025). A research vial of SS-31 is not that product and is supplied only for in vitro laboratory use.

What does SS-31 bind?

Cardiolipin, a phospholipid found almost exclusively in the inner mitochondrial membrane. Liposome, NMR and cross-linking mass-spectrometry studies show it interacts only with cardiolipin-containing membranes and sits near cardiolipin-binding proteins of the electron transport chain; it is not a ligand for any receptor.

Citations

1. Hashimoto Y, Niikura T, Tajima H, et al. A rescue factor abolishing neuronal cell death by a wide spectrum of familial Alzheimer's disease genes and Abeta. Proc Natl Acad Sci U S A. 2001;98(11):6336-6341. PMID 11371646
2. Guo B, Zhai D, Cabezas E, et al. Humanin peptide suppresses apoptosis by interfering with Bax activation. Nature. 2003;423(6938):456-461. PMID 12732850
3. Ying G, Iribarren P, Zhou Y, et al. Humanin, a newly identified neuroprotective factor, uses the G protein-coupled formylpeptide receptor-like-1 as a functional receptor. J Immunol. 2004;172(11):7078-7085. PMID 15153530
4. Harada M, Habata Y, Hosoya M, et al. N-Formylated humanin activates both formyl peptide receptor-like 1 and 2. Biochem Biophys Res Commun. 2004;324(1):255-261. PMID 15465011
5. Zhu S, Hu X, Bennett S, et al. The Molecular Structure and Role of Humanin in Neural and Skeletal Diseases, and in Tissue Regeneration. Front Cell Dev Biol. 2022;10:823354. PMID 35372353
6. Lee C, Zeng J, Drew BG, et al. The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis and reduces obesity and insulin resistance. Cell Metab. 2015;21(3):443-454. PMID 25738459
7. Kim KH, Son JM, Benayoun BA, Lee C. The Mitochondrial-Encoded Peptide MOTS-c Translocates to the Nucleus to Regulate Nuclear Gene Expression in Response to Metabolic Stress. Cell Metab. 2018;28(3):516-524.e7. PMID 29983246
8. Merry TL, Chan A, Woodhead JST, et al. Mitochondrial-derived peptides in energy metabolism. Am J Physiol Endocrinol Metab. 2020;319(4):E659-E666. PMID 32776825
9. Zhao K, Zhao GM, Wu D, et al. Cell-permeable peptide antioxidants targeted to inner mitochondrial membrane inhibit mitochondrial swelling, oxidative cell death, and reperfusion injury. J Biol Chem. 2004;279(33):34682-34690. PMID 15178689
10. Birk AV, Chao WM, Bracken C, et al. Targeting mitochondrial cardiolipin and the cytochrome c/cardiolipin complex to promote electron transport and optimize mitochondrial ATP synthesis. Br J Pharmacol. 2014;171(8):2017-2028. PMID 24134698
11. Chavez JD, Tang X, Campbell MD, et al. Mitochondrial protein interaction landscape of SS-31. Proc Natl Acad Sci U S A. 2020;117(26):15363-15373. PMID 32554501
12. Mitchell W, Tamucci JD, Ng EL, et al. Structure-activity relationships of mitochondria-targeted tetrapeptide pharmacological compounds. Elife. 2022;11:e75531. PMID 35913044
13. Thompson WR, Manuel R, Abbruscato A, et al. Long-term efficacy and safety of elamipretide in patients with Barth syndrome: 168-week open-label extension results of TAZPOWER. Genet Med. 2024;26(7):101138. PMID 38602181

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