SS-31 and MOTS-c: Two Roads Into Mitochondrial Research

SS-31 and MOTS-c both get filed under "mitochondrial peptides". That label hides how different they are. One was designed in a lab to stick to a lipid in the inner mitochondrial membrane. The other is written into the mitochondrial genome itself and was only described in 2015. One has a prescription-drug relative with an FDA decision behind it. The other has never finished a controlled human trial. This post sets the SS-31 peptide and the MOTS-c peptide next to each other so the differences are easy to see.

Side by side first

Question	SS-31 (elamipretide)	MOTS-c
What it is	Synthetic Szeto-Schiller tetrapeptide, D-Arg-Dmt-Lys-Phe-NH2	16-amino-acid peptide encoded in mitochondrial DNA
Where it came from	Designed in the lab; also called MTP-131	Found in a short open reading frame of the 12S rRNA gene
Proposed target	Cardiolipin in the inner mitochondrial membrane	Skeletal muscle metabolism, with AMPK activation
Human research	Registered clinical trials; one FDA accelerated approval for a prescription product in a rare disease	Observational data on natural levels; no completed controlled trial of MOTS-c itself
Stage (as reported)	Approved as a drug for one indication; other uses investigational	Mostly preclinical

SS-31: a peptide aimed at a lipid

Origin and structure

SS-31 is one of the Szeto-Schiller peptides, named for the two researchers who designed the series. It is four residues long: D-Arg-Dmt-Lys-Phe-NH2. Two things make it unusual. The first residue is a D-amino acid, the mirror image of the form found in ordinary proteins. The second, Dmt, is a modified tyrosine (2',6'-dimethyltyrosine) that proteins do not normally contain. The design alternates charged and aromatic groups, and the point of that pattern is to make the peptide accumulate at the inner mitochondrial membrane. You will see it called elamipretide and MTP-131. All three names mean the same molecule.

Mechanism in the lab

SS-31 binds cardiolipin, a phospholipid found in the inner mitochondrial membrane, through electrostatic and hydrophobic interactions. Szeto, 2014 describes the consequences in laboratory systems: the interaction limits cardiolipin's conversion of cytochrome c into a peroxidase while preserving its role carrying electrons, and is associated with preserved cristae structure and oxidative phosphorylation. The FDA label for the prescription product uses a short phrase for this: a mitochondrial cardiolipin binder.

What the trials looked at

Elamipretide has an unusually long clinical record for a research-shelf peptide. Registered trials have studied Barth syndrome muscle strength, primary mitochondrial myopathy, heart failure with reduced ejection fraction and age-related macular degeneration. These are areas studied, not outcomes achieved. The Barth syndrome work ran longest. Thompson et al., 2024 reports the 168-week open-label extension of the TAZPOWER study: ten patients enrolled, eight reached week 168, and the authors are affiliated with the sponsor.

In September 2025 the prescription product FORZINITY (elamipretide hydrochloride) received FDA accelerated approval, with an indication in Barth syndrome for patients weighing at least 30 kg. The decision rested on an intermediate endpoint, knee extensor strength, and continued approval depends on confirmatory trials. The FORZINITY prescribing information and the Drugs@FDA record for NDA 215244 are public. Two plain points follow. The approval belongs to a specific manufactured drug product in one rare disease. It does not extend to research-grade SS-31, and every other use remains investigational.

MOTS-c: a peptide the mitochondrion writes itself

Origin and structure

MOTS-c stands for mitochondrial open reading frame of the 12S rRNA type-c. It is a 16-amino-acid peptide encoded by a short open reading frame inside the 12S ribosomal RNA gene of mitochondrial DNA. Nobody designed it. Lee et al., 2015 first described it, which makes it a young research compound. The version on a research shelf is a synthetic copy of that natural sequence.

Mechanism in the lab

In the original cell and mouse work, MOTS-c acted mainly on skeletal muscle. It inhibited the folate cycle and the de novo purine synthesis linked to it, and that led to activation of AMPK, a cellular energy sensor. Later studies reported that it can regulate nuclear genes tied to metabolism and proteostasis. Kong et al., 2023 reviews this chain of findings and its connection to diabetes and age-related disease research.

What the animal work and human observation looked at

The mouse studies cover insulin sensitivity and metabolic homeostasis, skeletal muscle metabolism, diet-induced obesity and age-related physical decline. Reynolds et al., 2021 is the key paper on the last of these: it reports on physical capacity in aging mice and, in people, shows that exercise is associated with higher natural MOTS-c in muscle and blood. That human part is observational. It measures the body's own peptide, not a supplied one. No completed controlled trial of MOTS-c itself was found.

Where the two differ

1. Origin. SS-31 is engineered, with a D-amino acid and a non-natural residue. MOTS-c is a natural sequence copied from mitochondrial DNA.
2. Target. SS-31 is proposed to bind a lipid, cardiolipin. MOTS-c is proposed to work through a metabolic pathway, the folate cycle and AMPK, and through gene regulation.
3. Evidence stage. Elamipretide has registered trials, a long open-label extension and an FDA decision for one rare disease. MOTS-c has cell and mouse data plus observational human measurements.
4. What gets measured. SS-31 studies measure mitochondrial structure and function and, in trials, muscle strength. MOTS-c studies measure metabolic markers and physical capacity in mice, and circulating peptide levels in people.

Both remain research questions. For a lab, the difference is in what a well-designed experiment could add. For SS-31, that is mechanism in new tissue models. For MOTS-c, it is the first controlled data.

Lot testing

SS-31 is a good example of why mass spectrometry matters, and of its limits. The dimethyltyrosine gives the peptide a mass that differs from a version built with ordinary tyrosine, so MS confirms that the modified residue is present. The D-arginine has exactly the same mass as L-arginine, so MS cannot see that difference; it is fixed at synthesis and documented there. HPLC then shows the main peak and any related impurities. MOTS-c, at 16 residues, is long enough for truncated or deletion sequences to appear during synthesis, so the HPLC trace carries more weight. Each lot's certificate states its measured purity and identity. We link that certificate from the product page before a lot is listed.

Storage and formats

Item	SS-31	MOTS-c
Format	Lyophilised powder, crimp-sealed glass vial	Lyophilised powder, crimp-sealed glass vial
Sizes	10 mg, 30 mg, 50 mg	10 mg, 40 mg
House codes	PTY-SS31-10, PTY-SS31-30, PTY-SS31-50	PTY-MOTS-10, PTY-MOTS-40
Storage	-20 °C, desiccated	-20 °C, desiccated
Documentation	Per-lot certificate, HPLC and MS	Per-lot certificate, HPLC and MS

Frequently asked questions

Is research-grade SS-31 the same as FORZINITY?

Same molecule, different product. FORZINITY is a manufactured prescription drug with an FDA decision attached to it for one rare disease. Research-grade SS-31 is laboratory material with a lot certificate. The approval belongs to the drug product, not to the peptide in general.

Why is MOTS-c called a mitochondrial-derived peptide?

Because the gene for it sits in mitochondrial DNA, inside the 12S rRNA region, rather than in the cell nucleus. Most peptides a cell makes are encoded in the nucleus; MOTS-c is one of the few exceptions.

Has MOTS-c been studied in a controlled human trial?

Not in anything completed that we could find. The human data are observational: measurements of natural MOTS-c in muscle and blood, including its association with exercise. Controlled data would be new.

Which of the two has more human evidence?

Elamipretide, by a wide margin, with registered trials and a 168-week open-label extension. Even so, the numbers are small; the extension enrolled ten patients. Both compounds leave plenty of open questions.

See it in the catalogue: SS-31 and MOTS-c.