SS-31 (Elamipretide): Cardiolipin Targeting, Mechanism and Laboratory Use

A designed peptide of only four residues, SS-31 (D-Arg-Dmt-Lys-Phe-NH2) was created so that it would concentrate on the inner membrane of the mitochondrion and bind the phospholipid cardiolipin, which occurs essentially nowhere else. There is no gene for it anywhere, so it stands apart from the peptides that mitochondrial DNA encodes. It belongs to the Szeto-Schiller series of cationic aromatic peptides, and it is characterised by its destination rather than by any receptor it turns on. Sold under several names — elamipretide, plus MTP-131 and Bendavia — it is further along in clinical development than any other compound we list for mitochondrial research, having been taken into a registrational programme.

We supply SS-31 (Elamipretide) freeze-dried in 10 mg, 30 mg and 50 mg vials, listed with the mitochondrial peptides.

Defining SS-31

The molecule is a tetrapeptide whose residues alternate between charged and aromatic: D-arginine, then Dmt (2',6'-dimethyl-L-tyrosine), then lysine, then phenylalanine, ending in an amide. Identifiers are CAS 736992-21-5, formula C32H49N9O5, molar mass 639.80 g/mol.

Every element of that design does a job. Putting D-arginine first defeats aminopeptidases; capping the other end as an amide removes the carboxylate that carboxypeptidases look for. A net charge of +3 pulls the peptide into the matrix compartment, and the two aromatic side chains give it enough lipophilicity to sit in a lipid bilayer. Dmt is not one of the twenty natural amino acids but a synthetic tyrosine analogue that adds aromatic character and resists metabolism. Taken together, these features let the peptide concentrate inside mitochondria purely through its physical chemistry — no receptor and no import sequence required.

Where the molecule came from

Development of the series began in the early 2000s in the hands of Hazel Szeto and Peter Schiller, who came to it from an unlikely direction — Dmt-bearing analogues of opioid peptides. What they found was that certain sequences combining positive charge with aromatic rings built up inside mitochondria at concentrations thousands of times the external level. The later compounds, SS-31 included, were engineered to preserve that targeting behaviour while dropping the opioid activity, and reports indicate SS-31 has no appreciable opioid effect at the concentrations mitochondrial studies use.

There is one important contrast with older targeting strategies: SS-31's uptake is independent of membrane potential. Triphenylphosphonium conjugates get in because the matrix side is electrically negative, which means they stop working in exactly the depolarised, damaged mitochondria they are meant to help. SS-31 accumulates whatever the potential, and that is the practical argument for using it in dysfunction models.

Proposed mechanism

Throughout the literature, the mechanism comes back to cardiolipin — a phospholipid with four acyl tails that is essentially confined to the inner mitochondrial membrane, where it curves the cristae and keeps respiratory complexes assembled into supercomplexes. SS-31 attaches to cardiolipin through a combination of charge and hydrophobic contacts.

Two consequences are described downstream. Normally cardiolipin keeps cytochrome c anchored; disturb that arrangement and cytochrome c takes on peroxidase activity, oxidises the cardiolipin around it and triggers the cascade of release that ends in apoptosis. Published work reports that bound SS-31 preserves the electron-carrier arrangement between the two and quiets the peroxidase activity. Independently of that, in models where cristae had lost their shape and supercomplexes their organisation, both are reported to recover, with respiration becoming more efficient and fewer electrons escaping to form reactive oxygen species.

For experimental design, the crucial point is that SS-31 is not an antioxidant in the radical-scavenging sense. Its reported effect is architectural: by restoring an organisational property of the membrane it reduces how many reactive species are generated at all, instead of removing them once formed. And there is no receptor to identify, because the mechanism does not need one.

Research to date

Ischaemia-reperfusion and cardiac studies

The biggest preclinical body of work involves rodent and large-animal models of cardiac ischaemia-reperfusion injury, where smaller infarcts and preserved mitochondrial respiration have been reported. A parallel literature exists for renal ischaemia-reperfusion.

Primary mitochondrial myopathy

Clinical development centred on primary mitochondrial myopathy, where a Phase 3 study failed to hit its primary functional endpoint despite encouraging earlier-phase results. That outcome is worth noting: a mechanism that looks convincing in preclinical models did not carry over cleanly into a functional clinical result.

Barth syndrome

Mutations in tafazzin, the enzyme that remodels cardiolipin, cause Barth syndrome — which makes it the indication that fits a cardiolipin-binding molecule best on mechanistic grounds. A crossover study and its open-label extension are in print, and the regulatory assessment of elamipretide in this rare condition has moved over the past few years. Wherever that stands today, an authorised medicine and research-grade substance are separate articles and must never be treated as the same thing.

Eye and age-related models

Trials in geographic atrophy and dry age-related macular degeneration examined retinal pigment epithelium function — a tissue chosen because its mitochondrial density is exceptionally high.

Aging and skeletal muscle

Rodent and ex-vivo human muscle studies have measured mitochondrial respiration, ATP output and resistance to fatigue in aged tissue, reporting gains in respiratory parameters. This remains preclinical work.

Formats and sizes

The three vial sizes span the range from plate-scale in-vitro series to larger preclinical studies. For the comparison with the mitochondrially encoded alternative, see MOTS-c vs SS-31.

Reconstitution and storage in the lab

As a short, strongly cationic peptide, SS-31 is about as easy to dissolve as peptides get: it goes into water or bacteriostatic water without trouble. Direct the solvent onto the glass rather than the cake and swirl instead of shaking.

Bench arithmetic: 3 mL of diluent in a 30 mg vial produces 10 mg/mL (10,000 mcg/mL), so 0.1 mL — the 10-unit line on a U-100 syringe — carries 1,000 mcg. With a molar mass of 639.80 g/mol, 1 mg is about 1.56 µmol; that conversion is worth having ready, because most published in-vitro studies of this compound quote molar rather than mass concentrations. The procedure is in our reconstitution guide.

Unopened freeze-dried vials: −20 °C, sealed, dry, dark. Reconstituted material can sit at 2-8 °C for the length of a study, or be aliquoted and frozen for longer programmes. Since Dmt is a phenol and thus prone to oxidation, keeping the peptide away from light and air matters just as much as it does for any tyrosine-containing sequence. See how to store peptides for general practice.

Purity and reading the COA

A mass-spec value of 639.80 g/mol establishes identity. The harder specification, and the one a routine certificate is worst at proving, is chirality: residue 1 must be the D form of arginine, and no mass measurement can separate D from L. Where that purity is load-bearing for an experiment, ask for chiral chromatography or amino acid analysis — a certificate that includes either is telling you something real about the batch. Otherwise the usual entries apply: main-peak area for purity, the counter-ion (reversed-phase work generally leaves material as a TFA salt), the lot number and the date it was made. Each section is unpacked in our guide to peptide certificates.

Regulatory position

For a rare mitochondrial disorder, elamipretide has gone through extensive clinical study and regulatory review, and where it stands has changed more than once in recent years. Regardless, two things stay constant: the research-grade substance sold here was not made to pharmaceutical standards and is not a pharmaceutical, and it is not interchangeable with any licensed or investigational drug of the same name. Nothing about it is approved for use.

Related compounds and further reading

The gene-encoded members of this field are Humanin and MOTS-c, mitochondrial-derived peptides that act by signalling rather than by targeting a membrane. Among cofactors, NAD+ works on redox capacity instead of membrane architecture. Our peptides for energy and mitochondria research overview gives the wider picture.

Matching catalogue section: Mitochondrial Peptides