SS-31 Research Peptide

Cardiolipin is an unusual phospholipid. It carries four fatty-acid tails instead of the usual two, and in mammalian cells it sits almost entirely in the inner mitochondrial membrane. There it helps fold the membrane into cristae, the tightly packed ridges where ATP is made, and holds the respiratory complexes together in larger working units called supercomplexes. Cardiolipin also governs how cytochrome c behaves: normally bound, cytochrome c carries electrons along the respiratory chain, but when the lipid environment is disturbed it can turn into a peroxidase that attacks cardiolipin itself.

SS-31 research is built around this lipid. Early work from Hazel Szeto’s laboratory reported that peptides of this class concentrate around 1,000-fold in the inner mitochondrial membrane. A 2013 study in the Journal of the American Society of Nephrology then used a fluorescent SS-31 analogue to show high-affinity binding to cardiolipin. The SS-31–cardiolipin complex held back cytochrome c’s peroxidase activity, and in a rat model of kidney ischaemia the peptide helped preserve cristae and sped up the recovery of ATP.

SS-31 belongs to the Szeto-Schiller family, developed by Hazel Szeto and Peter Schiller around one simple design rule: alternate aromatic and positively charged residues. Its sequence, D-Arg-Dmt-Lys-Phe-NH2, follows that pattern exactly. The D-arginine at the front is the mirror-image form of the amino acid, which protein-cutting enzymes recognise poorly. Dmt (2′,6′-dimethyltyrosine) is a modified tyrosine. The amidated C-terminus removes a negative charge, leaving a net charge of +3 at physiological pH in a peptide small enough to cross cell membranes.

Biophysics has sharpened the picture. A 2020 Journal of Biological Chemistry study (Learn more) found that SS-31 settles into the interfacial region of lipid bilayers, with binding that tracks surface charge, and that it tunes the surface electrostatics of model and mitochondrial membranes, including where calcium ions gather. A 2022 eLife structure–activity study compared SS-31 with three analogues and found that changing the side chains or their order changed both membrane behaviour and activity in cell-stress assays.

Cardiolipin never works alone. Dozens of inner-membrane proteins depend on it, so researchers have asked which of them SS-31 actually meets. A 2020 study in PNAS answered that with chemical cross-linking and mass spectrometry in isolated mitochondria. Every SS-31 partner the team identified was a known cardiolipin-binding protein, and they fell into two groups: components of oxidative phosphorylation, the machinery that produces ATP, and enzymes of 2-oxoglutarate metabolism within the Krebs cycle.

Many of the cross-linked sites lay close to the regions where those proteins touch cardiolipin. That supports a model in which SS-31 works at the boundary between lipid and protein, steadying the local environment of the respiratory machinery rather than switching on a single receptor. It also explains why SS-31 is such a useful probe for anyone studying how membrane organisation shapes electron transport and ATP output.

Some of the most cited SS-31 work comes from ageing research. In a 2013 Aging Cell study, 27-month-old mice showed skeletal-muscle mitochondrial energetics restored toward young levels within one hour of SS-31, measured in living animals with optical and phosphorus magnetic resonance spectroscopy. Young muscle was unaffected, and after eight days of treatment whole-animal endurance improved.

The heart tells a similar story. A 2020 eLife study reported that eight weeks of SS-31 substantially reversed diastolic dysfunction in old mice, alongside reduced proton leak and lower mitochondrial reactive oxygen species in heart muscle cells. Old mice engineered to express a mitochondria-targeted catalase gained nothing extra from SS-31, which points to mitochondrial oxidative stress as the shared mechanism. Together, these studies made SS-31 a reference compound for asking whether age-related decline in mitochondrial function can be reversed late in life, not just slowed.

SS-31 stands out among research peptides because the same molecule, as elamipretide, has completed the journey to an approved drug. Developed by Stealth BioTherapeutics, and earlier known in trials as MTP-131 and Bendavia, elamipretide received accelerated approval from the US Food and Drug Administration on 19 September 2025 under the brand name Forzinity. It is approved for Barth syndrome in patients weighing at least 30kg, and was the first FDA-approved medicine for that condition, a rare inherited disorder caused by faults in the TAFAZZIN gene needed to build mature cardiolipin. The approval rests on improved knee-extensor muscle strength, and the FDA requires a confirmatory trial; Stealth’s parent company, Mighty Therapeutics, announced in July 2026 that the Phase 4 confirmatory study had enrolled its first patient.

Not every programme has succeeded. The MMPOWER-3 trial in primary mitochondrial myopathy showed no significant overall benefit, though a subgroup with nuclear DNA variants prompted further analysis, and a Phase 3 study in dry age-related macular degeneration is under way. Crown Peptides’ SS-31 is a research-grade peptide, separate from the licensed medicine.