SS-31

<p><strong>SS-31 (Elamipretide; D-Arg-dimethylTyr-Lys-Phe-NH₂)</strong> is a cell-permeable, mitochondria-targeted tetrapeptide developed by the Szeto-Schiller (SS) peptide chemistry program. Its alternating aromatic-cationic structure allows SS-31 to accumulate selectively in the inner mitochondrial membrane (IMM) by electrostatic attraction to the membrane negative potential, where it binds cardiolipin — a unique phospholipid almost exclusively present in the IMM that plays critical roles in mitochondrial electron transport chain (ETC) function, cristae architecture, and apoptotic cytochrome c release. SS-31 stabilizes cardiolipin and prevents cardiolipin peroxidation, producing downstream effects on ETC efficiency, ATP production, and reactive oxygen species (ROS) generation that have made it a leading candidate for research into mitochondrial dysfunction-driven disease models.</p>

<h2>Mechanism of Action</h2>

<h3>Cardiolipin Binding and IMM Stabilization</h3>

<p>Cardiolipin is a dimeric phospholipid with four fatty acid chains that localizes almost exclusively to the IMM in eukaryotic cells. In the IMM, cardiolipin stabilizes the supercomplexes (respirasomes) formed by electron transport chain complexes I, III, and IV, maintains cristae curvature, and anchors cytochrome c in a non-mobile pool that supports ETC electron shuttling. When cardiolipin is oxidized (by ROS derived from the ETC itself), it dissociates from cytochrome c, releases cytochrome c into the intermembrane space as a pro-apoptotic signal, and destabilizes respiratory supercomplexes — reducing ETC efficiency and increasing superoxide production in a self-amplifying cycle.</p>

<p>SS-31 binds cardiolipin via electrostatic interactions between its basic residues (Arg, Lys) and the negatively charged phosphate headgroups of cardiolipin, and via hydrophobic/pi-stacking interactions between its aromatic residues (dimethylTyr, Phe) and the cardiolipin acyl chains. This interaction stabilizes the IMM, prevents cardiolipin peroxidation by scavenging adjacent lipid peroxyl radicals, and maintains the cytochrome c-cardiolipin interaction that supports electron shuttling. The result is improved ETC complex assembly, increased coupling efficiency, reduced proton leak, and lower ROS production from complexes I and III.</p>

<h3>Mitochondrial Targeting Mechanism</h3>

<p>SS peptides exploit the large inside-negative electrochemical gradient across the IMM (approximately −180 mV) to concentrate in the IMM at 1,000- to 5,000-fold higher than cytoplasmic concentrations. Unlike triphenylphosphonium (TPP)-based mitochondrial targeting strategies, SS-31 is not permanently cationic and does not accumulate to toxic levels in hyperpolarized mitochondria — an important safety advantage in research models. The alternating aromatic-cationic motif is itself sufficient for IMM targeting without the need for a cleavable mitochondrial targeting sequence.</p>

<h2>Research Applications</h2>

<ul> <li><strong>Cardiac ischemia-reperfusion injury models</strong> — mitochondrial permeability transition pore (mPTP) opening prevention, cardioprotection during simulated ischemia-reperfusion in isolated cardiomyocytes and perfused heart models; ischemia-reperfusion-induced ROS quantification using mitosox and related dyes</li> <li><strong>Heart failure and cardiac remodeling</strong> — SS-31 in pressure-overload (TAC) and myocardial infarction models; improvement in left ventricular ejection fraction and mitochondrial respiratory capacity; ATP/ADP ratio measurements in failing vs. treated cardiomyocytes</li> <li><strong>Aging and mitochondrial bioenergetics research</strong> — age-associated mitochondrial dysfunction, cardiolipin remodeling with age, decline in ETC complex activity, and ATP production capacity in aged rodent tissues</li> <li><strong>Renal and kidney injury models</strong> — cisplatin-induced acute kidney injury (AKI), ischemia-reperfusion nephropathy, ROS-driven proximal tubule cell death; SS-31 cytoprotection in kidney mitochondria</li> <li><strong>Neurodegenerative disease research</strong> — mitochondrial dysfunction in Alzheimer disease (Amyloid-beta cardiolipin peroxidation), Parkinson disease (Complex I dysfunction), ALS; neuronal apoptosis models driven by cytochrome c release</li> <li><strong>Skeletal muscle mitochondrial function</strong> — mitochondrial respiration assays (Seahorse XF) in primary myocytes or muscle fibers; muscle fatigue, post-exercise recovery, and mitophagy research</li> <li><strong>Cancer biology</strong> — mitochondrial metabolism in Warburg-effect cancer cells; differentiation between respiratory and glycolytic metabolism; mitochondria-targeted drug delivery models using SS peptide vectors</li> <li><strong>Electron transport chain and respiratory supercomplex research</strong> — supercomplex stability assays (Blue Native PAGE), Complex I/III/IV activity measurements (spectrophotometric), and oxygen consumption rate (OCR) in intact cells and isolated mitochondria</li> </ul>

<h2>Clinical Research History</h2>

<p>SS-31/elamipretide has advanced into human clinical trials — a relatively rare milestone for a research peptide. Phase 2 trials in heart failure with reduced ejection fraction (HFrEF) demonstrated improvements in exercise capacity and cardiac function in a 28-day IV infusion protocol. A separate phase 2 trial in Barth syndrome (a mitochondrial cardiomyopathy driven by tafazzin mutations causing cardiolipin remodeling abnormalities) showed improvement in exercise tolerance and quality of life endpoints over 12 weeks of subcutaneous dosing. These clinical results validate the cardiolipin-interaction mechanism in human disease contexts and motivate ongoing research into SS-31 in mitochondrial disease, aging-associated organ dysfunction, and ischemia-reperfusion protection.</p>

<h2>Market Context</h2>

<p>SS-31 is a moderately specialized research tetrapeptide with strong demand from cardiovascular and mitochondrial biology research groups. Peptides.SO tracks <strong>59 active supplier listings</strong> with pricing from near $0 (sample/promo listings) to approximately $1,093 per listing, with a platform average near $73. Typical research quantities sold are 5mg to 25mg. SS-31 is a tetrapeptide (4 amino acids) and is synthesized by standard SPPS; the D-amino acid configuration at the N-terminus (D-Arg) and the N-methylated tyrosine (dimethylTyr) are non-standard elements that require specific resin/coupling chemistry and add to synthesis cost relative to standard L-amino acid tetrapeptides.</p>

<p>When procuring SS-31 for mitochondrial functional studies, specify: correct stereochemistry (D-Arg at position 1, dimethylTyr at position 2), amidated C-terminus (-NH₂ not -OH), HPLC purity ≥98%, and endotoxin testing if using in cell-based or animal models. The D-amino acid configuration is essential for proteolytic stability — linear L-amino acid analogs would be rapidly degraded before reaching mitochondria.</p>

<h2>Frequently Asked Questions</h2>

<p><strong>How does SS-31 differ from MitoQ and other mitochondria-targeted antioxidants?</strong><br/> MitoQ and similar TPP-conjugated antioxidants accumulate in mitochondria based on the membrane potential gradient, but carry permanently cationic TPP moieties that can depolarize mitochondria at high concentrations and accumulate to toxic levels in highly polarized mitochondria. SS-31 is not permanently cationic, does not cause mitochondrial depolarization at research concentrations, and acts specifically at the IMM via cardiolipin binding rather than general ROS scavenging — a more targeted mechanism that preserves ETC function rather than simply intercepting ROS downstream.</p>

<p><strong>What is the typical SS-31 concentration used in cell culture studies?</strong><br/> Most published in vitro studies use SS-31 at 1–100 nM, with 10–100 nM being the most common effective range for cytoprotection in ischemia-simulated cardiomyocytes. The compound is remarkably potent — effective at nanomolar concentrations — which means that microgram-quantity vials provide many experimental doses for cell culture work.</p>

<p><strong>Can SS-31 be used in intact cell studies, or only in isolated mitochondria?</strong><br/> SS-31 is cell-permeable and accumulates in mitochondria of intact living cells without requiring cell permeabilization. This makes it uniquely useful compared to many mitochondria-targeting compounds that must be added to isolated organelles. In intact cell Seahorse XF assays, SS-31 pretreatment followed by ETC stress protocols (sequential oligomycin, FCCP, rotenone/antimycin A injections) is the standard approach for measuring SS-31 effects on mitochondrial bioenergetics.</p>

<p><strong>Does SS-31 need to be protected from light during storage?</strong><br/> Yes. The dimethyltyrosine (dimethylTyr) residue can undergo photodegradation. SS-31 should be stored in amber or foil-wrapped tubes at -80°C for long-term storage, protected from UV light during reconstitution and use.</p>

<h2>Cited Research</h2>

<ul> <li>Szeto HH. "First-in-class cardiolipin-protective compound as a therapeutic agent to restore mitochondrial bioenergetics." <em>Br J Pharmacol.</em> 2014. PMID: <a href="https://pubmed.ncbi.nlm.nih.gov/23991970/" rel="noopener">23991970</a></li> <li>Dai DF, et al. "Mitochondrial targeted antioxidant Peptide ameliorates hypertensive cardiomyopathy." <em>J Am Coll Cardiol.</em> 2011. PMID: <a href="https://pubmed.ncbi.nlm.nih.gov/21527152/" rel="noopener">21527152</a></li> <li>Birk AV, et al. "The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin." <em>J Am Soc Nephrol.</em> 2013. PMID: <a href="https://pubmed.ncbi.nlm.nih.gov/23274953/" rel="noopener">23274953</a></li> <li>Gibson CM, et al. "EMBRACE STEMI study: a Phase 2a trial to evaluate the safety, tolerability, and efficacy of intravenous MTP-131 on reperfusion injury in patients undergoing primary percutaneous coronary intervention." <em>Eur Heart J.</em> 2016. PMID: <a href="https://pubmed.ncbi.nlm.nih.gov/26578203/" rel="noopener">26578203</a></li> <li>Sabbah HN, et al. "Chronic Therapy With Elamipretide (MTP-131), a Novel Mitochondria-Targeting Peptide, Improves Left Ventricular and Mitochondrial Function in Dogs With Advanced Heart Failure." <em>Circ Heart Fail.</em> 2016. PMID: <a href="https://pubmed.ncbi.nlm.nih.gov/26699388/" rel="noopener">26699388</a></li> </ul>

<p><em>For research purposes only. SS-31/elamipretide is not approved for therapeutic use in humans or animals in this context, is not a drug or dietary supplement, and any clinical applications must be conducted under appropriate regulatory authorization. All in vitro and in vivo experimental use must comply with institutional biosafety and ethical guidelines.</em></p>