What mitochondrial peptides are
For decades, mitochondria were taught as the cell's power plants and little else. That picture has changed with the discovery that the small circular mitochondrial genome contains short open reading frames encoding tiny peptides, called mitochondrial-derived peptides (MDPs). These peptides are produced from the mitochondria and can travel through the cell and even circulate in the blood, acting as signals.
The best-studied MDPs are humanin, encoded in a region of the mitochondrial 16S ribosomal RNA, and MOTS-c, encoded in the 12S ribosomal RNA region. A useful distinction to keep in mind is that 'mitochondrial peptide' is often used loosely to include compounds that act on mitochondria without being encoded by them, such as SS-31.
Humanin: the first mitochondrial-derived peptide
Humanin was the first MDP identified, originally found in the context of protecting neurons. In laboratory models it is broadly cytoprotective and anti-apoptotic, meaning it helps cells resist programmed death under stress. Much of the early attention came from neurodegeneration research, where humanin protected neurons against toxicity associated with amyloid-beta.
Beyond the brain, humanin has been studied for metabolic and cardioprotective effects in models, including improvements in insulin sensitivity. A recurring observation is that circulating humanin levels tend to decline with age, which has made it a candidate biomarker as well as a potential effector of healthy aging. Most of this evidence is preclinical or observational.
MOTS-c and AMPK-driven energy metabolism
MOTS-c is the mitochondrial peptide most associated with whole-body metabolism. Its signature action is activation of AMPK, the AMP-activated protein kinase that serves as the cell's energy sensor. AMPK switches on when cellular energy is low, promoting glucose uptake, fatty-acid oxidation, and mitochondrial biogenesis. It sits at the center of the same network engaged by exercise, caloric restriction, and metformin.
In animal work, MOTS-c improved insulin sensitivity and protected against diet-induced obesity, and one widely cited finding is that it acted as an exercise mimetic, enhancing physical performance and metabolic health even in aged mice. MOTS-c also appears able to translocate to the cell nucleus under metabolic stress. These results are striking, but they are principally from cell and rodent studies.
SS-31: a mitochondria-targeted peptide, not an MDP
SS-31, also known as elamipretide, is grouped with mitochondrial peptides because of where it acts, but it is not a mitochondrial-derived peptide. It is a synthetic Szeto-Schiller tetrapeptide designed to concentrate in the inner mitochondrial membrane. There it binds cardiolipin, a lipid essential to the structure of the cristae and the efficiency of the electron transport chain, helping to stabilize energy production and reduce the leak of reactive oxygen species.
Because SS-31 targets a core structural component of mitochondrial energy machinery, it has been the furthest developed clinically of the compounds discussed here, entering human trials in conditions with mitochondrial dysfunction. The clinical record, however, has been mixed, with some trials failing to meet their primary endpoints. That trajectory is a useful reality check.
An exciting but unproven field
The excitement around mitochondrial peptides is easy to understand. Mitochondrial dysfunction is one of the recognized hallmarks of aging, so molecules that come from or repair mitochondria and that engage master regulators like AMPK are natural longevity candidates.
The honest counterweight is that the evidence base is early. Humanin and MOTS-c rest largely on cell and animal studies plus human observational data, with no approved therapy. SS-31 has the most human data but a mixed clinical record. NAD+ is often mentioned in the same breath — rightly so as context — but it is a coenzyme rather than a peptide. This guide is educational only and not medical advice.