SS-31.

Mitochondria-derived and mitochondria-targeted peptides in metabolism and stress responses.

Mitochondria are not only the site of oxidative phosphorylation; they are also signalling organelles that communicate their status to the rest of the cell and, through circulating factors, to the rest of the body. Two distinct classes of peptide are grouped here, and the distinction matters because they are quite different things. The first class is peptides encoded within mitochondrial DNA itself, in short open reading frames inside genes normally read as ribosomal RNA. These are genuinely mitochondria-derived and act as signalling molecules. One has been described as a stress-responsive regulator that activates the AMP-activated protein kinase energy-sensing pathway, influences folate and methionine one-carbon metabolism, and under metabolic stress translocates to the cell nucleus where it is proposed to help direct adaptive antioxidant gene expression. Another acts both inside and outside the cell: intracellularly it binds and antagonises pro-apoptotic Bcl-2 family proteins, preventing their translocation to mitochondria and suppressing programmed cell death, and extracellularly it signals through a cytokine-like receptor complex that activates STAT3 and engages formyl peptide receptors. It also interacts with insulin and IGF-1 signalling. Circulating levels of both have been described as declining with age, which is the observation driving interest in them. The second class is not mitochondria-derived but mitochondria-targeted by design. A peptide with an alternating aromatic and cationic motif carries a net positive charge that drives selective accumulation in the inner mitochondrial membrane, where it binds reversibly to cardiolipin, an anionic phospholipid found almost exclusively there. By associating with cardiolipin it is proposed to stabilise cristae architecture, protect the interaction between cardiolipin and cytochrome c, and support electron-transport-chain organisation and membrane potential. This is a structural stabilisation mechanism rather than a receptor-mediated signal, and it is why the compound is described as improving mitochondrial efficiency rather than stimulating it. Honest assessment: this is an active and legitimate research field with well-characterised molecular interactions, but the characterisation is predominantly in cell and animal systems. Human clinical evidence is limited, and the mitochondria-targeted compound has been through clinical trials in specific conditions without producing a clearly positive result in every setting. None of these is an approved medicine, and none should be described as an established treatment for age-related decline or metabolic disease.

Handling reactive oxygen species, detoxification conjugation, and cellular redox state.

Cells continuously generate reactive oxygen species as a by-product of aerobic metabolism, and they maintain an elaborate system to keep those species in check. Glutathione is the centre of that system: a tripeptide of glutamate, cysteine, and glycine, present in millimolar concentrations inside cells, and the largest pool of non-protein thiol in the body. Its reactive cysteine sulfhydryl group is what does the work. Glutathione operates in several ways at once. It neutralises radicals directly, and it serves as the substrate for glutathione peroxidase enzymes that reduce hydrogen peroxide and lipid peroxides to harmless products, becoming oxidised to glutathione disulfide in the process and then being regenerated by glutathione reductase using NADPH. The ratio between reduced and oxidised forms is one of the standard measures of a cell's redox state. Glutathione also regenerates other antioxidants including vitamins C and E, so its depletion degrades the whole antioxidant network rather than just one component. Separately, glutathione S-transferases conjugate it to toxins, drugs, and electrophilic metabolites during phase II detoxification, making them water-soluble for excretion, which is why hepatic glutathione depletion is the central event in certain overdose toxicities. A proposed cosmetic application rests on different chemistry: inhibition of tyrosinase, the rate-limiting enzyme in melanin synthesis, and a shift of pigment production from darker eumelanin toward lighter pheomelanin. A second compound is included here because it addresses oxidative damage at its source rather than downstream. By stabilising the inner mitochondrial membrane and the organisation of the electron transport chain, it is proposed to reduce the leakage of electrons that generates reactive oxygen species in the first place, which is conceptually upstream of scavenging them. The honest complication with glutathione is delivery. Oral glutathione is largely broken down into its constituent amino acids in the gut, and the extent to which any intact molecule reaches cells is a long-standing point of contention. Raising intracellular glutathione by supplying cysteine precursors is a better-established route than supplying the tripeptide itself. Intravenous and injected use for skin lightening has been the subject of regulatory safety warnings in several countries, is not an approved indication, and has been associated with serious adverse events. The underlying biochemistry is textbook; the interventions built on it are not.

Elamipretide received accelerated FDA approval in 2025 for Barth syndrome on the basis of a muscle-strength endpoint reasonably likely to predict benefit.

The large phase 3 trial in primary mitochondrial myopathy missed both co-primary endpoints, with only post hoc subgroup signals.

Trials in both indications missed their primary endpoints.

Cardiolipin binding and bioenergetic effects are well characterized in cell and tissue models; aging outcomes in humans are untested.

The trial safety record belongs to the quality-controlled prescription product; research-use material carries unresolved purity, sterility and identity questions.