These three appear together in catalogs and in the NAD-adjacent kits, and the grouping conceals a real distinction. Two of them are mitochondrial-derived peptides, a defined class. The third is a synthetic compound designed to localise to mitochondria. Those are different things.
Mitochondrial-derived peptides
Most of the proteins that operate in mitochondria are encoded in nuclear DNA. A small number of short open reading frames sit within the mitochondrial genome itself, and the peptides they encode are termed mitochondrial-derived peptides. They are a relatively recent addition to the literature and are studied as signalling molecules rather than as structural components.
- Humanin was the first described, encoded within the mitochondrial 16S rRNA region. Published work examines cytoprotective signalling in cell models.
- MOTS-c is encoded within the 12S rRNA region. It runs to 16 residues and the literature centres on metabolic regulation and its interaction with nuclear gene expression.
The interesting structural fact about this class is that the genome encoding them uses a variant genetic code, which is why these sequences were not identified by standard nuclear-genome annotation for a long time.
[SS-31](/p/ss-31) is a different kind of molecule
SS-31, also appearing as elamipretide, is not encoded anywhere. It is a synthetic tetrapeptide, and its design is the point: it carries an alternating aromatic-cationic motif that causes it to concentrate at the inner mitochondrial membrane, where published work describes an interaction with cardiolipin.
SS stands for Szeto-Schiller, after the researchers who described the series. The compound is studied in the context of mitochondrial bioenergetics and membrane integrity.
So the grouping is: two short natural sequences read out of the mitochondrial genome, and one designed four-residue molecule that goes there on purpose.
Handling and analysis
The cationic character of SS-31 has practical consequences. Highly basic peptides can adsorb to glass and some plastics, which matters at low working concentrations, and they behave differently on reversed-phase columns than neutral sequences of similar length. A method statement is worth reading on this compound.
MOTS-c and humanin are conventional short-to-medium sequences by comparison. Humanin is the longer of the two at 24 residues and is the more demanding synthesis.
What to check
- Which variant a humanin lot is, since analogues of the parent sequence exist and are listed under similar names.
- Observed mass against theoretical, particularly for SS-31 where the short four-residue sequence makes the expected mass unambiguous.
- Net peptide content, where the counterion fraction is proportionally larger on a very short peptide than on a long one.
That last point is worth emphasising for SS-31 specifically: on a tetrapeptide the counterion and residual water can account for a substantially larger share of vial mass than on a 40-residue chain, so net peptide content is not a formality here.
All material is supplied strictly for laboratory research.
Why these are harder to characterise than most
Short peptides encoded outside the nuclear genome sit awkwardly in standard reference databases, and naming has lagged the literature. The practical consequence for a buyer is that a catalogue name may not map cleanly onto a database entry, which makes the certificate rather than the name the thing to work from.
A calculated mass from the published sequence is the most useful cross-check available, and it is arithmetic rather than a lookup. Molecular weight covers how to do it and the two reasons a correct number can look wrong.
Handling notes that follow from structure
Members of this group vary in length and in which residues they carry, so handling advice does not generalise across the class. The useful habit is to read the sequence for oxidation-prone residues and for any disulfide, then apply the handling that those imply rather than the handling the category implies.
How peptides degrade maps the routes to the residues that enable them, which is the shortest path from a sequence to a storage decision.
Why the group is defined by origin rather than by function
What these peptides have in common is where their coding sequence sits, not what they do. That is an unusual basis for a category and it has a practical consequence: members of the group do not share a mechanism, a receptor or a handling profile, so advice that applies to one does not automatically apply to another.
For a buyer the useful move is to stop treating the category as a unit. Read each compound's sequence, mass and modifications on its own terms, and let the certificate rather than the grouping tell you what you have.
What to establish before ordering
The sequence as the supplier states it, the calculated mass that follows from it, and the mass the certificate reports. Those three should reconcile, and where they do not the gap is usually an end modification rather than a problem, which molecular weight covers.
Then the identity method. For short sequences a single intact mass is weaker evidence than it looks, because peptides sharing a composition share a mass. A method that fragments the molecule and reads the ladder is the stronger answer, and mass spectrometry and peptide identity sets out the difference.
A note on naming in this group
Designations in this area are still settling, and the same molecule can appear under more than one name across suppliers and papers. Work from the sequence and the mass rather than the designation, which is the general defence against loose naming set out in reading peptide nomenclature.
For where this sits among the other molecules a catalogue carries, not everything in a peptide catalog is a peptide covers how the classes differ.