MOTS-c: What Mitochondria-Derived Actually Means
Two compounds in this catalogue get called mitochondrial peptides, and the word means something completely different in each case. One is named for where its gene sits. The other is named for where the molecule was designed to go. Confusing them is easy and it leads to expecting the wrong things from a certificate.
What mitochondria-derived actually means
MOTS-c is short for mitochondrial open reading frame of the twelve-S ribosomal RNA type-c, which is a description of its genetic origin rather than of the molecule. Almost every protein that works inside a mitochondrion is encoded in nuclear DNA and imported. MOTS-c is not: its reading frame sits inside the mitochondrial genome itself, within the 12S rRNA gene, in a frame that overlaps a sequence whose main job is structural rather than protein-coding.
That is the interesting part. The mitochondrial genome is small and was long assumed to encode a short, fixed list of products. A peptide whose reading frame is nested inside one of its RNA genes is a different kind of finding from another analog off a synthesiser, and it is why this class attracted attention in the first place.
The sequence
MOTS-c is sixteen residues: Met-Arg-Trp-Gln-Glu-Met-Gly-Tyr-Ile-Phe-Tyr-Pro-Arg-Lys-Leu-Arg. Its molecular formula is C101H152N28O22S2 for a mass of roughly 2174.6 g/mol, against CAS registry number 1627580-64-6.
Two details in that formula do the analytical work. The S2 is two sulfur atoms, and the aromatic content is unusually high for a peptide of this length.
Two methionines, and therefore two oxidation peaks
Both sulfurs are methionine thioethers, at positions 1 and 6, and a methionine thioether oxidises to the sulfoxide readily on exposure to air, light or a peroxide-contaminated solvent. Each oxidation adds one oxygen atom: 16 daltons.
With two methionines there are two steps rather than one. A peak 16 Da above the expected mass is singly oxidised material; a peak 32 Da above it is doubly oxidised. Both are different compounds from the one on the label, and neither is necessarily a synthesis fault — oxidation can happen after manufacture, which means a clean certificate describes the material as it was tested rather than as it will remain. Minimising repeat vial entries and keeping the powder dry and dark is the discipline that follows; how peptides degrade covers the routes.
Detection is the easy part here
The sequence carries a tryptophan, two tyrosines and a phenylalanine. That is a great deal of chromophore, so MOTS-c responds strongly at 280 nm as well as at the usual 214 nm, and a purity figure at either wavelength is meaningful.
That is not true of every peptide. A sequence with no aromatic residues, such as Selank, is nearly invisible at 280 nm and a purity figure measured there means very little. It is worth knowing which situation you are in before reading the number; what a third-party peptide test actually measures works through it.
Three arginines and a lysine
MOTS-c is strongly basic. Four positively charged residues against a backbone with substantial hydrophobic content — isoleucine, leucine, phenylalanine — gives it mixed character on a reversed-phase column, and basic peptides are prone to peak tailing from interaction with residual silanols on older column chemistries. A tailing peak inflates or deflates an integrated area depending on where the integration baseline is drawn, which is one of the ordinary reasons two laboratories return different numbers for the same vial. Why two certificates can disagree sets out the rest.
SS-31, by contrast, is targeted rather than derived
SS-31 is four residues, synthetic, and encoded nowhere. Its connection to mitochondria is that the molecule was designed with an alternating aromatic-cationic motif that associates with cardiolipin, a phospholipid largely confined to the inner mitochondrial membrane. It is a delivery property, not a genetic one.
So MOTS-c is mitochondrial by origin and SS-31 is mitochondrial by destination, and nothing about one tells you anything about the other. SS-31 and cardiolipin covers that mechanism, and mitochondrial peptides covers the research literature on both.
What a certificate should show
The measured mass against the calculated 2174.6, with attention to anything at 2190 or 2206 — the singly and doubly oxidised forms. The detection wavelength. The chromatogram rather than a summary figure, so tailing and shoulders are visible. And net peptide content, which accounts for counter-ion and residual water and is the figure that says how much peptide is in the vial — see net peptide content. Every report we hold is published in full on the certificates of analysis page.
How it is written
The compound appears as MOTS-c, MOTS-C, MOTSc and as the sequence MRWQEMGYIFYPRKLR. All refer to the same sixteen-residue peptide.
What we supply
MOTS-c is stocked as a 10mg vial and a 20mg vial, lyophilised, with identity data published on each product page. It is supplied for laboratory research use only. We publish no dosing information, no protocols involving human or veterinary subjects, and no statements about what the compound does in a person, and we do not provide them on request.
