A Szeto-Schiller peptide is any member of a small group of synthetic peptides, four amino acids long, that cross cell membranes on their own and collect in mitochondria. There are only a handful of them. One, SS-31, became a medicine. The others are mostly remembered as footnotes, which is a pity, because you cannot really understand SS-31 without knowing where it sits in the family.
We sell SS-31, and the question of what the SS stands for comes up constantly. The answer is two surnames and a good story about paying attention to results you did not expect. Here is the family, member by member, and what each one teaches.
Table of Contents
Table of Contents
Who Szeto and Schiller are
Hazel Szeto is a physician and pharmacologist who spent most of her career at Weill Cornell Medical College in New York. Her background was in opioids and how drugs reach the brain and the fetus.
Peter Schiller is a peptide chemist who ran a laboratory at the Clinical Research Institute of Montreal for decades. His speciality was designing opioid peptides: small synthetic molecules that act on the same receptors as morphine but with different properties.
They began working together in the 1990s. Schiller's lab made the molecules. Szeto's lab tested what they did in cells and animals. The aim was better painkillers. Nobody involved was thinking about mitochondria.
The idea behind every Szeto-Schiller peptide
Before going through the compounds one by one, it helps to see what they share. Every Szeto-Schiller peptide is built on the same plan.
- Four amino acids.
- Aromatic and positively charged residues in alternation.
- A net charge of plus three at body pH.
- A D-amino acid in the first or second position.
- An amide cap at the far end.
Aromatic residues are oily and like to sit in membranes. Charged residues are the opposite, and normally keep a molecule out of membranes altogether. Alternating the two in such a short chain produces something that can do both: dissolve freely in water and still slip across a lipid barrier.
That was not the textbook expectation. The rule of thumb in drug design was that three positive charges would stop a molecule getting into cells at all. These peptides broke the rule, and working out why is what opened up the whole field.
SS-01: the starting point
The first compound in the line has the sequence Tyr-D-Arg-Phe-Lys-NH2. Schiller's group published it in the late 1980s under the name DALDA. It was designed as a highly selective activator of the mu opioid receptor, the one responsible for most of morphine's painkilling effect.
DALDA was meant to stay out of the brain. With three positive charges it was not expected to cross the blood-brain barrier, which made it interesting as a painkiller that would act only in the body, without sedation or addiction.
In the numbering used later, DALDA became SS-01. It is an opioid first and foremost. It has a tyrosine with a free phenol group, which gives it some ability to react with free radicals, but that was not noticed or cared about at the time.
SS-02: the surprise
The next step was a small change. Replace the tyrosine at position one with 2,6-dimethyltyrosine, abbreviated Dmt. This is a common trick in opioid chemistry. The two extra methyl groups usually increase potency.
The result, Dmt-D-Arg-Phe-Lys-NH2, was an extraordinarily potent painkiller in animals, far stronger than morphine when given into the spinal fluid. It was called [Dmt1]DALDA, and later SS-02.
Then came the unexpected findings. Szeto's lab found that this highly charged peptide was taken up by cells. Not slowly, and not by any transporter they could identify. It simply went in. When they looked at where it went inside the cell, it was in the mitochondria, concentrated hundreds to thousands of times over.
A second finding followed. Dmt is a much better radical scavenger than plain tyrosine. So here was a molecule that delivered an antioxidant group directly to the place in the cell where most reactive oxygen species are made.
That was interesting in its own right. But SS-02 was still a powerful opioid. You could not study its mitochondrial effects in an animal without also sedating the animal. The opioid activity had to go.
SS-31: the same parts, rearranged
Opioid peptides need their tyrosine, or Dmt, at the very first position, with a free amine on it. That is how the receptor recognises them. Move it and the opioid activity vanishes.
So that is what they did. Take the same four building blocks and shuffle them: D-Arg-Dmt-Lys-Phe-NH2. The Dmt is now second. The alternating pattern is preserved, the three charges are preserved, the radical-scavenging phenol is preserved. The opioid activity is gone.
This is SS-31. It kept everything that made SS-02 interesting to mitochondria researchers and dropped the one thing that made it unusable. A 2004 paper in the Journal of Biological Chemistry laid out the results: uptake into cells and mitochondria, reduction of reactive oxygen species, prevention of mitochondrial swelling, and protection of heart tissue from injury.
Of every Szeto-Schiller peptide made, this is the one that went furthest. It was licensed to a company, renamed several times, tested in thousands of patients and eventually approved in 2025 for a rare mitochondrial disease. We tell that story in our article on elamipretide, and we cover the naming in MTP-131 and its other names.
SS-20: the control that turned out to be more
The fourth compound was designed to answer a question. Is the benefit of SS-31 down to the antioxidant Dmt group, or to something else about the Szeto-Schiller peptide?
SS-20 has the sequence Phe-D-Arg-Phe-Lys-NH2. No tyrosine, no Dmt, no phenol. It cannot scavenge radicals. It has no opioid activity either. But it has the same alternating pattern and the same charge, and it goes to mitochondria just the same.
The expectation was that SS-20 would do nothing, proving that the antioxidant group was the active part. That is not what happened. In a number of models SS-20 was protective too. It reduced reactive oxygen species even though it could not react with them directly.
This was an important clue. If a Szeto-Schiller peptide with no scavenging ability still reduced oxidative damage, then scavenging could not be the whole mechanism. Something about simply being at the inner mitochondrial membrane was doing the work, perhaps by making the electron transport chain run more cleanly so that fewer reactive species were produced in the first place.
That line of thinking led, a few years later, to the discovery that each Szeto-Schiller peptide bind cardiolipin. We explain that in detail in our piece on MTP131 and cardiolipin.
SS-20 is still used in research as a comparison compound. It was also taken into early development by the same company under a separate code.
The family at a glance
It can be hard to keep them straight, so here they are side by side.
- SS-01, Tyr-D-Arg-Phe-Lys-NH2. Opioid activity: yes. Radical scavenging: weak. Goes to mitochondria: yes.
- SS-02, Dmt-D-Arg-Phe-Lys-NH2. Opioid activity: very strong. Radical scavenging: yes. Goes to mitochondria: yes.
- SS-31, D-Arg-Dmt-Lys-Phe-NH2. Opioid activity: none. Radical scavenging: yes. Goes to mitochondria: yes.
- SS-20, Phe-D-Arg-Phe-Lys-NH2. Opioid activity: none. Radical scavenging: none. Goes to mitochondria: yes.
Read down the last item in each line. Every one of them reaches mitochondria. That property comes from the shared architecture, not from any single residue. The other properties were switched on and off by changing which amino acid sat where.
It is a tidy piece of medicinal chemistry. Four compounds, each differing from the next by one deliberate change, each answering a specific question.
How a Szeto-Schiller peptide gets into mitochondria
This puzzled people for a while, and the full answer is still debated. What is known is this.
Uptake into cells is fast, taking minutes. It does not need energy. It is not blocked by inhibitors of the usual transport routes. It happens at low temperature. All of that points to the Szeto-Schiller peptide passing directly through the membrane and not being carried by a protein.
Once inside, it concentrates at the inner mitochondrial membrane. Compounds like MitoQ get there by being pulled in by the membrane's electrical potential, and they end up in the innermost compartment. The Szeto-Schiller compounds depend much less on that potential. Even when mitochondria are depolarised, a large share of the Szeto-Schiller peptide still gets in and stays bound.
The reason is cardiolipin. The inner membrane is rich in this doubly negative lipid, and a peptide with three positive charges and two aromatic rings has a strong affinity for it. The peptide is not so much pumped in as drawn to a surface it sticks to.
This has a practical upside. Sick mitochondria tend to have low membrane potential. A compound that relies on potential will reach them poorly. A Szeto-Schiller peptide still finds them.
Why only SS-31 is widely sold
If you go looking to buy a Szeto-Schiller peptide for research, you will find SS-31 everywhere and the others almost nowhere. There are simple reasons.
SS-01 and SS-02 are opioids. SS-02 in particular is extremely potent. That brings regulatory complications that most suppliers and most labs would prefer to avoid, and it makes them unsuitable for studying mitochondria in isolation.
SS-20 is useful mainly as a control. Demand is small, so few suppliers keep it in stock. It can be made to order.
SS-31 has the published literature, the clinical history and the clean pharmacology. It is the one researchers ask for. It is the only one we carry as a stock item, in four grades:
- Standard Grade, 98 percent or better.
- High Purity Grade, for everyday cell work.
- Ultra-Pure Research Grade, 99.5 percent or better with HPLC and mass spec.
- Bulk Research Grade, in vials up to 500 mg.
If your study design needs SS-20 alongside it as a control, write to us through the contact page. We will tell you honestly whether we can source it for you and how long it would take.
What the family teaches about experimental design
Even if you only ever use SS-31, the history of the other three is a lesson in controls.
The original researchers did not just show that their compound worked. They made a version without the opioid activity to rule that out. They made a version without the antioxidant group to test whether that was the mechanism. Each new Szeto-Schiller peptide was a question put to the previous one.
You can apply the same thinking at the bench.
- If you see an effect with SS-31, ask whether a general antioxidant such as N-acetylcysteine reproduces it. If not, scavenging is unlikely to be the explanation.
- Ask whether a mitochondria-targeted antioxidant of a different class reproduces it.
- If you can get SS-20, see whether it does the same. If it does, the effect is about membrane localisation and not about Dmt.
- Test in both stressed and unstressed conditions. The peptides do little in healthy mitochondria.
We go further into models and controls in our piece on Szeto Schiller 31 research models.
Handling is the same across the family
Chemically these compounds are close cousins. All are highly water-soluble, all are supplied as freeze-dried salts, all are stable when kept dry and cold. The practical advice for one applies to the rest: store at minus 20, warm the vial before opening, dissolve in water or simple buffer, split into single-use portions.
The routine is laid out in our SS-31 peptide storage guide, and the points about salt forms and net content in elamipretide peptide chemistry apply equally.
Ordering from us
We supply wholesale, from a minimum of 50 units with no upper limit. You can combine SS-31 with our other compounds, BPC-157, GHK-Cu, Retatrutide and TB-500, to reach the minimum.
To order, open the SS-31 peptide page, press Order beside the grade and size you want, and fill in the quantity and delivery address. We reply by email with the total, usually on the same day. We take Apple Pay, Zelle, Chime, Cash App, USDT and Bitcoin, and every order ships with the Certificate of Analysis for its lot.
Everything we sell is for laboratory and in-vitro research only. It is not for human or animal use.
Why four residues, and why this pattern
People sometimes ask whether a Szeto-Schiller peptide could be longer, shorter or arranged differently and still work. The original researchers asked the same thing and tested it.
Four residues seems to be close to the smallest unit that carries the pattern. You need at least two aromatic and two charged groups to alternate, and that takes four positions. Shorter and the pattern is gone.
Longer chains were not pursued in the same way, partly because the short ones already worked and partly because each extra residue makes a peptide harder to get into cells and easier for enzymes to cut.
The order matters in subtle ways. A Szeto-Schiller peptide can start with an aromatic residue, as SS-02 and SS-20 do, or with a charged one, as SS-31 does. Both arrangements reach mitochondria. What changes is everything else: receptor activity, scavenging ability, and probably the fine detail of how the Szeto-Schiller peptide lies on the membrane.
The D-amino acid is there for stability, as is the amide at the end. Without them, the peptides would be broken down in minutes. With them, they last for hours in blood and tissue.
So each Szeto-Schiller peptide is a compromise worked out by trial: as small as possible, as stable as possible, with the charge and oiliness balanced so that it dissolves in water and still crosses membranes. That balance is narrow, which is one reason there are only a few of them.
What came after the original four
The family did not stop growing in 2004. Once the cardiolipin mechanism was understood, the company that licensed the compounds began designing new molecules on the same principle. These later compounds are not all peptides in the strict sense. Some replace parts of the backbone with other chemistry to improve how they are absorbed or how well they reach particular tissues such as the brain.
They carry different code names and are at earlier stages of testing. We mention them only so that you are not confused if you meet them in the literature. They are descendants of the Szeto-Schiller peptide idea, not members of the original set, and they are not available as research chemicals in the way SS-31 is.
Other groups have made their own variations as well, attaching the four-residue motif to dyes for imaging or to other active groups to carry them into mitochondria. In that sense the Szeto-Schiller peptide has become a delivery tool as well as a compound in its own right.
Mix-ups we see with the Szeto-Schiller peptide names
A few recurring confusions, briefly.
SS-31 and SS-20 are not two strengths of the same thing. They are different sequences with different properties.
SS-31 is not thirty-one amino acids long. The number is a serial number. It has four.
The SS does not stand for a chemical term. People have guessed at sulfur-sulfur, as in a disulfide bond, or at some abbreviation for a class of drug. It is just Szeto and Schiller.
A Szeto-Schiller peptide is not the same as a cell-penetrating peptide in the usual sense. Classic cell-penetrating peptides such as TAT are longer, rely on different uptake routes and do not home to mitochondria.
And not every mitochondria-targeted peptide is a Szeto-Schiller peptide. Other research groups have developed separate families with their own designs. The name belongs to this particular series.
Why the history is worth knowing as a buyer
This might all sound academic. It has a practical edge. A supplier who can tell you how SS-31 differs from SS-20, and why one is an opioid relative and the other is not, understands what they are selling. One who cannot is reselling a powder. When you are deciding whom to trust with a study, that is a fair thing to test, and a two-line email will do it.
A quick test of understanding
If you can answer three questions, you have the family straight. Which Szeto-Schiller peptide is an opioid? SS-01 and SS-02. Which Szeto-Schiller peptide cannot scavenge radicals? SS-20. Which one do researchers actually order? SS-31. Everything else in this article is detail hung on those three answers.
Questions about the Szeto-Schiller peptide family
How many Szeto-Schiller peptides are there?
Four are commonly discussed: SS-01, SS-02, SS-20 and SS-31. Others were made and tested but did not become widely used.
Is every Szeto-Schiller peptide an antioxidant?
No. SS-20 has no radical-scavenging group. That is the point of it.
Is SS-31 an opioid?
No. Its relatives SS-01 and SS-02 are, but in SS-31 the residue the opioid receptor needs has been moved out of position. It has no meaningful opioid activity.
Why are the numbers not in order?
They reflect the order in which compounds were made in the lab, and many in between were not pursued.
Where can I read the original research?
A good start is to search PubMed for Szeto-Schiller peptides. Hazel Szeto has written several clear review articles.
Closing thought
The story of the Szeto-Schiller peptide family is a reminder that the most useful result in a project is sometimes the one that does not fit. Two researchers were making opioids. One of their compounds turned up in the wrong part of the cell. They asked why, changed one thing at a time, and ended up founding a new class of mitochondrial compounds.
SS-31 is the member of that class that lasted. If you would like to work with it, our essential facts about SS-31 is the place to start, and the SS31 buying guide will help you choose a grade. A general overview is also on Wikipedia.