Frog-skin painkiller peptide (D-Met2-Deltorphin)
A natural peptide from a Brazilian tree frog's skin that binds opioid receptors and is used to study pain pathways in the lab, a research tool, not an approved drug.
A researcher, an agent, or an algorithm wrote down the sequence and picked a target to hit.
An AI model like OpenFold3 or AlphaFold built a 3D structure and scored how well it fits the binding site.
A second contributor repeated the computation on their own hardware and the scores matched.
A chemistry service or a researcher ordered the sequence, it was manufactured, and mass spectrometry confirmed the right molecule was produced.
A binding or activity measurement confirmed that it actually does what the computer predicted — or didn't.
What this is
D-Met2-Deltorphin is a seven-amino-acid opioid peptide originally isolated from the skin glands of the Jandaia leaf frog (Phasmahyla jandaia), a phyllomedusine tree frog endemic to the southern Espinhaço mountain range in Brazil. It belongs to the deltorphin family — a group of naturally occurring peptides notable for carrying an unusually rare D-amino acid at the second position of their sequence, a feature that gives them exceptional binding properties at opioid receptors. The peptide has no approved medical use and is used primarily as a pharmacological research tool.
The stored sequence YDMFHLMD is the bare eight-residue backbone. The active form carries a C-terminal amide (-NH₂) not visible in the raw sequence, and the methionine at position 2 is in the D-configuration (D-Met²) — encoded as a standard M in the stored letters. Both features are central to the peptide's receptor-binding behaviour.
History
The deltorphin family was first brought to scientific attention in 1989 through near-simultaneous reports from several groups. Kreil and colleagues coined the name "deltorphin" for these delta-receptor-preferring peptides isolated from Phyllomedusa sauvagei skin, and Erspamer and colleagues (PNAS 1989) characterized a family of heptapeptides from the same source with higher affinity and selectivity for delta-opioid binding sites than any natural compound known at the time. That same year, Lazarus and colleagues (JBC 1989) characterised a related variant, [D-Met²] dermorphin gene-associated peptide (DGAP), and found it carried high delta-selectivity — measurements showed an IC50 of 0.80 nM at delta receptors and greater than 1 µM at mu receptors, a selectivity ratio of approximately 1345.
Phasmahyla jandaia is a distinct frog species within the broader Phyllomedusinae subfamily. In 2011, Rates and colleagues published a systematic peptidomic survey of this frog's skin secretion, sequencing 57 peptides by tandem mass spectrometry and Edman N-terminal sequencing, and confirmed that the skin contains opioid peptides including D-Met2-Deltorphin alongside antimicrobial peptides (phylloseptins, dermaseptins, dermatoxins), bradykinin-related compounds, and several peptide families without similarity to any previously known molecules (Rates and colleagues, Toxicon 2011).
What it does
Deltorphins act primarily as agonists at delta-opioid receptors, triggering the inhibitory signalling cascade associated with reduced neuronal excitability. The common N-terminal sequence Tyr-D-Xaa-Phe shared by all amphibian opioid peptides appears to function as the "message" motif that engages opioid receptors; the C-terminal residues determine which receptor subtype is preferentially activated. In deltorphins, including the D-Met² form, the C-terminal region steers selectivity toward delta rather than mu receptors — in contrast to the related frog peptide dermorphin (/card/pep-10694), which carries a C-terminal sequence that confers mu selectivity.
Beyond direct receptor activation, D-Met2-Deltorphin has been used as a research tool to probe how opioid receptor subtypes differ in their intracellular fate after activation. Schulz and colleagues (JPET 2002) used fluorescently-tagged delta and mu receptor constructs in live cells to show that delta-opioid receptor activation triggers rapid translocation of G-protein-coupled receptor kinases (GRK2 and GRK3) to the cell membrane, with the result that delta receptors and GRKs cointernalize together into intracellular vesicles. Mu-opioid receptors, by contrast, also internalize upon activation but did not cointernalize with GRK2 or GRK3 in the same system — a mechanistically distinct profile that affects receptor resensitization and tolerance.
Evidence
- Human: No human trials identified for D-Met2-Deltorphin.
- Animal: The deltorphin family has been studied in rodent models of analgesia; the naturally occurring D-amino acid confers protease resistance relative to L-amino-acid sequences.
- In vitro: Binding studies report high delta-opioid receptor affinity for the D-Met² deltorphin variant (Lazarus and colleagues, JBC 1989). Schulz and colleagues (JPET 2002) demonstrated differential GRK2/3 cointernalization between delta and mu opioid receptors in transfected HEK293 and neuroblastoma-glioma cells.
Known effects
- Delta-opioid receptor agonism — Pharmacological characterization; high delta selectivity in binding assays (Lazarus and colleagues 1989)
- Differential receptor internalization — Delta receptors cointernalize with GRK2/3; mu receptors do not (Schulz and colleagues 2002)
- Protease resistance — Conferred by the D-amino acid at position 2; common to this peptide family
Mechanism
D-Met2-Deltorphin shares the conserved Tyr-D-Xaa-Phe motif at the N-terminus with other frog-skin opioid peptides, which serves as the minimal pharmacophore for opioid receptor engagement. The D-methionine at position 2 (D-Met²) stabilises a bioactive conformation that favours delta-opioid receptor binding over mu-opioid receptor binding. Delta-opioid receptors are class A GPCRs that signal primarily through Gαi/o proteins, inhibiting adenylate cyclase and reducing intracellular cAMP.
In live-cell fluorescence microscopy studies, delta-opioid receptor activation promoted Gβγ-dependent recruitment of GRK2 and GRK3 from the cytosol to the plasma membrane; treatment with phosducin (a Gβγ scavenger) blocked this translocation, confirming the G-protein dependence of the process (Schulz and colleagues, JPET 2002). The resulting cointernalization of delta receptors with GRKs distinguishes delta from mu receptors and has implications for how each receptor type desensitises and re-sensitises after repeated agonist exposure — a mechanistic difference relevant to tolerance research.
Related peptides
- Dermorphin — the mu-selective counterpart from Phyllomedusa sauvagei skin; shares the Tyr-D-Xaa-Phe N-terminal motif but carries a C-terminal sequence that confers mu rather than delta selectivity
Research directions for this peptide, selected from the current sources — hypotheses you can explore and model. None of it is proven yet; tap any one to see the full thinking.
Is D-Met2-Deltorphin actually a delta-opioid selective peptide that has been mislabeled as a mu-opioid ligand?
If true, this peptide would be correctly recognized as a precision tool for studying the delta opioid system, which is linked to mood and chronic pain, potentially guiding development of non-addictive pain treatments.
Does the frog peptide's resistance to being broken down make it drive receptor desensitization longer than the body's own opioid molecules?
If true, this would help explain why tolerance to delta-opioid drugs develops and could point to dosing strategies that minimize tolerance for people needing long-term pain management.
Does the mirror-image amino acid at position 2 force the peptide into a specific bent shape that lines up its two key aromatic residues (Tyr1 and Phe3) to fit the delta opioid receptor?
If confirmed, this could guide the design of smaller, more stable molecules that copy just the active turn of the frog peptide, a possible route to new options for chronic pain.
If the unusual D-amino acid at position 2 is swapped for other mirror-image building blocks, can the peptide's receptor preference be precisely controlled?
If the pattern holds, chemists could use this rule to create a family of stable, selective opioid peptides tailored to specific receptor subtypes, potentially improving pain treatment while reducing addiction risk.
Could this protease-resistant frog peptide, by activating delta opioid receptors on microglia, dampen harmful inflammation in the brain after injury?
If true, this peptide could be a starting point for treatments targeting brain inflammation after stroke or head injury, conditions with few effective options today.
▸full evidence table2 metrics
| metric | value | tool |
|---|---|---|
| ipTM | 0.9578285813331604 | boltz-2 |
| ranking score | 0.8095315098762512 | boltz-2 |
▸structural qualityopenfold3
| metric | value | note |
|---|---|---|
| gpde | 0.615 | global PDE — lower = better |
| disorder | NaN | fraction disordered |
▸3-letter notation
▸recipeboltz-2 1.0
| parameter | value |
|---|---|
| model | boltz-2 1.0 |
| weights | — |
| hardware | nvidia_nim_api |
| mlx version | — |
| python | — |
| random seed | — |
| msa strategy | none |
| diffusion samples | 1 |
| runtime | — |
| predicted by | mlx@peptide |
| predicted at | 2026-04-24 |
▸citationbibtex
@peptide{pep10695,
sequence = {YDMFHLMD},
target = {oprm1},
author = {peptidemodel},
year = {2026},
status = {synthesized}
}