Calculated with the residue masses used by the tools on this site. Dynorphin A values elsewhere on this page are for the C-terminally amidated form.
A receptor before its ligand
Morphine has been used for centuries, and by the early twentieth century it was clear that it and related compounds acted with great specificity. In 1973 three laboratories, those of Solomon Snyder and Candace Pert in Baltimore, Lars Terenius in Uppsala and Eric Simon in New York, independently demonstrated specific opiate binding sites in brain tissue.
This raised an obvious question, often credited to Avram Goldstein: why would a mammalian brain contain receptors for a compound made by a poppy? The answer had to be that the brain makes its own ligands for them. In 1975 John Hughes and Hans Kosterlitz, in Aberdeen, isolated two pentapeptides from pig brain that behaved like opiates in a bioassay and named them enkephalins, from the Greek for "in the head". Their sequences were YGGFM and YGGFL, differing only in the last residue.
Within months it was noticed that the Met-enkephalin sequence appears within a previously known pituitary peptide, β-lipotropin, at positions 61–65. That observation led to β-endorphin, the 31-residue fragment β-lipotropin 61–91. The word endorphin is a contraction of "endogenous morphine".
Three families, three precursors
The endogenous opioid peptides fall into three classical families, each cut from its own precursor protein by prohormone convertases, the processing route described in the guide to post-translational modifications.
| Family | Precursor | Main peptides | Preferred receptor |
|---|---|---|---|
| Enkephalins | Proenkephalin | Met-enkephalin, Leu-enkephalin and extended forms | δ (delta) |
| Endorphins | Proopiomelanocortin (POMC) | β-Endorphin | μ (mu) |
| Dynorphins | Prodynorphin | Dynorphin A, dynorphin B | κ (kappa) |
| Nociceptin | Pronociceptin | Nociceptin / orphanin FQ | NOP (ORL1) |
All four receptors are G protein-coupled receptors. The preferences above are relative rather than absolute: most of these peptides bind more than one receptor with different affinities.
The POMC precursor is a striking example of how much can be packed into one gene. Depending on which convertases a cell expresses, it yields β-endorphin, adrenocorticotropic hormone (ACTH), and the melanocyte-stimulating hormones. The same protein therefore contributes to opioid signalling, the stress axis and pigmentation, with the tissue deciding which products are made.
A single copy of proenkephalin contains four copies of the Met-enkephalin sequence and one of Leu-enkephalin, each flanked by pairs of basic residues that mark the cleavage sites.
The opioid motif
Nearly all of these peptides begin with the same four residues: Tyr-Gly-Gly-Phe. This N-terminal message sequence is what the receptor recognises, and the tyrosine is essential: removing it or modifying its phenolic hydroxyl group abolishes activity. The residues that follow differ between peptides and act as an address, tuning which receptor is preferred and how tightly it binds.
The same principle appears in an unrelated place. Dermorphin, from the skin of South American Phyllomedusa frogs, begins Tyr-D-Ala-Phe and is a potent opioid peptide; its D-alanine is discussed in the guide to D-amino acids. Fragments of food proteins also carry opioid-like motifs: β-casomorphins from milk casein and exorphins from wheat gluten bind opioid receptors in laboratory assays, though what they do in the body remains debated.
Leu-enkephalin as a laboratory standard
Beyond its biology, Leu-enkephalin has a second life as one of the most widely used test peptides in analytical chemistry. It is small, stable, commercially available in high purity, and ionises well, which makes it a standard for tuning and calibrating mass spectrometers.
It is the worked example in two guides on this site. The molecular weight guide uses it to show how residue masses and a single water add up to the peptide mass, and the mass spectrometry guide derives its complete set of b and y fragment ions. Entering YGGFL in the MW calculator reproduces the values in the fact sheet directly, since neither enkephalin carries any modification.
Met-enkephalin is less suitable as a standard for one specific reason: its C-terminal methionine oxidises readily to the sulfoxide, adding 15.995 Da, as described in the guide to peptide stability. The same +16 Da peak that signals a degraded sample would confuse a calibration.
Rapid degradation
Enkephalins survive only a few minutes in tissue. Two enzymes do most of the work: aminopeptidase N removes the N-terminal tyrosine, destroying the essential message residue, and neprilysin, also called enkephalinase, cleaves the Gly3–Phe4 bond. Neprilysin is the same enzyme that degrades substance P and bradykinin, described in the profiles of substance P and angiotensins and bradykinin.
This rapid breakdown is why enkephalins act locally and briefly, and why research analogues were among the earliest peptides designed for stability. DAMGO and DADLE, standard laboratory tools for studying μ and δ receptors, both carry a D-alanine at position 2, which blocks aminopeptidase cleavage. They are research reagents, not medicines.
β-Endorphin lasts considerably longer, which is consistent with its role as a circulating hormone released from the pituitary as well as a neuropeptide.
What endorphins are, and what they are not
Endorphins have acquired a large popular literature, much of which outruns the evidence. A few points are worth separating:
- The "runner's high" is not settled science. Blood levels of β-endorphin rise with sustained exercise, but β-endorphin in the blood comes largely from the pituitary and does not cross the blood–brain barrier well. Studies using brain imaging have reported changes in opioid receptor binding after exercise, while other work points to the endocannabinoid system. The question remains open.
- Endorphins are not a general "happiness chemical". The opioid peptide systems are involved in pain modulation, stress responses, reward and gut function, among others, and their effects depend on which peptide, which receptor and which brain region.
- The three families are not interchangeable. Dynorphins acting at κ receptors produce effects in animal studies that differ markedly from those of μ-receptor agonists, and are associated with aversive rather than rewarding states.
This page describes the chemistry and history of these peptides. It is not medical information, and nothing here concerns the use of opioid drugs.
Calculated properties
| Peptide | Length | Average MW | Monoisotopic | Calculated pI | Charge, pH 7.4 |
|---|---|---|---|---|---|
| Met-enkephalin | 5 | 573.66 | 573.2257 | 5.6 | −0.2 |
| Leu-enkephalin | 5 | 555.63 | 555.2693 | 5.6 | −0.2 |
| β-Endorphin | 31 | 3465.00 | 3462.8221 | 10.0 | +2.8 |
| Dynorphin A (amide) | 17 | 2146.51 | 2145.2072 | 12.2 | +4.8 |
| Dermorphin (amide) | 7 | 802.88 | 802.3650 | 8.9 | +0.8 |
Dynorphin A stands out: with four arginines and three lysines in seventeen residues it is strongly basic, and this positive charge is thought to contribute to its interaction with the κ receptor and with membranes.
Frequently asked questions
What is the difference between an endorphin and an enkephalin?
They come from different precursor proteins and differ in length. Enkephalins are pentapeptides from proenkephalin; β-endorphin is a 31-residue peptide from POMC. "Endorphin" is often used loosely in popular writing to mean any endogenous opioid peptide.
Why do so many of these peptides start with YGGF?
Because that motif is what the opioid receptors recognise, particularly the first tyrosine. The sequences that follow determine which receptor is preferred. Dermorphin achieves a similar arrangement with a different sequence and a D-amino acid.
Is Leu-enkephalin used for anything practical?
Yes, as an analytical standard. Its small size, purity and well-known fragmentation make it a common calibrant and test peptide in mass spectrometry and HPLC.
Do food-derived opioid peptides have effects in people?
β-Casomorphins and wheat exorphins bind opioid receptors in laboratory assays, but whether meaningful amounts survive digestion, cross into the blood and reach the brain is not established. The topic is an active area of research and much discussed beyond it.
References
- Hughes J, Smith TW, Kosterlitz HW, Fothergill LA, Morgan BA, Morris HR (1975) Identification of two related pentapeptides from the brain with potent opiate agonist activity. Nature 258:577–580.
- Pert CB, Snyder SH (1973) Opiate receptor: demonstration in nervous tissue. Science 179:1011–1014.
- Li CH, Chung D (1976) Isolation and structure of an untriakontapeptide with opiate activity from camel pituitary glands. PNAS 73:1145–1148.
- Goldstein A, Tachibana S, Lowney LI, Hunkapiller M, Hood L (1979) Dynorphin-(1-13), an extraordinarily potent opioid peptide. PNAS 76:6666–6670.
- Boecker H, Sprenger T, Spilker ME, et al. (2008) The runner's high: opioidergic mechanisms in the human brain. Cerebral Cortex 18:2523–2531.