Met-enkephalin-Arg-Gly-Leu

Where it comes from

Made in

Adrenal medulla, striatum and spinal cord

Sent out by

With the enkephalins

Acts on

• Opioid receptors

What it does

• An active opioid in its own right, released from striatal tissue alongside the heptapeptide and inactivated as fast ¹
• Mapped with antisera raised against its Arg-Gly-Leu tail, which is how its nerves can be told from met-enkephalin’s ²
• Trimmed to met-enkephalin in tissue, so the amount found depends on how quickly it is cut

In medicine

• No medicine copies it or blocks it, and no clinical laboratory measures it
• Its use is as a read-out of processing rather than of opioid tone: the ratio of octapeptide to pentapeptide differs between adrenal, striatum and gut, which reports on the cutting enzymes of each tissue ¹

Beyond the body’s own use

• Nothing is in trials, and no analogue of it has been given to a person
• Its one distinctive finding is anatomical: antisera to the octapeptide label a set of nerves in lung and pancreas that run with VIP rather than with the classic enkephalin pathways ²
• What is unknown is whether the extension does more than slow its breakdown. Oxidising its methionine turns it into a substrate for neutrophil elastase, so inflamed tissue can destroy it by a route the ordinary peptidases do not use ³