As if in protest, the stems of many species of mushroom instantly turn blue when they are plucked. The mechanism that underlies this ‘bluing’ is well known for some mushrooms, but bluing of psychedelic mushrooms that contain a compound called psilocybin has puzzled scientists for decades.

Dirk Hoffmeister at the Hans Knöll Institute in Jena, Germany, and his colleagues investigated the mushroom Psilocybe cubensis and discovered an enzyme that they named PsiP. When a mushroom is bruised or sliced, PsiP cuts off the phosphorus-containing portion of the psilocybin molecule, freeing the psychoactive molecule psilocin.

A second enzyme that the scientists named PsiL then destabilizes psilocin by stealing an electron from it. That forces individual psilocin molecules to fuse into pairs, trios and larger groupings. Some of the psilocin assemblies turn into blue compounds after losing hydrogen atoms. This process might explain the bluing of other psilocybin-laced mushrooms, such as Psilocybe azurescens.

Enzymes that behave like PsiP are also found in the human body. There, the psilocin produced by the enzymes creates psychedelic effects rather than a blue colour.