What is an indole alkaloid in simple terms?

It is a plant-derived nitrogen-containing compound whose chemistry is built around the indole ring system - a fused bicyclic aromatic ring with one nitrogen atom. Mitragynine and 7-OH are indole alkaloids.

Is mitragynine an indole alkaloid?

Yes. Mitragynine is a member of the corynantheine subclass of indole alkaloids.

Is 7-OH an indole alkaloid?

Yes. 7-Hydroxymitragynine retains the indole ring system of its parent compound mitragynine.

What other notable compounds are indole alkaloids?

Notable non-kratom indole alkaloids include the anticancer vinca alkaloids vincristine and vinblastine, the antihypertensive reserpine, strychnine, the ergoline alkaloids of ergot fungus, and psilocybin/psilocin from psychoactive mushrooms.

How does an indole alkaloid differ from an oxindole alkaloid?

An oxindole alkaloid contains an oxidized form of the indole ring system. The kratom alkaloids corynoxine and corynoxeine are oxindole-class compounds, distinct from mitragynine and 7-OH.

Alkaloids & Chemistry

Indole Alkaloid

At-a-glance comparison

Spec	Value
Defining structural feature	Indole ring system (fused benzene and pyrrole)
Notable subclasses relevant to kratom	Corynantheine, oxindole (corynoxine), pseudoindoxyl
Other notable indole alkaloids (non-kratom)	Vincristine, reserpine, strychnine, ergoline alkaloids, psilocybin (psilocin), serotonin (technically an indolamine biosynthetic precursor)
Plant families known for indole alkaloids	Apocynaceae, Loganiaceae, Rubiaceae (which includes Mitragyna speciosa)
Biosynthetic origin	Tryptophan amino acid pathway

What is an indole alkaloid?

An indole alkaloid is a nitrogen-containing organic compound of plant origin (an alkaloid) that is built around the indole ring system. The indole ring is a fused bicyclic aromatic structure consisting of a benzene ring fused to a five-membered pyrrole ring; it contains one nitrogen atom and is one of the most pharmacologically prevalent structural motifs in nature.

Indole alkaloids are biosynthetically derived from the amino acid tryptophan. The tryptophan-derived indole core is elaborated by plant biosynthetic pathways into thousands of distinct natural products. Many of these compounds have been historically consequential in pharmacology - examples include the vinca alkaloids vincristine and vinblastine (used as anticancer agents), reserpine (an antihypertensive), strychnine (a poison), the ergoline alkaloids of ergot fungus, and the psilocybin/psilocin alkaloids of certain mushroom species. Serotonin itself is an indolamine, biosynthetically related though not classified as an alkaloid by most authorities.

Indole alkaloids in Mitragyna speciosa

The great majority of alkaloids identified in Mitragyna speciosa are indole alkaloids of the corynantheine subclass. The dominant member, mitragynine, contains the indole core elaborated with a methoxy group, an ethyl group, and a methyl-acrylate moiety in a configuration characteristic of the corynantheine alkaloid family. 7-Hydroxymitragynine is a hydroxylated analog of mitragynine; mitragynine pseudoindoxyl is a rearrangement product in which the indole core has been transformed into a pseudoindoxyl skeleton.

Other corynantheine-class kratom alkaloids include paynantheine, speciogynine, speciociliatine, and corynantheidine. The genus also contains oxindole-class alkaloids such as corynoxine and corynoxeine, in which the indole ring system is oxidized into an oxindole - a closely related but distinct ring system.

Why the indole structural family matters pharmacologically

The indole ring system appears in many human signaling molecules - including the neurotransmitter serotonin and the hormone melatonin - and in many pharmacologically active natural products. As a result, indole-based natural products often interact with human receptor systems that have evolved to recognize indole-based endogenous ligands. The receptor activity of mitragynine and 7-OH at multiple receptor families (opioid, serotonergic, adrenergic) reflects this broader pattern.

Common questions about indole alkaloid

What is an indole alkaloid in simple terms?: It is a plant-derived nitrogen-containing compound whose chemistry is built around the indole ring system - a fused bicyclic aromatic ring with one nitrogen atom. Mitragynine and 7-OH are indole alkaloids.
Is mitragynine an indole alkaloid?: Yes. Mitragynine is a member of the corynantheine subclass of indole alkaloids.
Is 7-OH an indole alkaloid?: Yes. 7-Hydroxymitragynine retains the indole ring system of its parent compound mitragynine.
What other notable compounds are indole alkaloids?: Notable non-kratom indole alkaloids include the anticancer vinca alkaloids vincristine and vinblastine, the antihypertensive reserpine, strychnine, the ergoline alkaloids of ergot fungus, and psilocybin/psilocin from psychoactive mushrooms.
Where does the indole ring come from biosynthetically?: Indole alkaloids are biosynthetically derived from the amino acid tryptophan.
How does an indole alkaloid differ from an oxindole alkaloid?: An oxindole alkaloid contains an oxidized form of the indole ring system. The kratom alkaloids corynoxine and corynoxeine are oxindole-class compounds, distinct from mitragynine and 7-OH.

References

Brown PN, Lund JA, Murch SJ. (2017). A botanical, phytochemical and ethnomedicinal review of the genus Mitragyna korth. Journal of Ethnopharmacology.
Hassan Z, Muzaimi M, Navaratnam V, et al. (2013). From kratom to mitragynine and its derivatives. Neuroscience and Biobehavioral Reviews. PMID 23206666.
Dewick PM. (2009). Medicinal Natural Products: A Biosynthetic Approach (3rd ed.). Wiley.

Important safety information:

Products containing 7-hydroxymitragynine (7-OH) are sold for adult use only (21+). These statements have not been evaluated by the U.S. Food and Drug Administration. Products are not intended to diagnose, treat, cure, or prevent any disease. The FDA has raised safety concerns regarding concentrated 7-OH products; consult a qualified healthcare professional before use. Do not operate vehicles or machinery after use. Keep out of reach of children and pets. Laws vary by state, buyers are responsible for knowing applicable law.