Which alkaloid is the primary toxic constituent of Datura stramonium responsible for anticholinergic poisoning?

Explanation

## Datura Alkaloid Composition **Key Point:** Datura stramonium contains three main tropane alkaloids: hyoscyamine (primary, ~50%), scopolamine (~30%), and trace amounts of other tropanes. Atropine is NOT a naturally occurring constituent of Datura — it is the racemic form (dl-hyoscyamine) that forms as an artifact during extraction or as an in vivo metabolite of hyoscyamine. ### Mechanism of Toxicity Hyoscyamine (l-hyoscyamine) is the predominant naturally occurring alkaloid in Datura stramonium and acts as a potent competitive antagonist at muscarinic acetylcholine receptors. It is approximately twice as pharmacologically active as its racemate (atropine) at muscarinic receptors, making it the primary driver of the anticholinergic syndrome seen in Datura poisoning. ### Clinical Manifestation The anticholinergic toxidrome presents as: - Mydriasis (dilated pupils) - Dry mouth and skin ("dry as a bone") - Tachycardia ("hot as a pistol") - Hyperthermia ("red as a beet") - Agitation and delirium ("mad as a hatter") - Urinary retention **Clinical Pearl:** The mnemonic "Dry as a bone, red as a beet, mad as a hatter, hot as a pistol" captures the anticholinergic syndrome, which is primarily caused by hyoscyamine blocking muscarinic receptors. Scopolamine contributes additional CNS depression (sedation, amnesia), but hyoscyamine is quantitatively and pharmacologically the dominant alkaloid. **High-Yield:** Atropine (dl-hyoscyamine) is NOT present as a native alkaloid in Datura stramonium — it is formed from hyoscyamine during extraction (racemization) or metabolically in vivo. Cocaine is entirely unrelated, being a tropane alkaloid from Erythroxylum coca with sympathomimetic, not anticholinergic, effects. (Reference: Casarett & Doull's Toxicology; KD Tripathi Essentials of Medical Pharmacology, 8th ed.) ## Differential Alkaloids | Alkaloid | Source | Primary Effect | Datura Role | |----------|--------|----------------|-------------| | Hyoscyamine | Datura, Belladonna | Anticholinergic (potent) | **Primary natural alkaloid (~50%)** | | Scopolamine | Datura, Hyoscyamus | Anticholinergic + CNS depression | Secondary natural alkaloid (~30%) | | Atropine | Belladonna (racemic hyoscyamine) | Anticholinergic | NOT a native Datura constituent; artifact/metabolite of hyoscyamine | | Cocaine | Coca plant | Sympathomimetic | Unrelated to Datura |