Calabar Bean

A plant known for its toxic seeds used historically in trials by ordeal and medicine.

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Calabar Bean
Kingdom:	Plantae
Phylum:
Class:
Order:	Fabales
Family:	Fabaceae

The Calabar Bean (Physostigma venenosum) is a highly toxic seed from a plant native to tropical Africa, particularly Nigeria. It belongs to the family Fabaceae, which is commonly known as the legume, pea, or bean family. The plant is a perennial climbing vine that produces pods containing the seeds known as Calabar beans.

Description

The Calabar bean plant is a woody vine that can grow up to 15 meters in length. It has compound leaves with three leaflets and produces purple to pink flowers. The seeds are oblong, about 2-3 cm long, and have a hard, glossy coat.

Toxicity

The seeds of the Calabar bean contain a potent alkaloid called physostigmine, also known as eserine. Physostigmine is a reversible inhibitor of the enzyme acetylcholinesterase, which breaks down the neurotransmitter acetylcholine. By inhibiting this enzyme, physostigmine increases the levels of acetylcholine in the synapses, leading to prolonged muscle contraction and potentially fatal convulsions.

Historical Use

In the 19th century, the Calabar bean was used in West Africa as an ordeal poison in trials to determine guilt or innocence. The accused would ingest the bean, and survival was considered a sign of innocence, while death indicated guilt. This practice was based on the belief that the innocent would be protected by the divine.

Medical Applications

Despite its toxicity, physostigmine has been used in medicine. It was historically used to treat glaucoma by reducing intraocular pressure. It has also been used as an antidote for atropine and other anticholinergic drug overdoses. However, due to its narrow therapeutic index and the availability of safer alternatives, its use is now limited.

Cultivation and Harvesting

The Calabar bean plant is cultivated in its native regions for its seeds. The seeds are harvested when the pods mature and dry. Due to the plant's toxicity, handling and processing require caution.

Also see

• Physostigmine
• Acetylcholinesterase inhibitor
• Glaucoma
• Fabaceae

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