Which West African seed is the natural source of physostigmine, used in glaucoma treatment? The Calabar bean is the seed that naturally contains physostigmine.
The Calabar bean is the seed of Physostigma venenosum, a climbing legume native to tropical West Africa. It is also called the ordeal bean. The seed contains physostigmine, also known as eserine, a highly toxic reversible inhibitor of acetylcholinesterase.
Its history is unusually dramatic. In the Old Calabar region, in what is now southeastern Nigeria, the bean was used as an ordeal poison in accusations of witchcraft or other crimes. Western scientists studied it in the nineteenth century, and its ability to constrict the pupil and lower pressure inside the eye led to medical use. Physostigmine was first used to treat glaucoma in the 1870s.
The key distinction is between the plant and its active compound: Calabar bean is the natural source, while physostigmine is the alkaloid and drug. The seed is extremely poisonous, so it is not comparable to ordinary edible beans such as coffee, tonka or castor beans.