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(also eserine), C15H21O2N3, the chief alkaloid of Calabar beans, which are the seeds of the tropical African woody vine Physostigma venenosum. Physostigmine was discovered in 1864 by the German chemists Jobst and Hesse; the first synthesis of the alkaloid was carried out in 1935 by the American chemists P. Julian and J. Pikl. A weak base, the alkaloid yields salts that readily crystallize. It is soluble in alcohol, ether, and chloroform. The structural formula is
Physostigmine is highly poisonous. It is used in medicine, where it is classified as a reversible anticholinesterase. When administered medically, physostigmine blocks the enzyme cholinesterase, thereby protecting acetylcholine from a rapid hydrolytic breakdown. Physostigmine thus induces effects in the organism that are outwardly similar to those induced by acetylcholine and cholinomimetic agents (contraction of the pupils, slowed heartbeat, intensification of uterine contractions and of peristalsis in the stomach and intestine).
The protection of acetylcholine effected by physostigmine has made possible the discovery that acetylcholine is given off at the endings of the parasympathetic nerves and that nerve impulses are transmitted chemically. Physostigmine salicylate is used medically as an anticholinesterase. It is used mainly in ophthalmology, where in drop form it serves to contract the pupils and to reduce the intraocular pressure arising from glaucoma; in ointment form, it is effective against keratitis. Hypodermic injections of physostigmine are sometimes prescribed for neuromuscular diseases (myasthenia), enteroparesis, and paresis of the urinary bladder. Physostigmine is an antidote for atropine and curarine. Synthetic substitutes for physostigmine, for example, neostigmine, are available.
V. V. PARIN