Page 90 - Phytochemistry 2 (06-PG 605)

Physostigma alkaloids
The seeds of Physostigma venenosum (Leguminosae)

contains some indole alkaloids, the main one being physostigmine,

which is also known as eserine. Physostigmine is extremely

toxic, but it is a good parasympathomimetic agent. It acts

physiologically by inhibiting the enzyme choline-estrase. The chief

use of physostigmine is as a miotic in glaucoma, and also as a

diaphoretic in cases of kidney dysfunction.

CH3NHCOO CH3 HO CH3
NN Hydrolysis NN

OH -

CH3 CH3 CH3 CH3

Physostigmine (Eserine)

Eseroline

Physostigmine is very unstable in solutions, especially in

alkaline solutions. It is oxidized by exposure to air and in presence

of alkali giving the red dye stuff rubreserine (C13H16O2N2). So, it

should be protected from air and light. It is hydrolyzed in alkaline

solutions into the phenolic compound eseroline and methylamine

and CO2.

The base physostigmine occurs as colourless crystals which

are soluble in water and immiscible in organic solvents except

petroleum ether. It is laevorotatory. The salicylate is the salt

usually used in therapeutics as it is much stable and less

hygroscopic than other salts e.g. sulphate.

Tests for identification

1. Oxidation test (formation of rubreserine).

Shaking an alkaline solution red.

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