ISOFLAVONOIDS

In contrast to most other flavonoids, isoflavonoids have a rather limited taxonomic distribution, mainly within the Leguminosae. Most of our knowledge about the biosynthesis of isoflavonoids originates from studies with radioactive isotopes, by feeding labelled ¹⁴C cinnamates.

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Example of a BIOLOGICALLY ACTIVE ISOFLAVONOID

Rotenone comes from Derris root and Lonchocarpus species leaf (Family: Leguminosae)
It is an insecticide and also used as a fish poison.

* (blue): carbons derived from methionine.
(red): carbons derived from PRENYL (isoprenoid). 

Biochemical pathway to the formation of rotenone.

Six rotenoid esters occur naturally and are isolated from the plant Derris eliptica found in Southeast Asia or from the plant Lonchocarpus utilis or L. urucu native to South America.

Rotenone is the most potent. It is unstable in light and heat and almost all toxicity can be lost after two to three days during the summer. It is very toxic to fish, one of its main uses by native people over the centuries being to paralyze fish for capture and consumption. Crystalline rotenone has an acute oral LD50 of 60, 132 and 3000mg/kg for guinea pigs, rats, and rabbits (Matsumura, 1985). Because the toxicity of derris powders exceeds that of the equivalent content of rotenone, it is obvious that the other esters in crude preparations have significant biologic activity.

Acute poisoning in animals is characterized by an initial respiratory stimulation followed by respiratory depression, ataxia, convulsions, and death by respiratory arrest (Shimkin and Anderson, 1936). The anesthetic-like action on nerves appears to be related to the ability of rotenone to block electron transport in mitochondria by inhibiting oxidation linked to NADH₂, this resulting in nerve conduction blockade (O'Brien, 1967; Corbett, 1974). The estimated fatal oral dose for a 70kg man is of the order of 10 to 100g.

Rotenone has been used topically for treatment of head lice, sacbies, and other ectoparasites, but the dust is highly irritating to the eyes (conjunctivitis), the skin (dermatitis), and to the upper respiratory tract (rhinitis) and throat (pharyngitis).