Submitted to: Photochemistry and Photobiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 15, 1995
Publication Date: N/A
Interpretive Summary: A large number of compounds called furanocoumarins occur in wide variety of plant species. Many of these compounds are known as photosensitizers because they are activated by light and can transfer the light energy to biological tissues with harmful results. One of the major functions of plant furanocoumarins is to protect the plants from the action of insects that would otherwise feed on the plant tissue. Several furanocoumarins ar known to be powerful feeding deterrents against insects, and one of these is a compound known as isopimpinellin. Strangely enough, although isopimpinellin is a powerful insect antifeedant, some research has found it not to be active as a photosensitizer, while other research has shown some photosensitizing activity. Because previous reports are in conflict, the present studies were conducted using isopimpinellin that was very highly pure. The results showed that pure isopimpinellin is in fact not a photosensitizer, and it is almost certain that previous reports of isopimpinellin photoactivity can be attributed to other active furanocoumarins present as impurities in the isopimpinellin samples studied. This information is important to know because it indicates that in the case of isopimpinellin, photosensitization is not the mechanism by which insects are deterred by it. Some other mechanism is involved, and if this mechanism could be defined and exploited, it is possible that isopimpinellin or similar chemicals could have use as effective insect control agents for use in protecting crops and livestock from destructive insects.
Synthetic isopimpinellin (5,8-dimethoxypsoralen), confirmed to contain as impurities only trace quantities at most of psoralen, bergapten (5- methoxypsoralen), and xanthotoxin (8-methoxypsoralen), is not phototoxic when tested in a chick skin bioassay system. These findings are at variance with earlier studies showing isopimpinellin to be phototoxic against chick skin, and support the conclusion that isopimpinellin is photobiologically inactive. As recently proposed by others, the several reports of isopimpinellin photoactivity are most likely attributable to contamination by small amounts of highly active psoralens such as bergapten or xanthotoxin.