The psoralens are a family of chemical
s made by many plants, but most prevalent in (and named after) the genus Psoralea
, which includes celery
s and turnip
s. Plants produce psoralens as natural pest defense, since it can stop some infection
s in their tracks and is also deadly to insect
Generally, psoralen molecules are small and inert enough to permeate an exposed cell's nucleus without any problem. Psoralen does most of its action by getting between the two strands of the DNA double-helix, specifically between adenine and thymine bases. Without activation, this is not a problem as proteins can push the unbound psoralen aside and get to the DNA. When exposed to UVA -- like that found in sunlight, UV lamps, etc. -- a chemical reaction takes place that binds the psoralen molecule on both sides, effectively tying the DNA together. Any process that would need the DNA, like cell reaction or division, can't happen since the DNA is no longer usable. Psoralen stops the cell's activities without actually killing it.
In an interesting etymological twist, the psoralens had their name before it was discovered that they could be used to treat Psoriasis. As it says above, psoralen was named after the Psoralea genus. Psoraleads were named in reference to the appearance of their leaves, from Greek psoraleos which means "mangy", which in turn comes from Greek psora, meaning "itch". The term Psoriasis comes directly from the word psora, as itching describes its main symptom well. Thus two related concepts are named similarly for completely unrelated reasons.
The psoralens' current favored use is currently in PUVA (psoralen + UVA) treatments for psoriasis, vitiligo and other skin conditions. More information is available in the PUVA node, but essentially they speed up the skin's reaction to sunlight, helping it fight the infection. For psoriasis this consists of drying it out, while for vitiligo it speeds up tanning and thus colors the skin. Because of the psoralens' ability to speed up tanning, they were included in some tanning lotions in the 80s, but was withdrawn because of too many users getting sunburns. It's good that it was withdrawn, as it turns out the PUVA causes skin cancer quite well, to the point that doctors limit the number of PUVA treatments a single person may undergo in his lifetime. The PUVA effect is also responsible for the high incidence of skin problems and skin cancer in workers who pick celery, turnips, etc.
A really cool new use of psoralen, which will probably reach the mainstream in the next five years, is to disinfect donated blood. Psoralens are introduced into the whole blood in high enough concentration to insure that they get into all of the cells present, and the blood is exposed to UVA. While the useful components of transfered blood (red blood cells, plasma, and platelets) don't have any DNA to attach to and damage, the infectious agents (bacteria and viruses) do. With psoralen damaged DNA, the bacteria and viruses are unable to reproduce, and are thus not dangerous. White blood cells, which can cause a rejection response to transferred blood, are also rendered inert. It should be noted that this treatment does nothing to stop proliferation of prions (since they are proteins instead of organisms), which are transmitted by blood and can be as deadly as infectious agents.
Finally, psoralens sometimes show up used as pesticide. Since they occur naturally in plants, psoralen concentration is a good starting point to breeding pest-resistant plants. A strain of celery was once bred with eight times the psoralen concentration of normal celery, which made it virtually impervious to infection and insect damage. Unfortunately, field workers and produce department stockers both developed skin disorders, and the strain was removed from the market. Considering the psoralens' tendency to be carcinogenic, this is probably a very good thing. Curiously, while the US heavily regulates genetic modifications to food crops, they do very little to regulate breeding for certain traits. Had the FDA been on top of breeding for psoralen content, the dangerous celery would've probably never reached market.