Psoralen is a group of organic chemicals found in plants such as celery, carrots, and turnips. They can make skin highly reactive to UVA light and are used to treat skin conditions. Psoralen compounds are toxic in larger doses and can cause rashes, burns, and skin cancer. They are produced by plants as a defense mechanism and are highly toxic to insects and mammals. Psoralen has been used in Chinese medicine to stimulate melanin production in the skin during exposure to sunlight. It was also used extensively as tan activators in suntan lotions until it was banned in Switzerland in 1987. Psoralen is used to treat vitiligo and psoriasis and research is underway to use it as an ingredient to disinfect blood.
Psoralen is a name for a group of organic chemicals found in plants of the genus Psoralea, such as celery, carrots and turnips. The Chinese herb po gu zi, also known as bu gu zhi or Psorales corylifolia L., is also a member of the psoralen family like many other natural flowering plants. Together, they produce chemicals that can make skin highly reactive to ultraviolet A (UVA) light and, for this reason, are called PUVA compounds and are used to treat skin conditions such as psoriasis, vitiligo, and hair loss. They are toxic chemicals, however, in larger doses, as they interfere with deoxyribonucleic acid (DNA) production and can therefore cause rashes, burns and skin cancer in healthy individuals such as farm workers who harvest celery and parsnips that contain the compounds .
The family of parent compounds from which psoralen is derived is known as furocoumarins, which is a five-carbon ring furan compound similar to the six-carbon ring of benzene, combined with coumarin, which produces a fragrant odor and it is known as benzopyrone. These compounds are produced by plants as a defense mechanism, as they are natural pesticides that are highly toxic to insects and mammals. Plants also use them as an agent against infection by invasive molds and microorganisms. The molecules of a psoralen compound are small enough to pass through a cell wall and bind to the adenine and thymine bases in the DNA structure, where they are inert until exposed to UVA light, when they become active and stop the normal process of cell division.
In small doses, psoralen compounds have been used in Chinese medicine as a muscle injection or topical cream to stimulate melanin production in the skin during exposure to sunlight. This unique trait also led them to be used extensively as tan activators in suntan lotions, where light levels were reduced due to the climate, until they were banned for such use in Switzerland in 1987. Subsequent trials of their contribution to melanoma cases worldwide prompted a broader ban on chemicals for this use in 1996. Substitute compounds with ultraviolet B (UVB) blocking chemicals and psoralens have been introduced as substitutes in countries such as the France and promoted as safe tanning compounds for people who have had difficulty getting a tan, but claims about their safety remain controversial. A related compound used in tanning agents that has suspected photomutagenic qualities is umbelliferon, commonly found in carrots, coriander, and other plants.
The medical use of psoralens for skin conditions was pioneered in 1940s Egypt as a treatment for vitiligo and is considered preferable to steroid therapy, even though it has a high rate of side effects. In related skin conditions like psoriasis, it works by drying out the skin itself, which reduces skin infection rates. As of 2011, research is underway to use psoralen as an ingredient from blood banks to disinfect blood after exposing it to UVA radiation. It would have no harmful effect on red blood cells or plasma and would render the DNA of white blood cells inert in the blood.
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