| abstract | - Many lichens have been used medicinally across the world. A lichen's usefulness as medicine probably usually comes from the lichen secondary compounds that are abundant in most lichen thalli. Different lichens produce a wide variety of these compounds, most of which are unique to lichens. The exact use of these lichen compounds is still being debated, but some lichen compounds can act as antibiotics, fungicides, and herbivore deterrents (Lawrey 1986). This undoubtedly gives the lichen some protection, and probably endows the lichen with some medicinal characters as well. Sharnoff (1997) estimates that 50% of all lichen species have antibiotic properties. The scientific search for antibiotics in lichens started in 1944 when Burkholder found that extracts from 27 out of the 42 different species of lichen that he tested inhibited the growth of certain bacteria. Lichen compounds have been found to act as anti-tumor agents (Kupchan and Kopperman 1975; Takai et al 1979), antibiotics (Burkholder 1944; Vartia 1973), and anti-inflammatories (Handa et al. 1992; Skidmore and Whitehouse 1965). Research to develop pharmaceuticals from lichens continues, especially in Japan (Sharnoff 1997). There is currently work being done to genetically engineer lichens so that lichen products could easily be produced in the lab (Miao et al. 2001). Patent Number 6132984 (issued on October 17, 2000 to J. E. Davies, B. Walters, and G. Saxena from TerraGen Discovery Inc.) is for a method for inhibiting eukaryotic protein kinase activity (and thus the sporulation of Streptomyces) with vulpinic acid or usnic acid (two lichen compounds). Some of the most widely studied lichen compounds are usnic acid, vulpinic acid, atranorin, and protolichesterinic acid. Usnic acid is found in large quantities in Usnea spp., as well as in several other lichen genera. It is a fairly wide spectrum antibiotic and is the most active antibiotic to be characterized from lichens (Abo-Khatwa et al. 1996; Shibamoto and Wei 1984; Rowe et al. 1991; Dobrescu et al. 1993). Usnic acid and diffractaic acid (a derivative of usnic acid) have both been demonstrated to be analgesic when tested on mice (Okuyama et al. 1995). And a mixture of usnic acid and isolichenin has been demonstrated to have moderate activity against sarcoma 180 and Ehrlich tumor cells (Periera et al. 1994). There is some research to indicate that protolicheresterinic acid may be valuable in the treatment of ulcers and cancers, and in AIDS prevention. It has been documented that protolicheresterinic acid has in vitro activity against Helicobacter pylori (Ingolfsdottir et al. 1997) and DNA polymerase activity of human immunodeficiency virus-1 reverse transcriptase (Pengsuparp et al. 1995). Protolicheresterinic acid was also found to be antiproliferative and cytotoxic to T-47D and ZR-75-1 cell lines cultured from breast carcinomas, and to K-562 from erythro-leukemia (Ogmundsdottir et al. 1998). Protolichesterinic acid may perform these functions by inhibiting 5-lipoxygenase, and this would also contribute to protolichesterinic acid's reported anti-inflammatory actions (Ogmundsdottir et al., 1998). Vulpinic acid also has some mild antibiotic properties, but it is not as strong of an antibiotic as usnic acid. It is, however, a significant herbivore deterrent and has been found to be toxic to animals in large doses (Lawrey 1986). Atranorin has been found to be much less biologically active than the above mentioned compounds (Lawrey 1986), but it is still a bit of a herbivore deterrent (Abo-Khatwa et al. 1996). Another property of lichens that had them being used for medicines is their cool little shapes. According to the 'Doctrine of Signatures' of the 15th century Europe a plant could be used to treat whatever ailment it most looked like. This use was mostly obsolete be 1800 (Llano 1944b), but some of these uses have persisted. Some lichens commonly used according to the Doctrine of Signatures include species of Cladonia, Evernia, Lobaria, Parmelia, Peltigera, Pertusaria, Physcia, Roccella, Usnea, and Xanthoria. The importance of this use is evident when one looks at the origin of the word 'lichen'. 'Lichen' comes from the Greek word 'Leprous' and refers to the use of some lichens for treating cutaneous diseases due to their peeling-skin appearance (Llano 1944b).
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