Líf - og heilbrigðisvísindaráðstefna Háskóla Íslands 2023

Herbal exploitation of usnic acid-containing lichens

Maonian Xu, Elín Ólafsdóttir

Usnic acid (UA) is a major lichen secondary metabolite produced by multiple lineages of lichen-forming fungi, and it has two enantiomers – (+)- and (-)-UA. The lichens in the genera Usnea and Evernia have been traditionally used for antimicrobial personal care agents and cosmetic ingredients, respectively. It is not clear whether these lichens contain (+)- or (-)-UA or even a mixture. Interestingly, many other cosmopolitan lichens rich in UA were not used for herbal medicines.
UA enantiomer composition in lichens was characterized with chiral liquid chromatography. Both isomers were also purified and tested against multiple cell lines and microbial pathogens. Enantiomer composition was also mapped in a lichen phylogenetic framework to reflect its phylogenetic pattern.
Our data show that the genera Usnea and Evernia stands out with dominant (+)-UA production, while others are mostly (-)-UA producing. Bioassay data show little differences in antimicrobial activity between both isomers but (-)-UA tends to be more toxic to human being. Even though there are other lichens producing (+)-UA, they are limited in either geographic distribution, morphology or biomass production.
This study explains the reason why the lichen genera Usnea and Evernia were exploited for herbal medicines, instead of other lichens. Donimance in (+)-UA is the main factor, which is probably less toxic than (-)-UA. Other factors may include geographic distribution, morpholog or biomass production.


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