Biological activities of secondary lichen metabolites

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Figure 1. Biosynthetic pathways of lichen secondary metabolites [modified from ELIX andSTOCKER-WÖRGÖTTER (2008)].Figure 2. Lichen substances have several biological activities.
Antioxidant activityFree radicals play an important role in many chemical processes in the cells, but they are alsoassociated with unwanted side effects, causing cell damage. Since synthetic antioxidants areoften carcinogenic, finding natural substitutes is of great interest. Lichens have been found tocontain a variety of secondary lichen substances, which are strong antioxidant compounds.According to LUO et al. (2009), the extreme conditions in Antarctica increase oxidativestress, consequently, Antarctic lichens contain larger amounts of antioxidant substances andhave higher antioxidant activity than tropical or temperate lichens.AMO de PAZ et al. (2010) reported that methanol extracts of Xanthoparmelia camtschadalisand X. conspersa (Figure 3.), as well as their isolated lichen compounds (salazinic acid,stictic acid, and usnic acid) protected human astrocytes from hydrogen peroxide-induceddamage. Astrocytes are the first line of defense in the brain against neurotoxicity of reactiveoxygen species (ROS), thus salazinic acid, stictic acid, and usnic acid could act as antioxidantagents in those neurodegenerative disorders associated with oxidative damage (e.g.,Alzheimer’s disease and Parkinson’s disease).Figure 3. Xanthoparmelia conspersa contains the β-orcinol depsidone stictic acid as majorlichen compound in the medulla. This antioxidant lichen substance plays an important role inthe protection against oxidative stress.Effect on metal homeostasis and pollution toleranceLichen secondary metabolites are sensitive to heavy metal accumulation and might play ageneral role in metal homeostasis and pollution tolerance. According to BIAŁONSKA andDAYAN (2005) the levels of atranorin, physodic acid and hydroxyphysodic acid significantlydecreased in thalli of Hypogymnia physodes transplanted to the vicinity of a chemical plantproducing chromium, phosphorous and sulfur compounds. In contrast, the level of physodalicacid increased significantly, suggesting that this compound might be effective againstpollution stress.Usnic acid and divaricatic acid were both found to significantly increase the intracellularuptake of Cu 2+ in Evernia mesomorpha and in Ramalina menziesii (usnic acid only) (HAUCKet al. 2009), but intracellular uptake of Mn 2+ was reduced. The influence of the compoundsfacilitate the survival of the two lichen species.
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Biological activities of secondary lichen metabolites

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