Date Published: June 17, 2014
Publisher: Public Library of Science
Author(s): Jatinder Kumar, Priyanka Dhar, Amol B. Tayade, Damodar Gupta, Om P. Chaurasia, Dalip K. Upreti, Rajesh Arora, Ravi B. Srivastava, Gianfranco Pintus.
Fourteen saxicolous lichens from trans-Himalayan Ladakh region were identified by morpho-anatomical and chemical characteristics. The n-hexane, methanol and water extracts of the lichens were evaluated for their antioxidant capacities. The lichen extracts showing high antioxidant capacities and rich phenolic content were further investigated to determine their cytotoxic activity on human HepG2 and RKO carcinoma cell lines. The ferric reducing antioxidant power (FRAP), 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH) and nitric oxide (NO) radical scavenging capacities and β-carotene-linoleic acid bleaching property exhibited analogous results where the lichen extracts showed high antioxidant action. The lichen extracts were also found to possess good amount of total proanthocyanidin, flavonoid and polyphenol. The methanolic extract of Lobothallia alphoplaca exhibited highest FRAP value. Methanolic extract of Xanthoparmelia stenophylla showed the highest ABTS radical scavenging capacity. The n-hexane extract of Rhizoplaca chrysoleuca exhibited highest DPPH radical scavenging capacity. Highest antioxidant capacity in terms of β-carotene linoleic acid bleaching property was observed in the water extract of Xanthoria elegans. Similarly, Melanelia disjuncta water extract showed highest NO scavenging capacity. Among n-hexane, methanol and water extracts of all lichens, the methanolic extract of Xanthoparmelia mexicana showed highest total proanthocyanidin, flavonoid and polyphenol content. From cytotoxic assay, it was observed that the methanolic extracts of L. alphoplaca and M. disjuncta were exhibiting high cytotoxic effects against cancer cell growth. Similarly, the water extract of Dermatocarpon vellereum, Umbilicaria vellea, X. elegans and M. disjuncta and the methanolic extract of M. disjuncta and X. stenophylla were found to possess high antioxidant capacities and were non-toxic and may be used as natural antioxidants for stress related problems. Our studies go on to prove that the unique trans-Himalayan lichens are a hitherto untapped bioresource with immense potential for discovery of new chemical entities, and this biodiversity needs to be tapped sustainably.
Free radicals having one or more unpaired electrons are generated as a by-product in normal or pathological cell metabolism. Reactive oxygen species (ROS) react swiftly with free radicals to become radicals themselves thereby starting free radical chain reaction. Superoxide anion (O2–), hydrogen peroxide (H2O2), hydroxyl radical (HO•) and singlet oxygen (1O2) are the various forms of reactive oxygen species , . Excess ROS in our body damages biological molecules leading to the development of degenerative diseases such as premature aging, heart diseases, cancer, inflammation, diabetes, genotoxicity, arthritis and many more , . Exogenous sources of free radicals viz. tobacco smoke, ionizing radiation, certain pollutants, organic solvents, pesticides etc. trigger the process of generation of ROS within the body , . The most efficient way to exterminate free radicals that cause the oxidative stress is through antioxidant supplementation.
Lichens, one of the prominent life-form in the trans-Himalayan Ladakh region contain natural antioxidants which provide protection from the damage caused by the climatic conditions of high altitude environments and support their adaptability in extreme environmental conditions like extreme habitats (rocks in high altitude cold desert), low temperature, drought, prolonged winter, high solar irradiance and ultraviolet radiations. Numerous extracellular deposits in lichen thallus may contribute to the activation of antioxidant defence system in these conditions. In addition, diverse secondary metabolites are synthesized by lichens that are unique and found rarely in other plants . In recent time, much attention has been paid on the biological roles of lichen secondary substances that have been found to have a lot of positive biological activities viz. antitumour, antibacterial, antifungal, antiviral, antiinflammatory and antioxidant capacities . Therefore the secondary metabolites of high altitude cold desert lichens might be a potential resource of natural antioxidants and deserved to be tested for their antioxidant capacities.
In the present study, fourteen saxicolous lichens from trans-Himalayan cold desert of Ladakh have been identified with their morpho-anatomical and chemical compositions and the antioxidant and free radical scavenging capacities of these lichen extratcts were reported for the first time. This study revealed that these lichen species have broad spectrum free radical scavenging effect and high antioxidant capacity. Moreover, the antioxidant capacity was found to be associated with the species and the polarity of solvent used for extraction. Our studies clearly showed that the high altitude saxicolous lichens can be an interesting source of new antioxidative substrates with the potential to be used for scavenging various types of free radicals by different scavenging mechanisms. The lichen extracts were found to contain considerable amount of phenolic compounds which were responsible for their high antioxidant and free radical scavenging ability and could be used as natural source of antioxidants to ameliorate oxidative stress related disorders. Among all extracts, eight lichen extracts having high antioxidant capacity and phenolic compounds were further investigated for cytotoxic action on human carcinoma cell lines and these extracts were found to exhibit anticancer and radioprotective effects that would be of great interest for further chemical investigations and could lead to the discovery of novel cytotoxic anticancer molecules from lichens of the trans-Himalayan region. With these present primary findings, further studies are also required to ascertain the biological activities of the lichen extracts in animal models.