Phytochemical and pharmacological investigations

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Lichen metabolites exert a wide variety of biological actions including antibiotic, antimycobacterial, antiviral, anti-inflammatory, analgesic, antipyretic, antiproliferative and cytotoxic effects. Even though these manifold activities of lichen metabolites have now been recognized, their therapeutic potential has not yet been fully explored and thus remains pharmaceutically unexploited. The utility of lichens is due to the of range of secondary compounds produced by them.

Antimicrobial activity

Antibacterial

Antibiotic properties of lichens are of special interest to scientists. According to one estimate, 50% of all lichens have antibiotic properties. Burkholder and co workers (1944) were pioneers initiating research on lichens as antibacterial agents. They tested 42 lichens for antibiotic properties and 27 were reported to inhibit growth of bacteria.

Parmelia physodes (L.) Ach. was reported to be an antibiotic. Usnic acid is a wide-spectrum antibiotic characterized from lichens. Vulpinic acid has mild antibiotic property. Atranorin has been found to be much less biologically active than usnic and vulpinic acids. Usnic , evernic and vulpinic acids inhibited the growth of gram positive bacteria Staphylococcus aureus, Bacillus subtilis, and Bacillus megaterium, but the acids had no effect on the gram negative bacteria Escherichia coli or Pseudomonas aeruginosa.

Usnic acid isolated from lichens from the south of Spain has high activity against Gram-positive bacteria. Acetone, diethyl ether and ethanol extracts of the lichen Cetraria aculeata (Schreber) Fr. and its active constituent protolichesterinic acid were tested positive against Escherichia coli, Staphylococcus aureus, Aeromonas hydrophila, Proteus vulgaris, Streptococcus faecalis, Bacillus cereus, Bacillus subtilis, Pseudomonas aeruginosa, and Listeria monocytogenes.

Alectosarmentin, (-)-usnic acid, physodic acid and 8'-O-ethyl-beta-alectoronic acid isolated from the alcoholic extract of the lichen Alectoria sarmentosa (Ach.) Ach. (Alectoriaceae) showed antimicrobial activity. Hypogymnia apinnata (atranorin), Letharia columbiana (vulpinic acid), Lobaria pulmonaria (Stictic acid, constictic acid, and norstictic acid) and Usnea filipendula (Usnic acid and salazinic acid) have been reported to have significant antibiotic activity against Micrococcus luteus, Staphylococcus aureus, Salmonella gallinarum and Serratia marcescens respectively.

The acetone and methanol extracts of Lasallia pustulata (L.) Meret. (Umbilicariaceae), Parmelia sulcata Taylor, and Umbilicaria crustulosa (Ach.) Frey (Umbilicariaceae), manifested antibacterial activity against the majority of species of bacteria tested, in addition to selective antifungal activity. Acetone, chloroform, diethyl ether, methanol, and petroleum ether extracts of Parmelia sulcata and its constituent (salazinic acid) demonstrated antibacterial activity against Aeromonas hydrophila, Bacillus cereus, Bacillus subtilis, Listeria monocytogenes, Proteus vulgaris, Yersinia enterocolitica, Staphylococcus aureus, Streptococcus faecalis, Candida albicans, Candida glabrata, Aspergillus niger, Aspergillus fumigatus, and Penicillium notatum.

3-hydroxyphysodic acid isolated from Hypogymnia tubulosa (Schaerer) Havaas (Parmeliaceae) showed antimicrobial activity against Aeromonas hydrophila, Bacillus cereus, Bacillus subtilis, Escherichia coli, Klebsiella pneumoniae, Listeria monocytogenes, Proteus vulgaris, Salmonella typhimurium, Staphylococcus aureus, Streptococcus faecalis, and Candida albicans.

Extracts of Xanthoparmelia pokornyi (Vainio) Hale (Parmeliaceae) and its constituents (gyrophoric and stenosporic acid) have been reported to be potential antimicrobials. Extracts of Pseudevernia furfuracea and its constituents (physodic acid, chloroatranorin, atranorin, and olivetoric acid) have significant antimicrobial activity. Hirtusneanoside isolated from Usnea hirta (L.) Wigg. (Parmeliaceae) showed growth inhibitory activities against Gram-positive bacteria.

Antifungal

Parietin, anthraquinone isolated from methanol extract of Caloplaca cerina (Ehrh. ex Hedwig) Th.Fr. (Teloschistaceae) has been reported to have significant antifungal activity. Extracts of Andean lichens Protousnea poeppigii (Nees & Flot.) Vain. (Parmeliaceae) and Usnea florida var. rigida Acharius demonstrated antimicrobial activity against the pathogenic fungi Microsporum gypseum, Trichophyton mentagrophytes and T. rubrum. Isodivaricatic acid, 5-propylresorcinol, divaricatinic acid and usnic acid were identified as antifungal agents.

Antiviral

Usnic acid isolated from Teloschistes chrysophthalmus (L.) Th. Fr. (Teloschistaceae) and parietin isolated from Ramalina celastri demonstrated antiviral activity against the arena viruses Junin and Tacaribe.

Antioxidant activity

Phenolic constituents from the lichen Parmotrema stuppeum (Nyl.) Hale (Parmeliaceae) including methyl orsenillate, orsenillic acid, atranorin and lecanoric acid showed moderate antioxidant activity. An animal study reported antioxidant activity of the lichen Cetraria islandica. Stictic acid derivatives from the lichen Usnea articulata (Ach.) Motyka were reported to have significant antioxidant activity.

Antitumor activity

Pannarin inhibited cell growth and induces cell death in human prostate carcinoma DU-145 cells.The orcinol derivatives tenuiorin and methyl orsellinate present in the extract of Peltigera leucophlebia (Nyl.) Gyeln. (Peltigeraceae) exhibited in vitro inhibitory activity against 15-lipoxygenase from soybeans. A correlation has been observed between 5-lipoxygenase inhibition and antiproliferative effects for related lichen metabolites. On this account, tenuiorin and methyl orsellinate were further tested for antiproliferative activity on cultured human breast, pancreatic and colon cancer cell lines. Methyl orsellinate lacked antiproliferative activity but tenuiorin depicted moderate activity. Bianthraquinone glycosides, colleflaccinosides isolated from Collema flaccidum (Ach.) Ach. (Collemataceae) collected in Israel and Russia, were reported to have antitumor activity.

Immunomodulator activity

Heteroglycans and a beta-glucan isolated from Thamnolia vermicularis var. subuliformis were tested for in vitro immunomodulating activity and reported to have various influences on the immune system.

Tyrosinase-inhibitory activity

Methanol extracts of Graphina glaucorufa (Vain.) Zahlbr., Graphina multistriata Müll. Arg., Graphina salacinilabiata Patw. & CR Kulk., Graphis assamensis Nagarkar & Patw., Graphis nakanishiana Patw. & CR Kulk, and Phaeographopsis indica (Patw. & Nagarkar) Sipman & Aptroot (Graphidaceae) have significant tyrosinase-inhibitory activity. Methanolic extracts of the edible and medicinal lichens, Umbilicaria (Gyrophora) esculenta and Usnea longissima manifested in vitro melanogenesis inhibitory. It was concluded that lichen extracts affected the activity of tyrosinase via the inhibition of tyrosinase glycosylation.

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