Photoprotector capacity of lichen metabolites assessed through the inhibition of the 8-methoxypsoralen photobinding to protein.

Lichens produce a diversity of phenolic compounds, some of which efficiently absorb ultraviolet radiation, 8-Methoxypsoralen (8-MOP), commonly used in the treatment of psoriasis, binds irreversibly to proteins in the presence of ultraviolet radiation by a mechanism that is not well established. In this paper we demonstrate the photoprotector capacity of three phenolic compounds-pannarin, 1'chloropannarin and atranorin-through the inhibition of 8-MOP-human serum albumin (HSA) photobinding. The method measures the UV-filtering capacity of lichen compounds by means of a double-tube compartment (thus, solubility and interaction with the reaction medium is avoided). Photobinding was determined by measuring the radioactivity of mixtures containing 8-(methyl-3H) MOP and HSA irradiated at 360 and 310 nm in the presence of increasing concentrations of lichen phenolics. Pannarin, l'-chloropannarin and atranorin at a concentration of 10 mM and irradiated at 360 nm, inhibited photobinding to HSA by 40.4%, 31.7% and 20.1% respectively. Pannarin (10 mM) irradiated at 310 nm inhibited the photobinding by 35.2%. The participation of singlet oxygen and hydroxyl radicals was demonstrated in the photoreaction process.