Protective Effects of Natural Lipophilic Antioxidants on UV-Induced Lipid Oxidation in Liposomes and Their Enhancement

Antioxidants, especially lipophilic antioxidants, absorb ultraviolet (UV) radiation and protect human skin from radicals that lead to oxidation reactions. The differences in the protective effects of carotenoids and α-tocopherol against UV radiation and the possible enhanced effects by the polar antioxidants, vitamin C and Trolox, need further investigation. Therefore, malondialdehyde was analyzed as a biomarker for lipid oxidation using the Thiobarbituric Acid-Assay (TBA-Assay) in liposomes irradiated with UV-C, UV-B, and UV-A radiation (254 nm, 320 nm, and 360 nm). In addition, antioxidant degradation was analyzed using HPLC with a diode array or fluorescence detector. The lipophilic antioxidants differ in their effect mainly due to their polarity and the associated different localization in the lipid bilayer. No pro-oxidative effect was observed at antioxidant concentrations close to saturation. The antioxidant effect was low at small concentrations, mainly due to aggregation of the antioxidants. The protective effect at higher antioxidant concentrations increased from up to 25-72% under UV-C, over 59-77% under UV-B, to 77-86% under UV-A radiation. Vitamin C proved to be 2-40 times less effective depending on the wavelength and the lipophilic antioxidant. Mixtures of lipophilic and hydrophilic antioxidants showed partially additive or synergistic effects. This appears to be dependent on concentration and ratio.