The mechanism of oxidation of ascorbic acid in mouse skin homogenates by UV light was investigated by measuring ascorbate free radical formation using electron spin resonance signal formation. Addition of vitamin E (alpha-tocopherol or alpha-tocotrienol) had no effect, whereas short-chain homologues (2,5,7,8-tetramethyl-6-hydroxychroman-2-carboxylic acid [Trolox] and 2,2,5,7,8-pentamethyl-6-hydroxychromane [PMC]) accelerated ascorbate oxidation. The similar hydrophilicity of ascorbate, Trolox and PMC increased their interaction, thus rapidly depleting ascorbate. When dihydrolipoic acid was added simultaneously with the vitamin E homologues, the accelerated ascorbate oxidation was prevented. This was due to the regeneration of ascorbate and PMC from their free radicals by a recycling mechanism between ascorbate, vitamin E homologues and dihydrolipoic acid. Potentiation of antioxidant recycling may be protective against UV irradiation-induced damage. The rate of ascorbate oxidation in the presence of vitamin E homologues was enhanced by a photosensitizer (riboflavin) but was not influenced by reactive oxygen radical quenchers, superoxide dismutase or 5,5-dimethyl-1-pyrroline-N-oxide. These experimental results suggest that the UV irradiation-induced ascorbate oxidation in murine skin homogenates is caused by photoactivated reactions rather than reactive oxygen radical reactions.
To evaluate ozone damage to hairless mouse skin, two parameters of oxidative damage, vitamin E depletion and malondialdehyde (MDA) production, were measured in vitamin E-enriched and in control skin from mice exposed to ozone (10 ppm). A 5% vitamin E solution (tocotrienol-rich fraction, TRF) in polyethylene glycol (PEG) was applied to 2 sites on the back of hairless mice, PEG to 2 sites. After 2 h, the sites were washed, one of each pair of sites covered and the mice exposed ozone for 2 h. Ozone exposure (compared with covered sites) increased epidermal MDA in PEG-treated sites, while vitamin E was unchanged. In contrast, ozone exposure significantly depleted vitamin E in TRF-treated sites, while significant MDA accumulation was prevented. This is the first demonstration that ozone exposure causes damage to cutaneous lipids, an effect which can be attenuated by vitamin E application.