Participation of singlet oxygen in ultraviolet-a-induced lipid peroxidation in mouse skin and its inhibition by dietary β-carotene: an ex vivo study

Dietary β-carotene acts as a photoprotective agent in the skin, but the exact mechanism of protection is unknown. This ex vivo study is focused on determining the mechanism of action of β-carotene against UV-A-induced skin damage by characterizing peroxidized phosphatidylcholine (PC) and β-carotene oxidation products. BALB/c mice were fed with basal or a β-carotene-supplemented diet, and homogenates from their dorsal skin were prepared after 3 weeks for UV-A irradiation. Analyses revealed that the degree of lipid peroxidation in the β-carotene group was significantly lower than that in the controls. The isomeric composition of hydroperoxy fatty acids, constituting peroxidized PC, was determined by thin-layer chromatography-blotting followed by gas chromatography/mass spectrometry (MS)/selected ion monitoring analysis. The 9- and 10-isomers of peroxidized PC, resulting from the reaction of singlet molecular oxygen (1O2) with oleic acid, were elevated in the UV-A-exposed control group compared to the experimental group. Similar results were obtained from methylene-blue-sensitized photooxidation of mouse skin lipids in vitro. Liquid chromatography/MS analysis of the homogenates confirmed the formation of β-carotene 5,8-endoperoxide, a specific marker for the 1O2 reaction. These results indicate that dietary β-carotene accumulates in the skin and acts as a protective agent against UV-A-induced oxidative damage, by quenching the 1O2.