Cytotoxicity caused by near-ultraviolet-irradiated soluble vitamins

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Near-ultraviolet light (UVA) has received much attention as a possible cause of skin aging and carcinoma. The study focused on the role of photoinduced cytotoxicity of endogenous substances, riboflavin and pyridoxine, which have absorption spectra in the UVA range. These soluble vitamins were found to become cytotoxic after UVA.The cytotoxicity lasted for a long time after irradiation. Thus, the cause of the cytotoxicity was thought to be stable photoproducts derived from the vitamins and/or hydrogen peroxide.The production of hydrogen peroxide after the irradiation of soluble vitamins was studied by the use of horseradish peroxidase (HRP). Its absorption peak at 400 nm shifts to 417 nm after its binding to hydrogen peroxide. Irradiated pyridoxine produced a little amount of hydrogen peroxide which, however, was not cytotoxic. Thus, the cause of pyridoxine photosensitization was thought to be stable photoproducts. On the other hand, a high concentration of hydrogen peroxide was produced in UVA-irradiated riboflavin solution. Photoinduced riboflavin cytotoxicity was eliminated by catalase treatment of the irradiated solution. This shows that the cytotoxicity was mostly caused by hydrogen peroxide.The cytotoxicity of irradiated soluble vitamins was studied under anaerobic conditions. In the case of pyridoxine, its cytotoxicity decreased because of slow down of its photo-degradation. In the case of riboflavin, its cytotoxicity increased dramatically. The increase was thought to be due to short-lived active oxygen species.