Abstract

Titanium dioxide (TiO 2 ) is used in sunscreens because it reflects and scatters UV light. However, TiO 2 can catalyze in vitro oxidative damage to DNA, to cultured human fibroblasts and can also inactivate the enzyme horseradish peroxidase (HRP) upon photoexcitation by UV light. The effects of photoexcited TiO 2 on two key enzymes involved in the skin's enzymatic antioxidant defense system, copper-zinc superoxide dismutase (CuZnSOD) and catalase, are reported here. Both enzymes are rapidly inactivated upon exposure to photoexcited TiO 2 , and their inactivation mechanisms are described. Hydroxyl radicals (OH · ), generated upon photoexcitation of TiO 2 , attack the active sites of the enzymes, leading to their inactivation. Copper release from CuZnSOD and heme destruction in catalase were observed, which explain the loss of activity. Encapsulation of TiO 2 in faujasite and mordenite, two different types of zeolites, greatly reduced the inactivation of CuZnSOD and catalase upon exposure to UVA light. These results suggest that encapsulation may lead to safer sunscreens based on supramolecular chemistry.