Structural and functional changes in catalase induced by near-UV radiation

Part one of this study shows that exposure of purified beef liver catalase in buffered solutions to BL lamps that provide a mixture of 99% UVA and 1% UVB (to be labeled UVA) alters its chemistry and enzymatic activity. Thus, its spectral absorbance lost detail, it aggregated and exhibited a lower isoelectric point and its enzymatic activity was substantially reduced. These photochemically induced changes were increased by irradiation in phosphate buffer or in physiological medium (minimal essential medium) containing riboflavin and tryptophan. Neither alpha-tocopherol nor deferoxamine were protective against these UVA-induced changes in pure catalase. We further investigated the effect of UVA radiation on the activity of catalase in cultured lens epithelial cells and the protective effects of antioxidants. Cultured lens epithelial cells of rabbits and squirrels were exposed to near-UV radiation with representation in the UVA region of 99% and 1% UVB. Catalase assays were done on homogenate supernatants of cells kept dark or UV exposed. In some instances, cells were cultured in medium containing alpha-tocopherol or deferoxamine prior to UV radiation. Comparisons were made between UV-exposed lens cell catalase activity when exposure was done with or without the antioxidants. The UVA radiation was strongly inhibitory to both rabbit and squirrel lens epithelial cell catalase activities. The range of fluxes of near UV radiation was compatible with that which could reach the lens from the sunlit environment. Catalase inactivation was lessened in cells preincubated with alpha-tocopherol and deferoxamine. This suggests that both singlet oxygen and hydroxyl radical formation may be involved in near-UV damage to lens epithelial cell catalase. Such inhibition of catalase by near-UV would enhance H2O2 toxicity and stimulate SH oxidation so as to damage the lens.