Impact of UVR-A on whole human lenses, supernatants of buffered human lens homogenates, and purified argpyrimidine and 3-OH-kynurenine

PURPOSE: Yellow chromophores and fluorescent compounds accumulate in the lens with age. Some of these compounds are photochemically active. The present study aimed to examine the photochemical effect of ultraviolet radiation-A (UVR-A) on the human lens.

METHODS: Intact human lenses and supernatants of buffered lens homogenates were exposed to UVR-A. The effect of UVR-A was evaluated by time-resolved and steady-state fluorescence spectroscopy, visual evaluation of colour and protein gel electrophoresis.

RESULTS: Intact lenses exposed to UVR-A showed no changes in time-resolved or steady-state fluorescence properties but the yellow coloration was visibly attenuated. The supernatants of buffered lens homogenates exposed to UVR-A demonstrated a reduction in time-resolved and steady-state fluorescent properties and protein cross-linking.

CONCLUSIONS: Exposure of the intact lens to UVR-A causes chromophore bleaching without affecting fluorescence, indicating that non-fluorescent chromophores have been destroyed. After homogenization, both chromophores and fluorophores from the lens suffer damage and proteins aggregate. This indicates that powerful mechanisms of protection against UVR-A found in the intact lens are disturbed by homogenization of the lens, suggesting that isolated lens proteins cannot be used as a model system for studying cataractogenesis. Hypothetically, the protective mechanism could be related to the rigidly packed three-dimensional structure of the lens proteins or to the abundance of antioxidative and free radical scavenging defence systems.