The in vitro photolysis of whole rat lenses using focused 290 nm laser radiation.

Whole rat lenses have been irradiated with a UV laser at 290 or 298 nm focused to a 0.08 mm diameter spot. The irradiated spot was analyzed using fluorescence spectroscopy and it was observed that the intensity of fluorescence (290 nm excitation, 335 nm emission) fell as the irradiation proceeded. These observations were interpreted in terms of a model which postulates photolysis of tryptophan, primarily present as residues in lens proteins, and formation of photoproducts which absorb the UV laser radiation to an ever-increasing extent as the irradiation proceeds. The effect is to produce a linear dependence of log If on log t, where If is the observed tryptophan fluorescence intensity after time, t, of exposure to the laser radiation. Evidence is also presented which indicates that an observed fast component of the tryptophan fluorescence decay results from local heating of the lens tissues due to energy dissipation by the laser. The most important conclusion from the present series of experiments is that tryptophan residues can be photolyzed by UV light in the whole lens, in vitro, in a fashion entirely analogous to that reported previously only for lens protein solutions. Hence the present work demonstrates that the photochemical behavior of lens protein solutions is indeed relevant to whole lens photolysis and that no special protective mechanism appears to be operative in the intact organ.