When analyzed by alkaline comet assay, A2E-containing RPE exposed to blue light were associated with the formation of distinct comet tails indicative of DNA damage
(Fig. 1) . With a 20 minute exposure, 95% to 100% of the nuclei exhibited comets (100 nuclei/experiment, five experiments). Conversely, in preparations from control cells (untreated cells, cells exposed to blue light only or A2E-laden cells not exposed to blue light) nuclei appeared as condensed spherical bodies resembling normal nuclei. Under these conditions, the occasional nucleus exhibiting a discrete corona, that likely represented the nonspecific effects of trypsinization and centrifugation during the processing of cells for the comet assay. Tail moment, a parameter which increases in proportion to the number of DNA strand breaks,
44 47 48 was dramatically elevated in A2E-containing cells which had been exposed to blue light
(Fig. 2) . With blue-light–irradiated cells, the magnitude of the increase in tail moment was found to be dependent on the time of exposure
(Fig. 2A) . Conversely, measurements of tail moment obtained from nonilluminated A2E-laden cells, and cells exposed only to light, were not different from untreated control cells
(Fig 2B) . We previously showed that the death of blue-light–illuminated A2E-containing RPE occurs through oxygen-associated mechanisms.
34 Moreover, we have demonstrated that on irradiation with blue light, A2E undergoes singlet-oxygen mediated photooxidation resulting in the formation of highly reactive epoxides.
33 Thus, in the current work we examined the ability of sodium azide, an efficient quencher of singlet oxygen, to prevent DNA damage in our model. Illumination of A2E-laden RPE in the presence of 1, 5, and 10 mM azide, resulted in a concentration-dependent decrease in mean comet tail moment with reductions of 20% (1 mM), 37% (5 mM), and 58% (10 mM) being observed compared with illumination in its absence
(Fig. 3) .