MEDICAL AND ENVIRONMENTAL EFFECTS OF UV RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 46

considerations indicate that the rate of photorepair depends on the substrate
concentration, with the maximal rate at a saturating'' concentration of the substrate,

i.e., the concentration such that further increases do not increase the reaction rate. Since
photolyase has two substrates, the dimer and the photon, the rate of photorepair depends
not only on the dimer concentration (frequency) in the cell but also on the wavelengths
and intensity of the photoreactivating light.
Thus we can summarize the factors affecting the net effect of UVA as: Net
effect of UVA = UVA-induced damage - UVA-mediated repair - Light independent
repair. The UVA-induced damage depends on the wavelengths and intensities at each
wavelength and the cross-section of the biological system for damage at each
wavelength. The UVA-mediated repair term depends on the number of photolyases in a
cell, the temperature, the photorepair action spectrum for that photolyase, the
wavelength and intensity of the light, as well as the level of dimers per cell. The light-
independent repair is dependent on the genotype of the cell, the dimer frequency, the
presence of other damages that are also substrate for the repair system, the level of
these enzymes in the cell and on the substrates required for that repair, e.g. ATP, used
as an energy source.
Thus, obtaining robust data on the effects of UVA is challenging. Even with
careful attention to biological, physical and biochemical factors, current light sources
are far from ideal, being limited in intensity and in wavelength. An ideal source for
UVA radiation would provide a continuous selection of wavelength, high intensity over
a substantial area, and the time structure of the radiation should avoid high peak
power/low inter-peak modes that produce multi-photon photochemistry. Free electron
lasers in the UV range are being developed at several institutions; at least two of these
should provide high time-average power in the UVA range [35]. They may offer
powerful new next-generation sources for UVA photochemistry and photobiology. The
elucidation of the actual biological role of UVA awaits the development of capability of
irradiation of biological systems at such facilities.

"

Acknowledgments

Research supported by the Office of Biological and Environmental Research of the US
Department of Energy.

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