MEDICAL AND ENVIRONMENTAL EFFECTS OF UV RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 45

lines (or wavelengths from lasers) in this region thus severely limits the resolution of
the spectra that can be obtained.
One possible solution is to use a Hg-Xe lamp, which provides a continuum of
radiation between the Hg lines. However, in attempting to obtain higher intensity, the
slits of the monochromator may be opened wider, thus increasing the range of
wavelengths in the incident beam. The pitfalls in using wider bandwidths to measure
action spectra in which the slope of the spectrum is changing rapidly in the wavelength
region of interest are discussed by Sliney in this volume. It is critical that scattered light
from other wavelength ranges be rigorously excluded. If they are not, the shorter
wavelength UV may dominate the observed biological effects, which would then be
erroneously perceived as resulting from the UVA.
An additional problem is the lack of intensity in the UVA region available from
most sources. Since the cross-section for most biological effects in this wavelength
range is small, this results in long exposures (many minutes or even hours) for
production of observable damage in many biological systems. In isolated molecules,
e.g. DNA in solution, as long as nucleases that could degrade the DNA are rigorously
excluded by the use of sterile buffers, the presence of EDTA, etc., long exposures are an
inconvenience, but do not affect the scientific result.
However, results from biological systems capable of repair may be significantly
affected by extended irradiation: simultaneous repair may reduce damages induced by
the UVA radiation so that the actual damage level is underestimated. Such repair could
be carried out by light-independent repair processes (nucleotide excision repair could be
carried out by light-independent repair processes (nucleotide excision repair or base
excision repair). Moreover, cells capable of photoreactivation present even more
complex responses. Photoreactivation is a one-enzyme repair path, in which a single
enzyme, photolyase, binds to a cyclobutyl pyrimidine dimer in DNA; upon absorption
by the photolyase-dimer complex of a photon in the visible or UVA range, the dimer is
monomerized and the photolyase is liberated to seek another dimer.
Figure 6. Photolyase repairs cyclobutyl pyrimidine dimers in DNA.
The first step—association of enzyme and substrate—is temperature and
concentration dependent [33]. However, the second step—dimer photolysis—is a
photochemical reaction, and virtually independent of temperature [34]. As a light-
driven, enzymatic reaction, the rate of photorepair depends not only on the number of
photolyases per cell but also on the wavelength of the light. Further, this enzyme has
two substrates: the dimer and the photolyzing photon. Simple biochemical