MEDICAL AND ENVIRONMENTAL EFFECTS OF UV RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 39

inactivated by UV and their inactivation action spectrum resembles their absorption
spectrum. Cystine is an important target for protein inactivation. Although its molar
extinction coefficient is only ~ 100 at 280 nm, its high quantum yield (0.05) and critical
role in maintaining protein three-dimensional structure make it the most
sensitive residue in producing protein inactivation. Relative to nucleic acids, most
proteins are resistant to inactivation by UV. A major source of this resistance is the
much lower absorption cross section of proteins, about one-twentieth of that of nucleic
acids at ~ 2700 nm. Although some data suggest that Photosystem II of the
photosynthetic apparatus is photolabile [10, 11], current data suggest that the effect may
not be biologically significant (see Nogues, this volume). RNA is also susceptible to
UV damage, forming pyrimidine hydrates as well as pyrimidine photodimers.
Photoproducts are also induced in DNA by UV radiation. Figure 2 shows the
induction of a cyclobutane pyrimidine dimers in DNA. UV also induces [6-4]
pyrimidine-pyrimidone photoproducts, plus other minority products, principally
photoproducts of pyrimidine bases.
Figure 2. Induction by UV of a cyclobutyl pyrimidine dimer in DNA.
The cyclobutyl pyrimidine dimer (CPD) is the numerically predominant class of UV
photolesion in DNA. The 6-4 photoadduct is induced at about 10-30% of the level of
CPD, and other photoproducts are formed much lower levels.
Other factors that must be considered are the cellular numerology, and
importance of function of the specific molecule. Numerology involves the number of
the specific molecules present in a cell. If a thousand of copies of a protein or RNA are
present in a cell, and 10% of them are inactivated by UV, the remaining 900 may be
quite adequate for cell function. In addition, protein and RNA turnover are normal in
cells, and there are well-developed and coordinated paths for metabolism—including
salvage paths—of damaged proteins and of RNA. Further, proteins and RNA can be
replaced by de novo synthesis, as long as the DNA and cellular machinery for protein
synthesis are intact.
4. Measuring DNA Damage
Many approaches have been developed for measuring DNA damage, including those
based on chromatography, immunology, and various properties of undamaged and
damaged DNA. In the latter category is gel electrophoresis/electronic imaging/number
average length analysis. It is a powerful approach of DNA lesion quantitation that stems
from physical chemical methods for characterization of polymers. It provides absolute
measurements of lesion frequencies (lesions per kilobase or per megabase or even per
gigabase). With appropriate experimental modifications, it can be used to quantify
lesion frequencies over some six orders of magnitude. It does not require that the lesions
have any specific distribution on the DNA, and, in specific, does not require a random
distribution of lesions.