in mammals.^90 The various types of DNA repair will be considered in order of increasing molecular com-
plexity, with primary emphasis on the chemistry of the repair processes (Sections 8.11.1 and 8.11.2). The
repair of double strand breaks and other biological features of DNA repair are presented elsewhere
(Section 6.7.2).
8.11.1 Direct Reversal of Damage
Photoreactivation is catalysed in bacteria by a photolyase(Section 10.5.4) that separates cross-linked
pyrimidines by transfer of an electron from an excited FADH radical to the pyrimidine dimer. The pyrim-
idine dimer radical formed is unstable and decays to cleave the cyclobutane ring and transfer the electron
back to the co-factor in the enzyme. The energy for the excitation of the FADH co-factor comes from a
second chromophoric co-factor (5,10-methylenetetrahydrofolate or 7,8-didemethyl-8-hydroxy-5-deazari-
boflavin, depending on the source of the enzyme), which harvests blue-light photons. Interestingly, there
is no evidence for DNA photolyase activity in humans. Thomas Carell has obtained an X-ray structure for
a photodimer bound to the photolyase repair enzyme.^91
Demethylation is effected by two distinct types of repair system.^92 The first is carried out by an
O^6 -methylguanine-DNA methyltransferase(Table 8.1)^93. This sacrificial demethylation enzyme uses the
SH group of a Pro-Cys-His sequence near its C-terminus as a methyl acceptor (Figure 8.40a). The same
enzyme can also dealkylate other O^6 -alkyl guanines including ethyl, 2-hydroxyethyl and 2-chloroethyl
species. The S-alkylcysteine protein formed is inactive and cannot be reactivated. It thus follows that one
molecule of the enzyme can only repair one O-methylated base. Human lymphoid cells that are resistant
to alkyl-nitrosoureas have 10,000–25,000 copies of this enzyme per cell while mutants deficient in the
repair of O^6 -MeG have no enzyme. It is easy to see that the threshold of tolerance for alkylating agents
may vary greatly between different species of cells and this contributes to the selective cytotoxicity of
some anticancer alkylating agents, e.g.Temozolomide(Section 8.7). It has also led to the development of
analogues of O^6 -methylG, especially O^6 -benzylG, as specific inhibitors of this methyltransferase with the
aim of enhancing the biological activity of anticancer alkylating agents.
326 Chapter 8
Figure 8.39 Most common DNA-damaging agents, lesions and repair pathways