The antioxidant vitamins C and E

implicated as a causative agent in numerous cancers (28). One mechanism by which
iron could be involved in the initiation or promotion of cancer is through the oxidation
of DNA, causing mutations. DNA can be modified by free radicals resulting in singleand
double-strand breaks, depurination, and depyrimidation, or chemical modification
of the bases or phosphate-sugar backbone (42). Several ROS have been shown to
oxidatively modify DNA, including • OH, singlet oxygen, and ONOO − (43). In contrast,
O 2 •− and H2 O 2 are not capable of oxidizing DNA in the absence of adventitious
metals (42), suggesting that the role of O 2 •− in DNA oxidation is simply as a constituent
of the metal-driven Haber-Weiss reaction to produce • OH. In addition, it has
been demonstrated that the addition of any chemical that will act as an alternate reactant
for • OH, such as organic-based buffers or “ • OH traps,” inhibits the oxidation of
DNA (38). Conversely, the presence of chemicals that increase the iron-mediated production
of • OH will promote DNA oxidation (38).
Some researchers have postulated that the iron-mediated oxidation of DNA is a
site-specific process (41,42). They propose that iron or an iron chelate binds to the
DNA, either at phosphate on the backbone or to the purine or pyrimidine bases, where
the iron can serve as a center for recurring formation of • OH, resulting in modification
of the DNA (42,44,45). Experiments using purified DNA or isolated nuclei (46–48)
confirm that in the presence of added metal ions, ascorbate acts as a prooxidant in
vitro. In the absence of added metal ions, however, vitamin C inhibits oxidative DNA
damage in purified DNA and cells (47,49–53), although there are a few exceptions
(54–56). The latter are likely explained by “contaminating” metal ions in the cell culture
media used.
The bleomycin-iron complex was the first well-studied system to damage DNA
site specifically in a metal-dependent manner (44,45). Bleomycin-iron cleaves DNA
to release N-propenal–substituted derivatives of thymine, cytosine, adenine, and guanine,
which are believed to be responsible for some of the cytotoxic effects of
bleomycin (45). The bleomycin-mediated cleavage of DNA is proposed to occur via a
ternary bleomycin-DNA-iron complex. It is not known whether bleomycin binds to
DNA first and then Fe(II) binds to the bleomycin-DNA complex, or whether a
bleomycin-Fe(II) complex forms and then binds to DNA. Either way, oxidation of the
complexed Fe(II) results in site-specific oxidation of DNA, presumably via • OH production
(57). In the presence of reducing agents such as ascorbate, Fe(III) is reduced
back to Fe(II), thus continuing the oxidation of the DNA.
Protein oxidation. Studies of the metal-mediated denaturation of proteins were an
outgrowth of investigations into the regulation of protein turnover in bacteria (35).
These studies led to the discovery that degradation occurs when the protein has been
oxidized (35). Similar to DNA, iron-mediated oxidation of protein may be a site-specific
process. This notion is supported by the findings that the oxidation of proteins by
MCO systems involves modification of only a few amino acid residues, in particular
proline, histidine, arginine, lysine, and cysteine, whereas reactions of proteins with
ROS generated by ionizing radiation are more or less random events, leading to the
modification of many or all amino acid residues. Furthermore, metal ion-catalyzed
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