Chemistry Reference
In-Depth Information
are also able to cross the cell membrane, but rely solely on passive diffusion and
are therefore taken up approximately three orders of magnitude more slowly than
Cr(VI) ions.
24
Once in the cell, Cr(VI) is quickly reduced by a variety of intracellular
reductants including ascorbic acid and reduced thiols, such as cysteine and the ubiq-
uitous tripeptide glutathione.
25
Ascorbic acid is the most kinetically favoured of
these reductants
26
and is thought to be the major intracellular reductant of Cr(VI).
However, the exact contribution of the different reductants towards intracellular
chromate reduction and genotoxicity is not known. Irrespective of the exact nature
of the reductant, the ensuing rapid intracellular reduction of Cr(VI) to Cr(V),
Cr(IV) and fi nally to Cr(III) creates a concentration gradient across the cell mem-
brane that favours the continual uptake of Cr(VI) from the extracellular matrix. To
adequately understand the mechanism of how chromium interacts with DNA it is
fi rst necessary to defi ne the DNA reactive intermediates that are formed during the
intracellular reduction process of Cr(VI).
17.1.3 Reactive Intermediates Formed During Reduction of Chromate
The determination of a mechanistic pathway of cancer formation by Cr(VI) has
been complicated by the wide variety of reactive species formed during the intracel-
lular metabolism of this metal. These pathways are further complicated by the fact
that many of these reactive intermediates may give rise to similar DNA-damage
products, making it a challenge to associate specifi c reactants with the fi nal premuta-
genic DNA lesion. As stated above, there are three intracellular chromate reduct-
ants that have been widely studied and are considered to be the primary players in
chromate metabolism and the ensuing DNA damage. These are ascorbate, glutath-
ione and, to a lesser degree based on low physiological concentration, hydrogen
peroxide.
Reduction of chromate by all three of these compounds forms both radical
species and high valent metal species. While most research has focused on the
radical-mediated pathways, it is becoming clear that a direct or metal-mediated
pathway may play as great or greater role in DNA damage induced by chromate.
C r ( VI ) Reactions with Ascorbate
The
in vitro
reaction of Cr(VI) with ascorbate has been extensively studied. Ascor-
bate is kinetically one of the best physiological reductants of chromate and was
found to be the major reductant in rat liver, lung and kidney ultrafi ltrates.
27
Cr(VI)
reduction by ascorbate has been found to form a number of different radical species
including the ascorbyl radical and the carbon dioxide radical cation (Figure 17.2;
I
and
II
respectively) as well as high valent ascorbate-ligated Cr(V) and Cr(IV)
species (Figure 17.3A). The relative yield of each of these reactive species is depend-
ent upon ascorbate to Cr(VI) ratios.
28,29
However, since Cr(VI) uptake is essentially
steady-state from the extracellular milieu, and ascorbate concentrations in human
cells are variable depending on dietary status, defi ning a Cr(VI):ascorbate ratio and
the likely reactive intermediates that will be formed is unpredictable at best. Gross
DNA-damage studies using plasmid DNA however, have implicated the high-valent
metal species generically shown in Figure 17.3A as the likely reactive intermediates
