efsa-opinion-chromium-food-drinking-water

Chromium in food and drinking water
Figure 15: Cysteine-Cr(III)-DNA cross-link structure as determined by analysis of crystal structure
(de Meester et al., 1977; Madafiglio K et al., 1990)
The information from several studies indicate that all cellular Cr-DNA adducts are ternary cross-links.
Reductive metabolism of Cr(VI) in vitro usually generates a large number of binary Cr(III)-DNA
adducts (Zhitkovich et al., 1996, 2000; Quievryn et al., 2002), but the presence of these DNA
modifications in cells has not yet been established and is expected to be strongly inhibited due to the
abundance of intracellular ligands capable of rapid coordination to Cr(III) prior to its binding to DNA.
In cells in culture (human A549 cells) the restoration of physiological concentrations of ascorbic acid
is required to detect ascorbate-Cr(III)-DNA adducts (Quievryn et al., 2002). The authors concluded
that the availability of intracellular ascorbate for Cr(VI) reduction may be key to the amount of Crinduced
DNA damage observed.
DNA-protein cross-links (DPC) have also been detected in vitro during Cr(VI) reduction (Salnikow et
al., 1992) as well as in various Cr(VI)-treated cells (Costa et al., 1996) and tissues in vivo (Hamilton,
1986; Coogan et al., 1991b; Zhitkovich and Costa, 1992) as well as in vitro during Cr(VI) reduction
(Salnikow et al., 1992). In particular, Coogan et al. (1991b) reported the induction of DPC in male
Fischer 344 rat liver following 3-6 weeks of exposure via drinking water to potassium chromate at the
lowest effective dose of 100 mg Cr(VI)/L. In contrast, no DPC were reported by De Flora et al. (2008)
in forestomach, glandular stomach and duodenum cells of female SKH-1 hairless mice administerd
with sodium dichromate dihydrate in drinking water at concentrations up to 20 mg Cr(VI)/L for
9 months.
Although DPC represent only a very small fraction of initially formed DNA adducts in cultured cells
(about 0.1 % according to calculations by Zhitkovich group), DPC have been broadly utilized as a
biomarker of Cr-exposure in human populations (Costa et al., 1993). However, it is important to note
that the currently used methodologies do not allow differentiating between Cr(VI)-induced and other
forms of DPC. Macfie et al. (2010) have recently proposed a three-step mechanism for Cr(VI)-induced
DPC involving (i) reduction of Cr(VI) to Cr(III), (ii) Cr(III)-DNA binding and (iii) protein capture by
DNA bound Cr(III).
In vitro reduction of Cr(VI) by ascorbate (O’Brien et al., 2002; Bridgewater et al., 1994) or cysteine
(Zhitkovich et al., 2000) also produces a small number of Cr(III)-mediated interstrand DNA crosslinks.
The most extensive DNA cross-linking was always observed under conditions of limited reducer
concentrations. On the basis of the steric considerations and the fact that the yield of interstrand crosslinks
had the exponential dose dependence, Zhitkovich et al (2000) proposed that Cr(III) oligomers,
not monomeric Cr(III), are the cross-linking species.
Mutagenic and cytotoxic properties of Cr adducts
The fact that chromium binds preferentially to the N7 position of guanine on DNA was originally
suggested by in vitro studies where DNA polymerases of different origin produced guanine-specific
EFSA Journal 2014;12(3):3595 108