HIGH-VALENT Cr, Mn, AND Fe SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 295

HGPRT locus [145, 146]. However, pretreatment with vitamin E, a free radical
scavenger, showed a significant decrease in Cr(VI)-induced single-strand
breaks and cytotoxicity [151]. Importantly, there was no difference observed
in Cr(VI) uptake, Cr(VI)-induced DNA-protein cross-links (DPCs), and the
levels of GSH and GSH reductase activity [151]. An indication of the depen-
dence of antioxidant regulation on the concentration of Cr(VI) has also been
suggested by in vitro studies. Considering the relative ratio of the concentra-
tion of available intracellular reductants and the total dosage of Cr(VI) over
time as well as the dose rate may clarify the species involved in Cr(VI)-
induced oxidative stress [9, 129].

6.1.6 Carcinogenesis

Mutagenicity is considered an initiation step in Cr(VI)-induced carcinogenesis
[152]. A number of Cr(V)-amino acid and peptide complexes with ligands
(Ala, Ala 3 , Gly, and Gly 3 ) were found to be mutagenic against Salmonella
typhimurium with an order of reactivity as Cr(V)-Ala 3 > Cr(V)-Ala > Cr(V)-
Gly > Cr(V)-Gly 3 [106]. Complexes of Cr(V) with Gly 4 and Gly 5 were not
mutagenic to the species. Comparatively, complexes of Cr(III) with AibH (α-
aminoisobutanoic acid), Ala 3 , Gly 3 , Gly 4 , and Gly 5 were also not mutagenic to
S. typhimurium [106, 153].
Cr(VI) produced structural genetic lesions such as DPCs, DNA inter- and
intrastrand cross-links, DNA-strand breaks, DNA adducts, and oxidized bases.
High levels of DPC have been determined in peripheral blood lymphocytes
among chrome platers, welders, and leather tanners [154]. A good correlation
of DPCs with the levels of Cr in red blood cells has been observed [139]. The
formation of DPCs in vitro has been studied in detail [154, 155]. The mecha-
nism of DPC first involves the reduction of Cr(VI) to Cr(III), followed by the
formation of DNA-Cr(III) adducts, which subsequently capture proteins
(reaction 6.45):

VI III III III

.

(6.45)

Cr → Cr → Cr -DNA protein-Cr →

The role of ascorbic acid and GSH as reductants of Cr(VI) in the formation
of DPC has been examined in A549 cells (Fig. 6.8) [154]. The GSH inhibitor,
L-buthionine-[S,R]-sulfoxime (BSO), was applied during preincubation to
reduce the level of GSH from 4.2 ± 0.6 mM to 0.15 ± 0.2 mM ( n = 3). As shown
in Figure 6.8A, this resulted in lower levels of DPC immediately and 18 hours
after exposure to Cr(VI). When the cellular reducing capacity was restored
using physiological ascorbic levels (0.9 ± 0.1 mM) prior to treatment with
Cr(VI), the yield of DPC was increased (Fig 6.8B). However, no significant
changes in the uptake of Cr(VI) in addition to ascorbic were observed (Fig
6.8C). It appears ionic interactions between negatively charged phosphate
groups of DNA and positively charged Arg or Lys side chain groups are