Chromium - Toxicology of Metals - page 494

The occurrence of these malformations was low in the
controls, but statistical signifi cance was not tested. As
shown by Kanojia et al . (1998), when feeding female
rats before gestation at doses 250, 500, and 700 ppm
of K 2 Cr 2 O 7 in drinking water, a signifi cant reduction
in the number of implantations and the number of
fetuses was observed. Also, skeletal abnormalities
(i.e., reduced ossifi cation in parietal, interparietal, and
caudal bones) were observed in the fetuses.

et al., 1996). DNA-protein crosslink levels correlated well
with Cr RBC levels and with external measurements of
exposure. The induction of DNA-protein crosslinks rep-
resents DNA damage occurring in the lymphocytes of
humans exposed to chromate, and, thus, is a biomarker
of chromate exposure and effect.

9 CELLULAR MECHANISM OF TOXICITY
AND CARCINOGENICITY

8 BIOLOGICAL MONITORING

9.1 Molecular Toxicology of Cr
As mentioned previously, Cr(III) compounds enter
cells poorly because they do not resemble essential
molecules that we need in the body, as does chromate.
At physiological pH, Cr(VI) resembles phosphate and
sulfate because it exists as an oxyanion with an overall
−2 charge (Boyd, 1986). Thus CrO 4 −2 is taken up into
the cells by the sulfate and phosphate transport sys-
tem and accumulates into cells through this molecu-
lar mimicry mechanism (Tsapakos, 1983). The uptake
of all Cr(III) compounds is probably not negligible if
the trivalent Cr is stably complexed with a ligand as is
the case of Cr picolinate (Zhitkovich, 2005). However,
if the Cr(III) complex is maintained in water solution
(e.g., CrCl 3 ), it can form polymeric species that may
eventually be too large to enter the cell (Zhitkovich,
2005). Regardless of whether the Cr(III) enters the
cell or not, if it is stably complexed to a ligand, it will
generally be nonreactive. Thus, the most important
toxicological species is the hexavalent form of Cr that
enters the cell and is immediately subject to reduc-
tion by means of one or two electron reducers. The
reduction of hexavalent Cr depends on the pH and
will generally occur much faster at acidic pH than at
higher pH values (Sugden, 2002). It also depends on
the level of the reducing agent. Inside the cell there are
three major reducers of Cr(VI): ascorbic acid, glutath-
ione, and cysteine (O'Brien, 2003; Zhitkovich, 1996;
2000). If the level of the reducing agent is very high
relative to the concentration of Cr(VI), then reduc-
tion to Cr(III) will proceed quickly. However, if the
level of the reducing agents are low compared with
the amount of Cr(VI), then reduction may proceed by
means of Cr(V) and Cr(IV) intermediates (Zhitkovich,
2005). The major toxic effect of hexavalent Cr in the
cell is due to the reaction of these intermediates with
protein and DNA (Zhitkovich, 1996). Attack can occur
by means of the intermediates of Cr(III) (Zhitkovich,
1996). The reduction of hexavalent Cr by these reduc-
ing agents also generates oxygen radicals in the cell,
which has also been implicated in the cellular injury
induced by hexavalent Cr (Kawanishi, et al ., 1994; Shi,
1999a,b).

8.1 Biomarkers of Exposure
There are considerable background levels of Cr(III)
in foods, and many individuals supplement their diet
with Cr picolinate and other trivalent Cr nutritional sup-
plements, thus contributing to the high body burden of
Cr(III). This background results in a high level of uri-
nary Cr, making it diffi cult to use urine as a biomarker
of exposure to Cr(VI). However, as mentioned earlier
in this chapter, Cr(III) is barely (not) able to enter the
cell, and thus, measurement of intracellular Cr levels
will yield an excellent measure of human exposure to
Cr(VI). Any Cr inside the red blood cell (RBC) is likely
derived from Cr(VI) that survives extracellular reduc-
tion (Lukanova et al ., 1996), but the short lifespan of
RBCs limits the use of the RBC Cr-level as a marker.
However, measurement of Cr in RBCs may be used at
least as biomarker for high exposure to Cr(VI) shortly
after exposure (1-3 months).

8.2 Biomarkers of Effects
At high levels of exposure, perforation of the nasal
septum may occur after only 2-3 weeks of exposure
and may be considered a warning symptom of more
serious outcome effects. Lung X-ray screening among
workers previously exposed to Cr(VI)—at high risk of
cancer—has been shown to have some success (Bid-
strup, 1983) and seems even more promising when
combined with frequent sputum cytology, as shown
by Nishiyama et al ., (1988) who found that nine of
nine screening-detected and subsequently treated
lung cancer cases were alive 20 months to 8 years after
treatment. Such secondary screening may prove use-
ful among groups having accumulated very high can-
cer risk (i.e., an a priori risk of lung cancer exceeding
800 × 10 −5 /year) provided supervision by experienced
personnel.
A number of small pilot studies examined DNA-pro-
tein crosslinks in peripheral blood lymphocytes of chrome
electroplaters and as a function of air pollution in Bulgaria
as a biomarker of chromate exposure and effect (Costa