Reduction Of Carcinogenic Cr(VI) On The Skin Of Living Rats
1Ke Jian Liu, 1Karsten Mäder, 2Xianglin Shi, 1Harold M. Swartz
EPR Center for the Study of Viable Biological Systems, Department of
Diagnostic Radiology,
Dartmouth Medical School, Hanover, NH 03755, USA
INTRODUCTION: Chromate (Cr(VI)) compounds, widely used in industry,
have been shown to
have serious toxic and carcinogenic effects in humans. Since Cr(VI) does not
react with isolated
DNA, the reduction of Cr(VI) by cellular reductants to lower oxidation
states has been considered to
be an important step in the mechanism of Cr(VI)-induced carcinogenesis. It
is generally believed
that the Cr(V) intermediates are likely candidates for the "ultimate"
carcinogenic form of chromium
compounds.
Skin is one of the potential routes for chromium to enter into humans and
exert its
carcinogenicity. The purpose of present work is to determine whether Cr(VI)
is reduced to Cr(V)
after topical application to the skin, and whether the skin barrier has an
effect on the rate of the
reduction process. The development of in vivo EPR spectroscopy made it
possible to study this
problem directly in the living animal.
METHODS: Wistar rats weighing 200-220 gram were anesthetized and the
hair on the back of the
rat was shaved to expose the skin. Then 200 µl of 0.1 M aqueous solution of
sodium dichromate was
applied topically to the skin. To study the effect of barrier function of
the skin on the reduction of
Cr(VI), in a group of animals the stratum corneum of the skin was removed by
applying and
removing surgical tape 10 times before application of dichromate.
In vivo EPR measurements of Cr(V) on the skin of living rats were obtained
using a low
frequency (1.2 GHz) EPR spectrometer with an extended loop resonator. After
dichromate was
applied to the skin of the rat, the animal was immediately placed in between
the poles of the magnet
with the coil of the extended loop resonator positioned on top of the skin
at the region of interest.
The EPR spectra were collected every 60 seconds to record the formation and
decay of the Cr(V)
signal.
RESULTS: After application of an aqueous dichromate solution to the
skin of a living rat, an EPR
signal with a line width of 0.25 mT, and a g-factor of 1.979 was observed
from the treated area.
Since Cr(VI) is EPR silent, and the parameters of the recorded EPR signal
are identical to those
reported in the literature for Cr(V), we assign this signal to Cr(V).
The intensity of the Cr(V) signal grew with time, and reached a plateau at
approximately 60
min. Upon removing the Cr(VI) from the skin by washing with water, the
signal decayed with time.
Removing the stratum corneum (by stripping) increased the rates of both
formation and decay of
Cr(V), as well as the intensity of the Cr(V) signal when compared with the
non-stripped animals.
DISCUSSION AND CONCLUSIONS: These results provide the first direct
evidence on the
reduction of Cr(VI) to Cr(V) in the skin of the living rat. The results
demonstrate that skin represents
one of the routes for chromium to enter into animals.
The importance of the stratum corneum as a diffusion barrier was evident
because a higher
rate of formation of Cr(V) species, and the significantly increased signal
intensity of Cr(V) which
was observed in experiments when stratum corneum was partially removed by
stripping.
The results in the present study also illustrate that in vivo EPR
spectroscopy can be a very
useful and powerful tool to study paramagnetic reactive species in chemical
and biochemical
reactions occurring in/on the skin of both small and large animals. Because
this area is near the
surface of animals, it permits the technique of in vivo EPR to exploit its
unique advantage of directly
studying the paramagnetic species itself, and at the same time, avoids the
sensitivity problems
usually associated with the limited depth of penetration of microwaves at
L-band frequency (~ 1-2
GHz). Consequently, the technique can be readily applied to both small and
large animals, and
potentially, to humans.
1Department of Radiology, Dartmouth Medical School, Hanover, NH 03755, and
2Pathology and
Physiology Research Branch, National Institute For Occupational Safety and
Health, Morgantown,
WV 26505
REFERENCE:
1.Ke Jian Liu, Karsten Mäder, Xianglin Shi, Harold M. Swartz, Reduction of
carcinogenic
Cr(VI) on the skin of living rats, Magn. Reson. Med., Magn. 38:524-526
(1997).