Carcinogenesis, Vol. 20, No. 7, 1267-1275,
July 1999
© 1999 Oxford University Press
Carcinogenesis |
In vivo effects of ascorbate and glutathione on the uptake of chromium, formation of chromium(V), chromium–DNA binding and 8-hydroxy-2'-deoxyguanosine in liver and kidney of Osteogenic Disorder Shionogi rats following treatment with chromium(VI)
1 Department of Chemistry, Dartmouth College,
2 Department of Radiology and
3 Department of Pharmacology and Toxicology, Dartmouth Medical School, Hanover, NH 03755-3564, USA
Several previous in vitro studies have indicated that ascorbate and glutathione are the major reductants of Cr(VI) in cells. In order to evaluate the in vivo effects of ascorbate and glutathione on Cr(VI)-induced carcinogenesis, Cr uptake and the formation of Cr(V), Cr–DNA adducts and 8-hydroxy-2'-deoxyguanosine (8-OH-dG) were measured in the liver and kidney of Osteogenic Disorder Shionogi (ODS) rats that lack the ability to synthesize ascorbate. Despite a 10-fold difference in tissue ascorbate levels among different dietary ascorbate groups, the Cr(V) signal intensity, Cr uptake and total Cr–DNA binding were not affected in either organ. Treatment of ODS rats with Cr(VI) (10 mg/kg) had no substantial effect on the levels of ascorbate and glutathione in these tissues. The levels of Cr(V) and Cr–DNA binding were ~2-fold higher in the liver than in the kidney, although the levels of total Cr uptake were similar in both tissues. Cr uptake levels were significantly lower in the liver and kidney of ODS rats treated with high levels of ascorbate and a high dose of Cr(VI) (40 mg/kg), suggesting a detoxifying role played by plasma ascorbate. Similarly, modulation of glutathione levels by N-acetyl-L-cysteine, L-buthionine-S,R-sulfoximine or phorone in these animals by up to 2-fold had little or no consistent effect on Cr uptake, Cr–DNA binding, Cr(V) levels or 8-OH-dG formation in either organ. One possible explanation is that reduction of ascorbate and glutathione concentration to <10 and 50%, respectively, of normal in these two organs still provides threshold levels of these two reductants that are in excess of what is needed for significant reductive activation of Cr(VI). Alternatively, it is possible that ascorbate and glutathione do not play a major role in the formation of Cr(V), Cr–DNA binding or 8-OH-dG and that other cellular reductants, such as cysteine or other amino acids, might be more important reductants of Cr(VI) in vivo.
Abbreviations: BHT, butylated hydroxytoluene; BSO, L-buthionine-S,R-sulfoximine; DETAPAC, diethylenetriamine pentaaetic acid; dG, 2'-deoxyguanosine; DTNB, 5,5'-dithiobis-(2-nitrobenzoic acid); NAC, N-acetyl-L-cysteine; NPSH, non-protein sulfhydryl; 8-OH-dG, 8-hydroxy-2'-deoxyguanosine; ODS, osteogenic disorder Shionogi; PBS, phosphate-buffered saline; ROS, reactive oxygen species; TCA, trichloroacetic acid.
4 To whom correspondence should be addressed. Email: jim.liu{at}dartmouth.edu
Deceased.