Chromium cross-links glutathione and cysteine to DNA

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Abstract
The formation of chromium-DNA adducts, and chromium-mediated peptide-DNA or amino acid-DNA cross-links was measured after treatment of calf thymus DNA and defined DNA polynucleotides in vitro with potassium dichromate in the presence of glutathione or cysteine. The level of chromium bound to DNA after reaction with chromium (VT) in the presence of glutathione increased with increasing glutathione concentration to a level of ˜1.4 x 10−2 chromium per nucleotide. Glutathione and chromium were associated with the DNA in a 1:1 ratio. Reaction of chromhim(VI) with DNA in the presence of cysteine led to a maximal level of chromium binding that was 10-fold lower than that measured with glutathione, with 2–4 cysteine bound per chromium. The thiol-chromium-DNA complexes were stable to dialysis at room temperature against diethytenetriaminepentaacetic add, orthophenanthroline and ethylenediaminetetraacetie acid. However, when the chromium -DNA complexes were dialyzed against ethylenediaminetetraacetie acid at 37°C to chelate bound chromium, equivalent amounts of chromium and thiol were lost from the DNA, suggesting that thiol was associated with the DNA-bound chromium. These results suggest that chromium mediates cross-linking of cysteine and glutathione to DNA, to form glutathione-chromium-DNA and (cysteine)2–4-chromium-DNA complexes. In order to probe the DNA base and sequence specificity of glutathione-chromium-DNA adduct formation, chromium and glutathione binding to polynucleotides of defined composition was determined. Preferential binding of chromium to guanine-containing polynucleotides was observed. These results suggest that the interaction of chromium with thiol-containing amino acids and peptides may be important in chromium genotoxicity, and indicate that the thiol-activated chromium may target guanine bases in DNA.

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