Glutathione and free amino acids form stable complexes with DNA following exposure of intact mammalian cells to chromate

Abstract

Exposure of cells to carcinogenic Cr(VI) compounds results in the formation of several types of DNA lesions such as strand breaks, DNA-protein crosslinks and uncharacterized DNA-Cr adducts. Hexavalent chromium compounds are positive in most bacterial and eukaryotic mutagenic systems, although the nature of DNA modifications underlying the chromium-induced mutagenesis is not known. Hexavalent chromate(VI) is very active in cellular systems because it is actively transported into cells, but intracellularly it is ultimately reduced to Cr(III). Here we show that exposure of Chinese hamster ovary (CHO) cells to potassium chromate(VI) leads to the formation of stable complexes between DNA and amino acids or glutathione. Cysteine, glutamic acid and histidine were the major amino acids crosslinked to DNA in chromate-treated cells. Incubation of purified DNA in the presence of EDTA dissociated SDS stable amino acid—DNA complexes, which indicates that these DNA adducts are most likely to represent ternary coordination complexes mediated by Cr(III) rather than covalent linkage between amino acids/ glutathione and DNA. The amino acids that were found complexed with DNA purified from chromate-exposed cells did not orginate from previously crosslinked proteins during DNA isolation, but represented authentic reactions of free amino acids and glutathione with chromium and DNA in cells. Ternary complexes of glutathione or amino acids with Cr(III) and DNA were estimated to account for as much as 50% of DNA-bound chromium following exposure to ≦25 μM chromate.