Direct Oxidation of Guanine and 7,8-Dihydro-8-oxoguanine in DNA by a High-Valent Chromium Complex:  A Possible Mechanism for Chromate Genotoxicity

Intracellular reductive activation of the human carcinogen chromate, Cr(VI), is a necessary step in the formation of DNA lesions that lead to cancer. Reductive activation forms the transient metastable high-valent oxidation state of Cr(V) as a precursor to the final intracellularly stable oxidation state, Cr(III). In this study, we have used a model high-valent Cr(V) complex, N,N‘-ethylenebis(salicylideneanimato)oxochromium(V), Cr(V)−Salen, to probe the mechanism of interaction between this oxidation state of chromium and DNA. This interaction was found to be specific toward the oxidation of the nucleic acid base guanine in unmodified single- and double-stranded oligonucleotides as measured by an increased level of DNA strand cleavage at these sites following piperidine treatment. Replacement of a single guanine residue in DNA with a more readily oxidized 7,8-dihydro-8-oxoguanine (8-oxo-G) base allowed for site-specific oxidation at this modified site within the DNA strand by the Cr(V)−Salen complex. HPLC and ESI-mass spectrometry were used to identify the modified guanine base lesions formed in the reaction of this high-valent chromium complex with the 8-oxo-G-containing DNA substrate. Two of these modified base lesions, identified as guanidinohydantoin and spiroiminodihydantoin, were found in the reaction of the Cr(V)−Salen complex with 8-oxo-G-modified DNA, while only one, spiroiminodihydantoin, was formed from oxidation of the 8-oxo-G nucleoside. A primer extension assay using the exo- Klenow fragment demonstrated polymerase arrest at the site of these base modifications as well as a high degree of misincorporation of adenine opposite the site of modification. These results suggest that mutations arising from G → T transversions would predominate with these lesions. The mechanism of damage and base oxidation products for the interaction between high-valent chromium and DNA described herein may be relevant to the in vivo formation of DNA damage leading to cancer in chromate-exposed human populations. These results also suggest how high-valent chromium can act as a cocarcinogen with 8-oxo-G-forming xenobiotics.