Reaction of Cr(VI) with ascorbate and hydrogen peroxide generates hydroxyl radicals and causes DNA damage: role of a Cr(IV)-mediated Fenton-like reaction

Abstract

Incubation of Cr(VI) with ascorbate generated Cr(V), Cr(IV) and ascorbate-derived carbon-centered alkyl radicals, as well as formyl radicals. H2O2 caused generation of hydroxyl radicals (OH) and much higher levels of Cr(V), showing that .OH can be generated via a Cr(IV)-mediated Fenton-like reaction (Cr(IV) + H2O2-->Cr(V) + .OH + OH-). 1,10-Phenanthroline and deferoxamine inhibited the formation of both .OH and Cr(V) from the reaction of Cr(VI) with ascorbate in the presence of H2O2. Electrophoretic assays showed that ascorbate-derived free radicals caused DNA double-strand breaks. .OH radicals generated by Cr(V)- and Cr(IV)-mediated Fenton-like reactions also caused DNA double-strand breaks. HPLC measurements showed that .OH radicals generated by Cr(IV) and Cr(V) from H2O2 caused 2'-deoxyguanine hydroxylation to form 8-hydroxy-2'-deoxyguanine.

Reduction of hexavalent chromium by human cytochrome b5: Generation of hydroxyl radical and superoxide

“…This is in contrast to studies with chemical Cr(VI) reductants (e.g. ascorbate, glutathione, NADPH, cysteine) for which high concentrations (1 to 5 mM) of exogenous H 2 O 2 have typically been added [27,29,34,66]. Cr (VI) reduction by α-lipoic acid is an exception to these studies in that hydroxyl radical was detected without adding H 2 O 2 [25].…”

“…Hydroxyl radical was generated at all Cr(VI) concentrations tested (100, 50, and 10 μM). These are well below those used in most studies in which chromate/dichromate concentrations have typically been 1 to 2 mM [25][26][27][28][29][30]34,66]. One study with glutathione reductase did note hydroxyl radical generation when using insoluble PbCrO 4 at 50 μg per ml (equivalent to 155 μM) [67].…”

“…ascorbate, glutathione, α-lipoic acid, NADPH, cysteine) to assess hydroxyl radical generation, concentrations of reductants have typically been 2 to 50 mM [25,27,29,34,66]. In studies with other Cr(VI)-reducing enzymes, enzyme concentrations have often been given as units of activity based on specific enzyme substrates.…”

“…The generation of Cr(V) under aerobic conditions could potentially result in hydroxyl radical generation via Cr(V)-mediated Fenton-like chemistry [25][26][27][28][29][30]. Catalase was used as one of the tools (below) to determine if the reduction of H 2 O 2 by reactive Cr intermediates was involved in hydroxyl radical formation.…”

“…Cr(V), Cr(IV), reactive oxygen species), is likely a key component in the toxicity and carcinogenicity of Cr(VI) [18,[20][21][22][23][24]. Reactive Cr intermediates, Cr(V) and Cr(IV), generated by some reductants can participate in Fenton-like reactions with hydrogen peroxide to generate hydroxyl radical [25][26][27][28][29][30].…”