On the hydroxyl radical formation in the reaction between hydrogen peroxide and biologically generated chromium(V) species

Deferoxamine inhibition of Cr(V)-mediated radical generation and deoxyguanine hydroxylation: ESR and HPLC evidence

Electron spin resonance (ESR) and high-performance liquid chromatography (HPLC) techniques were utilized to investigate the effect of deferoxamine on free radical generation in the reaction of Cr(V) with H2O2 and organic hydroperoxides. ESR measurements demonstrated that deferoxamine can efficiently reduce the concentration of the Cr(V) intermediate as formed in the reduction of Cr(VI) by NAD(P)H or a flavoenzyme glutathione reductase/NADH. ESR spin trapping studies showed that deferoxamine also inhibits Cr(V)-mediated .OH radical generation from H2O2, as well as Cr(V)-mediated alkyl and alkoxy radical formation from t-butyl hydroperoxide and cumene hydroperoxide. HPLC measurements showed that .OH radicals generated by the Cr(VI)/flavoenzyme/NAD(P)H enzymatic system react with 2'-deoxyguanine to form 8-hydroxy-2'-deoxyguanine (8-OHdG), a DNA damage marker. Deferoxamine effectly inhibited the formation of 8-OHdG also.

Metal carcinogenesis: mechanistic implications

Cancer epidemiology has identified several metal compounds as human carcinogens. Recent evidence suggests that carcinogenic metals induce genotoxicity in a multiplicity of ways, either alone or by enhancing the effects of other agents. This review summarizes current information on the genotoxicity of arsenic, chromium, nickel, beryllium and cadmium compounds and their possible roles in carcinogenesis. Each of these metals is distinct in its primary modes of action; yet there are several mechanisms induced by more than one metal, including: the induction of cellular immunity and oxidative stress, the inhibition of DNA metabolism and repair and the formation of DNA- and/or protein-crosslinks.

The role of superoxide radical in chromium (VI)-generated hydroxyl radical: the Cr(VI) Haber-Weiss cycle

To understand the role of the superoxide (O-2) radical in chromate-related genotoxicity, we investigated whether Cr(VI) can catalyze the Haber-Weiss cycle in vitro: O-2 + Cr(VI)----Cr(V) + O2 Cr(V) + H2O2----Cr(VI) + .OH + OH-. ESR and spin trapping techniques were utilized to monitor the O-2 (produced using xanthine/xanthine oxidase), .OH, and Cr(V) species. Superoxide dismutase as well as catalase inhibited the .OH radical radical formation, attesting to the direct involvement of O-2 and H2O2 in the process. ESR measurements also provided direct evidence for the formation of Cr(V). Kinetic measurements were consistent with the role of Cr(V) and H2O2 as intermediates in .OH formation. These results indicate that in cellular media, especially during chromate phagocytosis, the O-2 radical can become a significant source of .OH radicals and hence a significant factor in the biochemical mechanism of cellular damage due to Cr(VI) exposure.

Role of physiological antioxidants in chromium(VI)-induced cellular injury

Chromium(VI) compounds are well known to be potent toxic and carcinogenic agents. Because chromium(VI) is easily taken up by cells and is subsequently reduced to the trivalent form, the formation of chromium(III) or other intermediate oxidation states such as chromium(V) and (IV) is believed to play a role in the adverse biological effects of chromium(VI) compounds. Recent in vitro studies have shown that this reduction process generates free radical species such as active oxygen radicals. Furthermore, physiological antioxidants are reported to modify the genotoxic and toxic effects of chromate. This article reviewed the recent in vitro and in vivo studies of the effects of antioxidants including active oxygen scavengers; glutathione; vitamins B2, E, and C, on chromate-induced injury such as DNA lesions; lipid peroxidation; enzyme inhibition; cytotoxicity; mutation; and so on. In addition, the mechanism of action of these antioxidants was discussed with respect to the formation of active oxygen radicals and paramagnetic chromium such as chromium(V) and (III). Such studies may help elucidate the mechanism of chromium(VI) toxicity as well as the mechanism of protection.

One-electron reduction of carcinogen chromate by microsomes, mitochondria, and Escherichia coli: identification of Cr(V) and .OH radical

Earlier studies have shown that a long-lived Cr(V) species is produced during the reduction of chromate (Cr(VI] by microsomes/NADPH, mitochondria, and other cellular constituents and that this Cr(V) species plays a significant role in the mechanism of Cr(VI) toxicity. The present work indicates that this species is a Cr(V) complex involving the diol moieties of NADPH as the ligand. Additionally, ESR spin trapping investigations show that the hydroxyl (.OH) radical is also generated in the reduction process. Hydrogen peroxide (H2O2) enhances the .OH generation but suppresses the Cr(V)-NADPH complex formation. Catalase decreases the .OH radical generation and enhances the Cr(V)-NADPH formation. Measurements under anaerobic atmosphere show decreased .OH radical generation, indicating that during the cellular Cr(VI) reduction process molecular oxygen is reduced to H2O2, which reacts with the Cr(V)-NADPH complex to generate the .OH radical via a Fenton-like mechanism.

Reduction of chromium(VI) in Chinese hamster V-79 cells

The cellular reduction of chromate(VI) was studied by electron spin resonance spectrometry. Incubation of Chinese hamster V-79 cells with Na2CrO4 resulted in the formation of both chromium(V) and chromium(III) complex in a manner dependent on time (30 min-2 h) and concentration (50-500 microM). Following removal of extracellular chromate, the level of chromium(V) complex decreased quickly during the first hour but more slowly for the next hour, whereas the level of chromium(III) remained unchanged, indicating that chromium(III) is the ultimate ion of this metal in cells. Alkaline elution studies demonstrated that treatment of cells with Na2CrO4 induced DNA single-strand breaks that decreased quickly and DNA-protein crosslinks that persisted for 2 h after removal of this metal. These results suggest that the cellular levels of chromium(V) and chromium(III) may be associated with the formation of DNA damage induced by chromium (VI).

Evidence for the generation of hydroxyl radicals from a chromium(V) intermediate isolated from the reaction of chromate with glutathione

The formation of hydroxyl radicals from a chromium(V) complex isolated from the reaction of glutathione with chromate has been demonstrated in spin trapping experiments using dimethylsulfoxide and 3,5-dibromo-4-nitrosobenzene sulfonate. Mechanisms for the formation of radicals in such systems are discussed. These results help to explain the ability of solutions containing chromate and glutathione to cause strand breaks in DNA.

Antioxidant behaviour of caffeine: efficient scavenging of hydroxyl radicals

Considerable controversy exists in the literature regarding the toxicity of coffee, including its possible carcinogenic and anticarcinogenic properties. This study reports on the reaction of 1,3,7-trimethylxanthine (caffeine) with the hydroxyl radical (.OH), as investigated by electron spin resonance (ESR) spin trapping. The .OH was generated by the Fenton reaction (Fe2+ + H2O2) as well as by the reaction of chromium(V) with H2O2. The results show that caffeine effectively scavenges .OH with a reaction rate constant of approximately 5.9 x 10(9) M-1 sec-1 that is comparable with those of other efficient .OH radical scavengers. ESR measurements provide evidence that a caffeine-derived oxygen-centred radical is formed in the reaction of caffeine with .OH and suggest a biochemical basis for the understanding of the reported anticarcinogenic properties of caffeine and related methylxanthine compounds.

NADPH-dependent flavoenzymes catalyze one electron reduction of metal ions and molecular oxygen and generate hydroxyl radicals

This study reports a new property of the important NAD(P)H-dependent flavoenzymes, glutathione reductase, lipoyl dehydrogenase and ferredoxin-NADP+ oxidoreductase, that can catalyze a one electron reduction of metal ions such as chromium(VI) and vanadium(V). During the enzymatic reduction process, molecular oxygen is reduced to H2O2, which reacts with the reduced metal complexes to generate hydroxyl radicals. Since the hydroxyl radicals have been suggested to play an important role in Cr(VI) toxicity, this study provides a basis for a recent observation that Cr(VI) mutagenesis is strongly oxygen dependent. These results also point to an enzymatic pathway for the metabolism of some metal ions and concomitant generation of hydroxyl radicals.

Reaction of vanadium(V) with thiols generates vanadium (IV) and thiyl radicals

The in vivo toxicity of vanadium(V) has been found to correlate with the depletion of cellular glutathione and related non-protein thiols. With a view to understanding the mechanism for this observation, we have investigated the oxidation of glutathione, cysteine N-acetylcysteine and penicillamine by vanadium(V), using electron spin resonance (ESR) and ESR spin trapping methodology. The spin trap used was 5,5-dimethyl-1-pyrroline 1-oxide (DMPO). It is found that the oxidation of these thiols by vanadium(V) generates the corresponding thiyl radicals and vanadium- (IV) complexes. The results suggest that free radical reactions play a significant role in the depletion of cellular thiols by vanadium(V) and hence in vanadium(V) toxicity.

Evidence for a Fenton-type mechanism for the generation of .OH radicals in the reduction of Cr(VI) in cellular media

Electron spin resonance (ESR) spectroscopy has been employed to examine the role of tetraperoxochromate (V) ions (CrO3-8) and other Cr(V) species in the generation of hydroxyl (OH) radicals in the reaction of Cr(VI) with H2O2 in biological media. In contrast to earlier suggestions, the present ESR studies using crystalline K3CrO8 as a source of CrO3-8 show that decomposition of CrO3-8 in water or in H2O2 does not generate significant amounts of OH radicals. Addition of NADH to a solution containing CrO3-8 yields a Cr(V)-NADH complex, which readily reacts with H2O2 to generate OH radicals. Similar results obtained from several other biological reductants, including vitamin B2, indicate that a vacant coordination site on a Cr(V) complex facilitates its reaction with H2O2 to generate .OH radicals. We thus suggest that in biological media, reaction [3] instead of [1] or [2] is the major pathway for the .OH radical generation: [formula, see text]