Cobalt-mediated generation of reactive oxygen species and its possible mechanism

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Abstract

Electron spin resonance spin trapping was utilized to investigate free radical generation from cobalt (Co) mediated reactions using 5,5-dimethyl-l-pyrroline (DMPO) as a spin trap. A mixture of Co with water in the presence of DMPO generated 5,5-dimethylpyrroline-(2)-oxy(1) DMPOX, indicating the production of strong oxidants. Addition of superoxide dismutase (SOD) to the mixture produced hydroxyl radical (OH). Catalase eliminated the generation of this radical and metal chelators, such as desferoxamine, diethylenetriaminepentaacetic acid or 1,10-phenanthroline, decreased it. Addition of Fe(II) resulted in a several fold increase in the OH generation. UV and O2 consumption measurements showed that the reaction of Co with water consumed molecular oxygen and generated Co(II). Since reaction of Co(II) with H2O2 did not generate any significant amount of OH radicals, a Co(I) mediated Fenton-like reaction [Co(I) + H2O2 → Co(II) + OH + OH] seems responsible for OH generation. H2O2 is produced from O2⋅− via dismutation. O2⋅− is produced by one-electron reduction of molecular oxygen catalyzed by Co. Chelation of Co(II) by biological chelators, such as glutathione or β-ananyl-3-methyl-L-histidine alters, its oxidation–reduction potential and makes Co(II) capable of generating OH via a Co(II)-mediated Fenton-like reaction [Co(II) + H2O2 → Co(III) + OH + OH]. Thus, the reaction of Co with water, especially in the presence of biological chelators, glutathione, glycylglycylhistidine and β-ananyl-3-methyl-L-histidine, is capable of generating a whole spectrum of reactive oxygen species, which may be responsible for Co-induced cell injury.

Introduction

Cobalt (Co) is an essential trace element for mammalian nutrition. It is toxic and carcinogenic at higher concentrations 1, 2, 3, 4. The International Agency for Research on Cancer has classified this metal as possibly carcinogenic to humans [5]. Occupational exposure to hard metals, including Co, causes “hard metal diseases”, such as cancer and asthma [6]. As stated in the National Occupational Research Agenda (NORA), investigation on occupational diseases caused by exposure to metals, such as cobalt, chromium and tungsten, is a NORA mission and appears on the National Toxicology Program priority list [6].

In epidemiological studies, workers exposed to cobalt in an electrochemical plant producing cobalt and workers exposed to cobalt-containing hard metal compounds exhibited significantly enhanced risk for lung cancer 7, 8, 9. In laboratory studies Co(II) caused direct induction of DNA damage 10, 11, DNA-protein crosslinking, and sister-chromatid exchange [1]. Besides the direct induction of DNA damage, Co(II) has been reported to interfere with DNA repair processes 1, 11. Co(II) enhances the frequency of UV induced mutations and sister-chromatid exchanges in V79 Chinese hamster cells [12]. Co(II) compounds have been shown to have a carcinogenic effects in animal studies 1, 13. Cobalt itself causes little or no DNA damage in vitro although Co(II) can bind to DNA [14]. While the mechanisms of cobalt-induced toxicity and carcinogenicity remains to be elucidated, cobalt-mediated free radical reactions have been suggested to be involved 10, 15. Co(II) alone does not efficiently generate hydroxyl radicals (OH) from H2O2. However, in the presence of nitrilotriacetic acid as a chelating agent, Hanna et al. [16]have reported the formation of OH radical from H2O2 by Co(II). Although nitrilotriacetic acid may not be a significant ligand for Co(II) in cells exposed to this cation, this study does suggest that the reactivity of Co(II) toward H2O2 could be enhanced by proper chelation. In a recent study [15], we have shown that Co(II) can generate OH and lipid hydroperoxide-derived free radicals from H2O2 and model lipid hydroperoxides in the presence of biologically relevant ligands, such as glutathione and anserine. The cobalt-mediated radical generation from these reactions was suggested to be involved in the mechanism of Co(II)-related toxicity and carcinogenicity 11, 15.

At present, most of the studies concerning the cobalt-mediated free radical generation have focused on Co(II). There is only limited study of free radical generation mediated by cobalt metal [17]. The goal of the present study is to investigate the possible free radical generation by cobalt metal and elucidate possible mechanisms of generation.

Section snippets

Materials

Cobalt metal powder (trade name, Xtra Fine Cobalt) was obtained from Newcomer Products (Latrobe, PA). The material is used as the metal binder in the industrial production of WC–Co hard metals. Xtra Fine Cobalt was analyzed at 99.3 purity of Co (O2, 0.6%; Ni, 0.1%; C, 0.3%; Na, 0.2%; Si, 0.01%; Ca, 0.01%; Mg, 0.01%; Mn, 0.01%; Cu, 0.01%; Fe, 0.01%; Mg, 0.01%). The range of particle size is 0.1–1.5 μm. Cobalt suspension, instead of solution, is used.

Diethylenetriaminepentaacetic acid (DTPA),

Results

Fig. 1 contains the scanning electron microscope image of cobalt particles. As shown by this figure, the particle sizes are distributed in the range of 0.1–1.5 μm. Fig. 2 shows a typical ESR spectrum obtained from an aqueous solution of 10 mg/ml cobalt particles at pH 7.4. The spectrum is centered at g=2.0065, which indicates oxygen involvement. Computer stimulation shows that the hyperfine splittings are aN=7.1 G and aH=4.2, where aN and aH denote hyperfine splitting constants of nitroxyl

Discussion

The results obtained in the present study show that reaction of metallic Co in aqueous suspension with dissolved oxygen is able to generate a strong oxidant as shown by the formation of DMPOX. According to Rosen and Rauckman [19], the DMPOX signal is indirect evidence for peroxy radical (ROO) generation and its trapping by DMPO. While further studies are required to elucidate the detailed mechanism, the steps outlined in Fig. 7 may best explain the pathways of DMPOX formation.

When SOD was

References (34)

  • D. Beyersmann et al.

    Toxicol. Appl. Pharmacol.

    (1992)
  • A. Hartwig et al.

    Mutation Res.

    (1991)
  • R.A. Floyd et al.

    Biochem. Biophys. Res. Commun.

    (1977)
  • G.R. Buettner

    Free Radical Biol. Med.

    (1987)
  • X. Shi et al.

    Arch. Biochem. Biophys.

    (1990)
  • M. Meyer et al.

    Chem. Biol. Interactions

    (1994)
  • Y. Sun et al.

    Free Radical Biol. Med.

    (1996)
  • M. Sugiyama et al.

    Arch. Biochem. Biophys.

    (1993)
  • B. Halliwell

    Free Radical Biol. Med.

    (1989)
  • R.J. Keller et al.

    J. Inorg. Biochem.

    (1991)
  • J.C. Health

    Nature

    (1954)
  • A.A. Shabaan et al.

    Lab. Anim.

    (1977)
  • G. Kazantzis

    Environ. Health Perspect.

    (1981)
  • International Agency for Research on Cancer, IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol....
  • National Occupational Research agenda, US Department of Health and Human Services, DHHS (NIOSH) Publication No. 96-115,...
  • G. Lasfargues et al.

    Am. J. Ind. Med.

    (1994)
  • C. Hogstedt et al.

    Scand. J. Work Environ. Health

    (1987)
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