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

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 O 2 consumption measurements showed that the reaction of Co with water consumed molecular oxygen and generated Co(II). Since reaction of Co(II) with H 2O 2 did not generate any significant amount of ⋅OH radicals, a Co(I) mediated Fenton-like reaction [Co(I) + H 2O 2 → Co(II) + ⋅OH + OH −] seems responsible for ⋅OH generation. H 2O 2 is produced from O 2 ⋅− via dismutation. O 2 ⋅− 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) + H 2O 2 → 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.