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

Electron spin resonance spin trapping was utilized to investigate free radical generation from cobalt (Co) mediated reactions using 5,5-dimethyl-1- 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.