FIRST ELECTRON-SPIN-RESONANCE EVIDENCE FOR THE PRODUCTION OF SEMIQUINONE AND OXYGEN-FREE RADICALS FROM ORELLANINE, A MUSHROOM NEPHROTOXIN

Orellanine is the tetrahydroxylated and di-N-oxidized bipyridine toxin from several Cortinarius mushrooms. The mechanism responsible for its lethal nephrotoxicity was unknown until now. Our present ESR spectros copic study of the redox properties of the toxin is an original contri bution to the knowledge of its toxicity. It was achieved in particular by comparison of the properties of orellanine to that of other bipyri dine compounds. After a one-electron oxidation (e.g., photochemical ox idation upon visible light), a radical form of orellanine is observed at physiological pH under aerobic or anaerobic conditions. This radica l, identified as ortho-semiquinone anion radical, can also be generate d by oxidation with biological oxidizing agents or enzymatic systems. Production of superoxide and hydroxyl radicals is shown by the spin-tr apping method using DMPO as a spin trap. Bioreducing agents like GSH a nd cysteine involve in vitro the semiquinone radical and orellanine in a redox cycling process resulting in the production of glutathionyl a nd oxygen free radicals. This process leads in vitro to a large oxygen consumption and to a dramatic depletion of glutathione level. The for mation of an apparently stable ortho-semiquinone anion radical and of reactive oxygen radical species is observed for the first time with a mushroom toxin. It is due to the catechol-like functions borne by the di-N-oxidized bipyridine structure of the toxin and may at least partl y explain the toxicity of orellanine.