Superoxide reaction with nitroxide spin-adducts

The reactions of superoxide radical with persistent nitroxide spin-adducts or with stable spin-labels were studied using ESR spectrometry. Superoxide radicals were produced enzymatically using xanthine - xanthine oxidase or chemically by dissolving potassium superoxide in DMSO. Hydroxyl and methyl spin-adducts of the spin-trap DMPO were preformed by sonolysis and subsequently reacted with superoxide radical. Superoxide-induced depletion of DMPO-OH obeyed second order kinetics. Contrary to previously published mechanisms, the reaction requires neither transition metal ions nor thiols. The depleted spin-adducts could not be restored by reoxidation with ferricyanide or copper +H₂O₂; thus, the superoxide-mediated destruction does not result in a mere one-electron reduction product. Superoxide also depletes other DMPO spin-adducts including DMPO-CH₃ and DMPO-H, but not PBN-CH₃. In addition, some 5-membered ring stable nitroxides are depleted by superoxide in a pseudo-zero order reaction. In studying systems which generate O₂ ^- and OH, the superoxide-induced destruction of DMPO-OH may well lead to erroneous conclusions regarding the primary radicals produced. In particular this reaction might be operative under circumstances where elevated rates of superoxide production take place, such as during oxygen consumption "burst" in phagocytosis, degranulation, or paraquat intoxication.