Structure - Activity relationship of cyclic nitroxides as SOD mimics and scavengers of nitrogen dioxide and carbonate radicals

Synthetic nitroxide antioxidants attenuate oxidative damage in various experimental models. Their protective effect reportedly depends on ring size and ring substituents and is greater for nitroxides having lower oxidation potential. The present study focuses on the kinetics and mechanisms of the reactions of piperidine, pyrrolidine and oxazolidine nitroxides with HO ₂^./O2^.-, ^.NO₂ and CO ₃^.- radicals, which are key intermediates in many inflammatory and degenerative diseases. It is demonstrated that nitroxides are the most efficient scavengers of ^.NO₂ at physiological pH (k = (3-9) × 10⁸ M^-1 s^-1) and among the most effective metal-independent scavengers of CO₃^.- radicals (k = (2 - 6) × 10⁸ M^-1 s^-1)- Their reactivity toward HO₂^., though not toward ^.NU₂ and CO₃^.-, depends on the nature of the ring side-chain and particularly on the ring-size. All nitroxide derivatives react slowly with O₂^.- and are relatively inefficient SOD mimics at physiological pH. Even piperidine nitroxides, having the highest SOD-like activity, demonstrate a catalytic activity of about 1000fold lower than that of native SOD at pH 7.4. The present results do not indicate any correlation between the kinetics of HO₂ ^./O₂^.-, NO₂ and CO₃ ^.- removal by nitroxides and their protective activity against biological oxidative stress and emphasize the importance of target-oriented nitroxides, i.e., interaction between the biological target and specific nitroxides.