Do nitroxide antioxidants act as scavengers of O-2(.-) or as SOD mimics?

Stable nitroxide radicals were reported to act as SOD mimics and catalyze the dismutation of O-2-radical-anion through two different catalytic pathways including reducive and oxidative reaction mechanisms (Samuni, A., Krishna, C. M., Riesz, P., Finkelstein, E, & Russo, A. (1988) J. Biol Chem. 263, 17921-17924). Recent studies directly monitoring O-2-radical-anion and employing kinetics analysis did not reveal SOD activity of nitroxides (Weiss, R. H., Flickinger, A. G., Rivers, W. J., Hardy, M. M., Aston, K. W., Ryan, U. S. & Riley, D. P. (1993) J. Biol. Chen. 268, 23049-23054). Such discrepancy may result in cases where distinction of stoichiometric scavengers from catalytic detoxifiers of O-2-radical-anion is not readily feasible. Nitroxides are effective antioxidants that protect against oxidative injury in various pathological processes. The distinction of their SOD mimic activity from O-2-radical-anion scavenging was established by examining the validity of direct and indirect methods employed to assay SOD-like catalytic activity. Kinetics analysis along with direct EPR monitoring were used to study the mechanism underlying nitroxide reactions with O-2-radical-anion. The nitroxide EPR signal decayed in the presence of NADH but otherwise did not decrease with time, thus substantiating its catalytic role in O-2-radical-anion dismutation. The catalytic rate constants for O-2-radical-anion dismutation, determined for the nitroxides tested, were found to increase with [H+], indicating that (OOH)-O-. rather than O-2-radical-anion is oxidizing the nitroxide. The results demonstrate the limitations associated with direct kinetics analysis in evaluating SOD mimic activity, underscoring the need for independent assays for valid discrimination of SOD mimics from stoichiometric scavengers of O-2-radical-anion.