DO NITROXIDE ANTIOXIDANTS ACT AS SCAVENGERS OF O-2(.-) OR AS SOD MIMICS

Stable nitroxide radicals were reported to act as SOD mimics and catal yze the dismutation of O-2-radical-anion through two different catalyt ic pathways including reducive and oxidative reaction mechanisms (Samu ni, 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 act ivity of nitroxides (Weiss, R. H., Flickinger, A. G., Rivers, W. J., H ardy, 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 di stinction of stoichiometric scavengers from catalytic detoxifiers of O -2-radical-anion is not readily feasible. Nitroxides are effective ant ioxidants that protect against oxidative injury in various pathologica l processes. The distinction of their SOD mimic activity from O-2-radi cal-anion scavenging was established by examining the validity of dire ct 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-r adical-anion dismutation, determined for the nitroxides tested, were f ound 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 vali d discrimination of SOD mimics from stoichiometric scavengers of O-2-r adical-anion.