Site-activity relationship of nitroxide radical's antioxidative effect

A relatively new strategy in preventing oxidative damage employs cyclic nitroxides. These stable radicals have been widely used as biophysical probes, spin labels, and are currently tested as contrast agents for nuclear magnetic resonance imaging. Nitroxides were found to protect cells, organs, and whole animals against diverse oxidative insults. The present study concentrated on comparing the antioxidative activity of nitroxides against oxidative damage, initiated either in the lipid or aqueous phase, to egg phosphatidylcholine acyl chains (13.4% polyunsaturated fatty acids) in small unilamellar vesicles. We determined the lipophilicity and liposome-membrane/aqueous-medium partition coefficient for several nitroxides and compared their specific protective effects. The aim was to study the relation between nitroxides' concentration, location in the lipid bilayer, and their protection against oxidative damage. Both 6-membered- and 5-membered-ring nitroxides were studied for: (i) partitioning between the lipid bilayer and the aqueous phase (nitroxides were quantified using EPR spectroscopy); (ii) the intrabilayer distribution, using three different fluorescent probes of known location of their fluorophors in the lipid bilayer; and (iii) the specific antioxidative effect (protection per concentration) against radicals formed in a liposomal dispersion. The radicals were generated using the thermolabile, radical-generating compounds 2,2′-azobis (2-amidinopropane) dihydrochloride (AAPH) in the aqueous phase, and 2,2′-azobis (2,4-dimethyl-valeronitrile) (AMVN) in the lipid phase. The results show that nitroxides react, in a concentration-dependent manner, with deleterious species at their formation sites, both in the aqueous and the lipid phase, and that their specific protective effects for the lipophilic target, the lipid bilayer, are similar for both the lipophilic and the hydrophilic nitroxides.