Biologically active metal-independent superoxide dismutase mimics

Abstract

Superoxide dismutase (SOD) is an enzyme that detoxifies superoxide (O2.-), a potentially toxic oxygen-derived species. Attempts to increase intracellular concentrations of SOD by direct application are complicated because SOD, being a relatively large molecule, does not readily cross cell membranes. We have identified a set of stable nitroxides that possess SOD-like activity, have the advantage of being low molecular weight, membrane permeable, and metal independent, and at pH 7.0 have reaction rate constants with O2.- ranging from 1.1 x 10(3) to 1.3 x 10(6) M-1 s-1. These SOD mimics protect mammalian cells from damage induced by hypoxanthine/xanthine oxidase and H2O2, although they exhibit no catalase-like activity. In addition, the nitroxide SOD mimics rapidly oxidize DNA-FeII and thus may interrupt the Fenton reaction and prevent formation of deleterious OH radicals and/or higher oxidation states of metal ions. Whether by SOD-like activity and/or interception of an electron from redox-active metal ions they protect cells from oxidative stress and may have use in basic and applied biological studies.

The chemistry and biology of nitroxide compounds

“…While the SOD mimetic activity of nitroxides was recognized in the initial studies, the multiple modes by which this class of agents provides protection was first reported in 1990 [13]. In this study, several nitroxides and their corresponding hydroxylamines were tested for protective effects in Chinese hamster V79 lung fibroblasts exposed to hypoxanthine/ xanthine oxidase under aerobic conditions.…”

“…Nitroxides can prevent generation of • OH radicals from Fenton reactions by accepting the electron from the reduced metal complex as shown in equation 6c [13].…”

“…These results suggest additional modes of protection by nitroxides other than SOD-mimetic activity. To evaluate this, several nitroxides were tested as protectors in V79 cells exposed to bolus concentrations of H 2 O 2 in the range 0-500 uM for 1 hour [13]. In the absence of protective agents, a H 2 O 2 dose-dependent cytotoxicity was observed.…”

Inhibition of in vitro lipid peroxidation by stable steroidic nitroxyl radicals

“…The cholestane and cholic acid nuclei were chosen as representatives of less (cholestane) and more polar (cholic acid) steroidic moieties. The IK-1/IK-2 activity was compared with that of known antioxidants: TEMPO, chosen for its ability to react mainly with primary radicals, (Mitchell et al, 1990;Voest et al, 1994;Nilsson et al, 1989) and h-tocopherol for its chain-breaking antioxidant capability (Niki, 1987;Halliwell et al, 1995).…”

“…Some of these have shown that simple nitroxyl radicals, previously used as spin labels for electron spin resonance spectroscopy, are able to inhibit lipid peroxidation in various ways. In fact these nitroxyl radicals react with, and dismutate, superoxide radical anions (O 2 − ) in a catalytic fashion that mimics that of superoxide dismutase (Mitchell et al, 1990;Samuni et al, 1991), competing with H 2 O 2 in the oxidation of Fe 2 + ions (Voest et al, 1994) and preventing hydroxyl radical (OH ) formation (Gelvan et al, 1991). However some of these simple nitroxyl radicals show, together with these favourable effects, undesirable properties like negative chronotropy and anaesthetic effects that prevent their use in pharmacology (Gelvan et al, 1991).…”