Reflections on the Theories of Aging, of Oxidative Stress, and of Science in General. Is It Time to Abandon the Free Radical (Oxidative Stress) Theory of Aging?

SIGNIFICANCE

Aging and oxidative stress are complex phenomena, and their understanding is of enormous theoretical and practical significance.

RECENT ADVANCES

Numerous hypotheses and theories that attempt to explain these phenomena have been developed. These hypotheses and theories compete with each other, with each claiming to be the correct one, while significantly contradicting each other.

CRITICAL ISSUES

It is important to develop a maximally correct theory that may then trigger significant practical breakthroughs.

FUTURE DIRECTIONS

None of these theories is entirely correct or close enough to the truth. However, most of them contain many correct elements (CE). Finding these CE is possible by analysis of these theories. Once the CE are found, they can be merged by synthesis in a better new theory. An analysis of some of the theories of aging followed by synthesis is attempted.

Free radicals: how do we stand them? Anaerobic and aerobic free radical (chain) reactions involved in the use of fluorogenic probes and in biological systems

Biologically significant conclusions have been based on the use of fluorogenic and luminogenic probes for the detection of reactive species. The basic mechanisms of the processes involved have not been satisfactorily elucidated. In the present work, the mechanism of the enzyme and photosensitized oxidation of NAD(P)H by resorufin is analyzed and appears to involve both aerobic and anaerobic free radical chain reactions. There are two major fallouts of this analysis. Many of the conclusions about the participation of radicals based on the use of probes such as resorufin and Amplex red need reevaluation. It is also concluded that anaerobic free radical reactions may be biologically significant, and the possible existence of enzymatic systems to eliminate certain free radicals is discussed.

Free radical paradoxes

Unlike bigger and more advanced animals, Caenorhabditis elegans does not generate NO, yet it was recently found that NO produced by chemical or biological sources exerts profound effects in that animal, leading to increased life span and thermotolerance. The biological source was Bacillus subtilis, a natural food for C. elegans. Yet once in the cell, NO might react with superoxide, leading to the production of the potentially toxic peroxynitrite. In this paper, a number of paradoxes that are involved in that situation are discussed. It is also argued that their solution might lead to a sizeable advancement of our knowledge of what constitutes oxidative stress and what role oxidative stress plays in the development of pathological processes and aging.

Superoxide dismutase mimics, other mimics, antioxidants, prooxidants, and related matters

A significant number of low molecular weight metal complexes as well as metal-free compounds that are capable of scavenging superoxide and/or other radicals and reactive species in simple systems have been proposed to be used as potential drugs in the case of various diseases and/or as antiaging agents. Some have been used or suggested to be used as diagnostic tools for the involvement of such species in biological processes. In the present work, analysis of such claims indicates that their use as specific detectors of superoxide or of other reactive oxygen species is unsupported and might be confusing. Many of these compounds exert beneficial effects by counteracting the toxic effects of oxidative stress in a significant number of models of pathological processes. However, it is concluded that these actions are more likely due to other effects including prooxidant actions and that their beneficial effects also may be exerted in pathological processes that do not practically involve reactive oxygen species. Adaptation may be a common mode of action explaining a sizable portion of the beneficial effect of the so-called mimics and other compounds including prooxidants.

Reactive oxygen species and the free radical theory of aging

The traditional view in the field of free radical biology is that free radicals and reactive oxygen species (ROS) are toxic, mostly owing to direct damage of sensitive and biologically significant targets, and are thus a major cause of oxidative stress; that complex enzymatic and nonenzymatic systems act in concert to counteract this toxicity; and that a major protective role is played by the phenomenon of adaptation. Another part of the traditional view is that the process of aging is at least partly due to accumulated damage done by these harmful species. However, recent workers in this and in related fields are exploring the view that superoxide radical and reactive oxygen species exert beneficial effects. Thus, such ROS are viewed as involved in cellular regulation by acting as (redox) signals, and their harmful effects are seen mostly as a result of compromised signaling, rather than due to direct damage to sensitive targets. According to some followers of this view, ROS such as hydrogen peroxide and superoxide are not just causative agents of aging but may also be agents that increase the life span by acting, for example, as prosurvival signals. The goal of this review is to recall that many of the effects of ROS that are interpreted as beneficial may actually represent adaptations to toxicity and that some of the most extravagant recent claims may be due to misinterpretation, oversimplification, and ignoring the wealth of knowledge supporting the traditional view. Whether it is time to abandon the free radical (oxidative stress) theory of aging is considered.

Mechanism of the peroxidase activity of Cu, Zn superoxide dismutase

In addition to its very efficient catalysis of the dismutation of superoxide ( O(2)(-) ) into O(2) plus H(2)O(2), Cu, Zn SOD acts less efficiently as a non-specific peroxidase. This peroxidase activity is CO(2) dependent although very slow peroxidation of some substrates occurs in the absence of CO2. The mechanism of that CO(2) dependence is explained by the generation of a strong oxidant at the copper site by two sequential reactions with H(2)O(2), followed by the oxidation of CO(2) to the carbonate radical that then diffuses into the bulk solution. This diffusible carbonate radical is then responsible for the diverse oxidations that have been reported. A different mechanism that involves the reduction of peroxymonocarbonate by the reduced superoxide dismutase to yield carbonate radical has been proposed. We will demonstrate that this mechanism is not supported by the available data. It seems likely that generation of the carbonate radical has relevance to the oxidative stress faced by aerobic organisms.

The effects of superoxide dismutase on H2O2 formation

Numerous reports of the effects of overproduction of SODs have been explained on the basis of increased H2O2 production by the catalyzed dismutation of O2-. In this review we consider the effects of increasing [SOD] on H2O2 formation and question this explanation.

Inactivation of copper, zinc superoxide dismutase by H2O2 : mechanism of protection

Cu,Zn SOD is known to be inactivated by HO(2)(-) and to be protected against that inactivation by a number of small molecules including formate, imidazole, and urate. This inactivation has been shown to be due to oxidation of a ligand field histidine residue by a bound oxidant formed by reaction of the active site Cu(II) with HO(2)(-). We now report that protective actions of both formate and NADH increase as the pH was raised in the range 8.0-9.5. This is taken to indicate increased accessibility of the Cu site with rising pH and/or increased reactivity of the bound oxidant toward exogeneous substrates at high pH. Formate appears to act as a sacrificial substrate that protects by competing with the endogenous histidine residue for reaction with the bound oxidant, or that repairs the damage by reducing the histidyl radical intermediate. The same is likely also true of NADH.

The role of CO2 in metal-catalyzed peroxidations

Transition metals, such as Cu(+2), Mn(+2), and Co(+2), have been seen to catalyze the bicarbonate enhanced oxidation of a variety of substrates by H(2)O(2). In several of these cases it has been demonstrated that CO(2), rather than bicarbonate, is the enhancing species. Mechanisms that are in accord with the data involve a hypervalent state that may be written (MO)(+n), or (MOH)(+n+1), or (M)(+n+2). This metal centered oxidant then oxidizes CO(2) to the carbonate radical; that is then the proximal oxidant of the various substrates. Whether a similar process has in vivo reality remains to be demonstrated.

The role of CO2 in cobalt-catalyzed peroxidations

Augmentation, by CO(2)/HCO(3)(-), of Co(II)-catalyzed peroxidations was explored to clarify whether the rate enhancement was due to CO(2) or to HCO(3)(-). The rate of oxidation of NADH by Co(II) plus H(2)O(2), in Tris or phosphate, was markedly enhanced by CO(2)/HCO(3)(-). Phosphate was seen to inhibit the Co(II)-catalyzed peroxidation, probably due to its sequestration of the Co(II). When CO(2) was used, there was an initial burst of NADH oxidation followed by a slower linear rate. The presence of carbonic anhydrase eliminated this initial burst; establishing that CO(2) rather than HCO(3)(-) was the species responsible for the observed rate enhancements. Both kinetic and spectral data indicated that Co(II) was converted by H(2)O(2) into a less active form from which Co(II) could be regenerated. This less active form absorbed in both the UV and visible regions, and is assumed to be a peroxy bridged binuclear complex. The rate of formation of this absorbing form was increased by HCO(3)(-)/CO(2). A minimal mechanism consistent with these observations is proposed.

Cross-compartment protection by SOD1

The absence of SOD1 in yeast has been found to result in inactivation of Lys4p. This [4Fe-4S]-containing dehydratase is in the pathway of biosynthesis of lysine, hence the oxygen-dependent lysine auxotrophy seen in this case. O(2)(-) is known to oxidize and thus destabilize the [Fe-4S] clusters of dehydratases; hence, this would make perfect sense were it not for the fact that SOD1 localizes to the cytosol and the intermembrane space of mitochondria, whereas Lys4p localizes to the mitochondrial matrix. How could SOD1 in one compartment protect against O(2)(-) attack in a different compartment? We suggest that the relatively high levels of O(2)(-) in the cytosol and intermembrane space of the SOD1 mutant may react with endogenous NO, forming HOONO that can diffuse into the mitochondrial matrix and there inactivate Lys4p and other [4Fe-4S]-containing dehydratases.

Carbon dioxide mediates Mn(II)-catalyzed decomposition of hydrogen peroxide and peroxidation reactions

Mn(II) can catalyze the decomposition of H(2)O(2) and, in the presence of H(2)O(2), can catalyze the oxidation of NADH. Strikingly, these processes depend on the simultaneous presence of both CO(2) and HCO(3)(-). This explains the exponential dependence of the rates on [HCO(3)(-)], previously noted by other workers. These processes are inhibited by Mn-superoxide dismutase, establishing the generation of O(2)(-) and its role as an essential reactant. A scheme of reactions, consistent with the known properties of this system, is proposed. The large rate enhancements provided by HCO(3)(-) + CO(2), and the abundance of both of these species in vivo, suggest that similar reactions have relevance to the oxidative stress imposed by O(2)(-) and H(2)O(2).

CO2 enhanced peroxidase activity of SOD1: the effects of pH

At pH 7.4, CO2, rather than HCO3-, markedly enhances the oxidation of diverse substrates by SOD1 plus H2O2. Since the concentration of CO2 would fall with rising pH in HCO3- buffers, it was of interest to explore the effects of pH on the peroxidase activity of SOD1 in the presence and in the absence of HCO3-. The rate of NADPH peroxidation in the HCO3- buffer was minimally affected by pH in the range of 8-10.5; in a pyrophosphate buffer, the rate increased markedly, such that at pH 10.5 the rates in the two buffers were nearly identical. Similar results were obtained when urate was used as the peroxidizeable substrate. These results are explicable on the basis of an increase in the rate with pH due to the ionization of H2O2 to the effective HO2- coupled with a decrease in [CO2] due to the ionizations of H2CO3, which displaces the hydration equilibrium to the right. These two opposing effects counteract in the HCO3(-)-buffered reaction mixtures; in the pyrophosphate buffer, only the effect of increasing [H02-] was seen.

CO2, not HCO3-, facilitates oxidations by Cu,Zn superoxide dismutase plus H2O2

The Cu,Zn superoxide dismutase catalyzes HCO(3)(-) -dependent oxidations by H(2)O(2). This activity has been shown to depend on the creation of a bound oxidant at the Cu(II) by interactions with H(2)O(2). The bound oxidant was then thought to oxidize HCO(3)(-) to CO(3)(.-), which diffuses into the bulk solution and there oxidizes diverse substrates. We now find that CO(2) rather than HCO(3)(-) facilitates the peroxidations catalyzed by Cu,Zn superoxide dismutase. This fact was shown by a lag in the rate of peroxidation of NADPH when NaHCO(3)(-) was added last and by a burst in the rate when aqueous CO(2) was added last. Both the lag and the burst were eliminated by carbonic anhydrase.

Bicarbonate-enhanced peroxidase activity of Cu, Zn SOD: is the distal oxidant bound or diffusible?

The Cu,Zn SOD catalyzes the bicarbonate-dependent oxidation of a wide range of substrates by H2O2. A mechanism in accord with this activity has been described. It involves the generation of a strong oxidant (Cu(I)O, Cu(II)OH, or Cu(III)) by reaction of the active site Cu with H2O2, followed by oxidation of bicarbonate to CO3-* that in turn diffuses from the active site to oxidize the various substrates in free solution. Recently, an alternative mechanism, entailing firmly bound HCO3- and CO3-*, has been proposed [J. Biol. Chem. 278 (2003) 21032-21039]. We present data supporting the diffusible CO3-* and discuss the properties of this system that can be accommodated in this way and that preclude bound intermediates.

Mutant Cu,Zn superoxide dismutases and familial amyotrophic lateral sclerosis: evaluation of oxidative hypotheses

FALS-associated missense mutations of SOD1 exhibit a toxic gain of function that leads to the death of motor neurons. The explanations for this toxicity fall into two broad categories. One involves a gain of some oxidative activity, while the second involves a gain of protein: protein interactions. Among the postulated oxidative activities are the following: (i) peroxidase action; (ii) superoxide reductase action; and, (iii) the enhancement of production of O2- by partial reversal of the normal SOD activity, which then leads to increased formation of ONOO(-). We will herein concentrate on evaluating the relative merits of these oxidative hypotheses and consider whether the experiments with transgenic animals that purport to disprove these oxidative explanations really do so.

The mode of decomposition of Angeli's salt (Na2N2O3) and the effects thereon of oxygen, nitrite, superoxide dismutase, and glutathione

The classical view of the aerobic decomposition of Angeli's salt is that it releases NO(2)(-) + NO(-)/HNO the latter then reacting with O(2) to yield ONOO(-). An alternative that has recently been proposed envisions electron transfer to O(2) followed by decomposition to NO(2)(-) + NO. The classical view is now strongly supported by the observation that the rates of decomposition of Angeli's salt under 20% O(2) or 100% O(2) were equal. Moreover, NO(2)(-), which inhibits this decomposition by favoring the back reaction, was more effective in the absence of agents that scavenge NO(-)/HNO. It is thus clear that Angeli's salt is a useful source of NO(-)/HNO for use in defined aqueous systems. The measurements made in the course of this work allowed approximation of the rate constants for the reactions of NO(-)/HNO with NO(2)(-), O(2), glutathione, or Cu, Zn superoxide dismutase. The likelihood of the formation of NO(-)/HNO in vivo is also discussed.

Reversal of the superoxide dismutase reaction revisited

Reversal of the superoxide dismutase (SOD) reaction was measured in terms of the reduction of tetranitromethane (TNM) by O2-. Cu,ZnSOD caused a biphasic reduction of TNM by H2O2. The rapid initial phase was stoichiometric with the enzyme and was followed by a slower catalytic phase that was oxygen dependent and was augmented by HCO3-. The reaction scheme explaining this behavior is presented and a rate constant for the reduction of O2 by the cuprous enzyme is estimated. This rate constant is so low that it precludes significant O2- production by the reduced enzyme under the conditions explored.

The Haber-Weiss cycle -- 70 years later: an alternative view

In a recent review published in this journal,(1) Koppenol traced the history of the Fenton reaction and of the catalytic decomposition of H(2)O(2) by iron salts. If his purpose was to shed light on current understanding of related chemistry in biological systems, he failed. Moreover, he managed to sow confusion by inaccurate reporting of the work of others. What follows is an attempt to point out these shortcomings and thus to clarify the situation.

Copper, zinc superoxide dismutase and H2O2. Effects of bicarbonate on inactivation and oxidations of NADPH and urate, and on consumption of H2O2

Copper,zinc superoxide dismutase (Cu,Zn-SOD) catalyzes the HCO(3)(-)-dependent oxidation of diverse substrates. The mechanism of these oxidations involves the generation of a strong oxidant, derived from H(2)O(2), at the active site copper. This bound oxidant then oxidizes HCO(3)(-) to a strong and diffusible oxidant, presumably the carbonate anion radical that leaves the active site and then oxidizes the diverse substrates. Cu,Zn-SOD is also subject to inactivation by H(2)O(2). It is now demonstrated that the rates of HCO(3)(-)-dependent oxidations of NADPH and urate exceed the rate of inactivation of the enzyme by approximately 100-fold. Cu,Zn-SOD is also seen to catalyze a HCO(3)(-)-dependent consumption of the H(2)O(2) and that HCO(3)(-) does not protect Cu,Zn-SOD against inactivation by H(2)O(2). A scheme of reactions is offered in explanation of these observations.

Superoxide and nitric oxide: consequences of varying rates of production and consumption: a theoretical treatment

Simple algebraic manipulations and steady state assumptions have been applied to the elucidation of the effects of superoxide dismutase (SOD) on [NO] and [O(2)(-)], and on the rates of production of H(2)O(2) and of ONOO(-). The deductions help explain seemingly discordant published results and predictions.

Nitroxyl (NO-): a substrate for superoxide dismutase

The interactions of Cu, Zn superoxide dismutase (SOD) with nitroxyl (NO-) and nitric oxide (NO), both of which are thought to be biologically significant, have been studied but remain undefined. Having previously noted that NO- can reduce Cu (II), Zn SOD aerobically, we now report that it also can do so anaerobically and that Cu, Zn SOD can catalyze the elimination of NO(-) in the absence of O2.NO- acts as a reductant of ferricytochrome c anaerobically, but in the presence of O2 causes the oxidation of ferrocytochrome c and NADPH. Equivalent fluxes of NO-, and NO + O2- were able to comparably oxidize NADPH, but the oxidation by NO + O2- was more than fivefold more sensitive to inhibition by Cu, Zn SOD than was the oxidation by NO-. Thus Cu, Zn SOD inhibited NADPH oxidation by NO- by a route independent of catalyzing the dismutation of O2. Plausible mechanisms for those observations are offered and rate constants are estimated.

Copper,zinc superoxide dismutase as a univalent NO(-) oxidoreductase and as a dichlorofluorescin peroxidase

Nitroxyl (NO(-)) may be produced by nitric-oxide synthase and by the reduction of NO by reduced Cu,Zn-SOD. The ability of NO(-) to cause oxidations and of SOD to inhibit such oxidations was therefore explored. The decomposition of Angeli's salt (AS) produces NO(-) and that in turn caused the aerobic oxidation of NADPH, directly or indirectly. O(2) was produced concomitant with the aerobic oxidation of NADPH by AS, as evidenced by the SOD-inhibitable reduction of cytochrome c. Both Cu,Zn-SOD and Mn-SOD inhibited the aerobic oxidation of NADPH by AS, but the amounts required were approximately 100-fold greater than those needed to inhibit the reduction of cytochrome c. This inhibition was not due to a nonspecific protein effect or to an effect of those large amounts of the SODs on the rate of decomposition of AS. NO(-) caused the reduction of the Cu(II) of Cu,Zn-SOD, and in the presence of O(2), SOD could catalyze the oxidation of NO(-) to NO. The reverse reaction, i.e. the reduction of NO to NO(-) by Cu(I),Zn-SOD, followed by the reaction of NO(-) with O(2) would yield ONOO(-) and that could explain the oxidation of dichlorofluorescin (DCF) by Cu(I),Zn-SOD plus NO. Cu,Zn-SOD plus H(2)O(2) caused the HCO(3)(-)-dependent oxidation of DCF, casting doubt on the validity of using DCF oxidation as a reliable measure of intracellular H(2)O(2) production.

2-methoxyestradiol does not inhibit superoxide dismutase

It has been reported in the literature that the endogenous estrogen metabolite 2-methoxyestradiol (2-ME) inhibits both manganese and copper,zinc superoxide dismutases (Mn and Cu,Zn SODs) and that this mechanism is responsible for 2-ME's ability to kill cancer cells. In fact, as demonstrated using several SOD assays including pulse radiolysis, 2-ME does not inhibit SOD but rather interferes with the SOD assay originally used. Nevertheless, as confirmed by aconitase inactivation measurements and lactate dehydrogenase release in human leukemia HL-60 cells, 2-ME does increase superoxide production in these cells and is more toxic than its non-O-methylated precursor 2-hydroxyestradiol. Other mechanisms previously suggested in the literature may explain 2-ME's ability to increase intracellular superoxide levels in tumor cells.

The Oxidation of 3-Hydroxyanthranilic Acid by Cu,Zn Superoxide Dismutase: Mechanism and Possible Consequences

Cu,Zn SOD, but not Mn SOD, catalyzes the oxidation of 3-hydroxyanthranilic acid (3-HA) under aerobic conditions. In the absence of O2, the Cu(II) of the enzyme is reduced by 3-HA. One plausible mechanism involves the reduction of the active site Cu(II) to Cu(I), which is then reoxidized by the O2- generated by autoxidation of the anthranilyl or other radicals on the pathway to cinnabarinate. We may call this the superoxide reductase, or SOR, mechanism. Another possibility invokes direct reoxidation of the active site Cu(I) by the anthranilyl, or other organic radicals, or by the peroxyl radicals generated by addition of O2 to these organic radicals. Such oxidations catalyzed by Cu,Zn SOD could account for the deleterious effects of the mutant Cu,Zn SODs associated with familial amyotrophic lateral sclerosis and of the overproduction or overadministration of wild-type Cu,Zn SOD.

Copper- and zinc-containing superoxide dismutase can act as a superoxide reductase and a superoxide oxidase

The copper- and zinc-containing superoxide dismutase can catalyze the oxidation of ferrocyanide by O(2) as well as the reduction of ferricyanide by O(2). Thus, it can act as a superoxide dismutase (SOD), a superoxide reductase (SOR), and a superoxide oxidase (SOO). The human manganese-containing SOD does not exert SOR or SOO activities with ferrocyanide or ferricyanide as the redox partners. It is possible that some biological reductants can take the place of ferrocyanide and can also interact with human manganese-containing superoxide dismutase, thus making the SOR activity a reality for both SODs. The consequences of this possibility vis à vis H(2)O(2) production, the overproduction of SODs, and the role of copper- and zinc-containing superoxide dismutase mutations in causing familial amyotrophic lateral sclerosis are discussed, as well as the likelihood that the biologically effective SOD mimics, as described to date, actually function as SORs.

Lucigenin: redox potential in aqueous media and redox cycling with O-(2) production

The use of lucigenin luminescence as a measure of ¿O-(2) has been questioned because lucigenin has been shown to be capable of mediating the production of O-(2). This being the case, lucigenin can signal the presence of O-(2) even in systems not producing it in the absence of lucigenin. The reduction potential of lucigenin should be in accord with its ability to mediate O-(2) production; but it has not heretofore been measured in aqueous media. The problems facing such measurement are the insolubility of the divalently reduced form, which deposits on the electrode, and the slow conformational transition that follows the second electron transfer and which interferes with reversibility. We have now used rapid scan cyclic voltammetry to determine that the reduction potential for lucigenin is -0.14 +/- 0.02 V versus the normal hydrogen electrode. This value applies to both the first and the second electron transfers to lucigenin and it is in accord with the facile mediation of O-(2) production by this compound.

On the role of bicarbonate in peroxidations catalyzed by Cu,Zn superoxide dismutase

The interaction of Cu,ZnSOD with H2O2 generates an oxidant at the active site that can then cause either the inactivation of this enzyme or the oxidation of a variety of exogenous substrates. We show that the rate of inactivation, imposed by 10-mM H2O2 at 25 degrees C and pH 7.2, is not influenced by 10-mM HCO3-; whereas the oxidation of 2,2'-azino-bis-[3-ethylbenzothiazoline sulfonate] (ABTS=) is virtually completely dependent upon HCO3-. The reduction of the active site Cu(II) by H2O2, which precedes inactivation of the enzyme, occurred at the same rate in phosphate buffer with or without bicarbonate added. These results indicate that HCO3- does not play a role in facilitating the interaction of H2O2 with the active site copper, but they can be accommodated by the proposal that HCO3- is oxidized to HCO3*, which then diffuses from that site and causes the oxidation of substrates, such as ABTS=, that are too large to traverse the solvent access channel to the Cu(II).

Superoxide and iron: partners in crime

Superoxide (O2-) poses multiple threats, which are diminished by a family of metalloenzymes, the superoxide dismutases. Among the damaging effects of O2- are direct oxidation of low-molecular-weight reductants; inactivation of a select group of enzymes; and reaction with NO to yield the strong oxidant, peroxynitrite. Of even greater import is the ability of O2- to univalently oxidize the [4 Fe-4 S] clusters of dehydratases, which causes release of iron. The "free" iron, which is kept reduced by cellular reductants, then reduces hydroperoxides to hydroxyl or alkoxyl radicals. Because the "free" iron will preferentially bind to anionic polymers, such as nucleic acids, or to anionic surfaces, such as cell membranes, these radicals will be generated adjacent to these vital targets and will preferentially attack them. O2- and iron can thus be viewed as partners in crime, and reciprocal regulatory effects between iron and O2- may be anticipated. These are discussed.

Induction of the soxRS regulon of Escherichia coli by superoxide

The soxRS regulon orchestrates a multifaceted defense against oxidative stress, by inducing the transcription of approximately 15 genes. The induction of this regulon by redox agents, known to mediate O-2 production, led to the view that O-2 is one signal to which it responds. However, redox cycling agents deplete cellular reductants while producing O-2, and one may question whether the regulon responds to the depletion of some cytoplasmic reductant or to O-2, or both. We demonstrate that raising [O-2] by mutational deletion of superoxide dismutases and/or by addition of paraquat, both under aerobic conditions, causes induction of a member of the soxRS regulon and that a mutational defect in soxRS eliminates that induction. This establishes that O-2, directly or indirectly, can cause induction of this defensive regulon.

Nitroreductase A is regulated as a member of the soxRS regulon of Escherichia coli

Nitroreductase A catalyzes the divalent reduction of nitro compounds, quinones, and dyes by NADPH. In this paper, nitroreductase A is induced in Escherichia coli by exposure to paraquat in a manner that depends on the expression of soxR. Nitroreductase activity was only slightly induced by paraquat in a strain bearing a mutational defect in the gene encoding nitroreductase A, but it was approximately 3-fold induced in the parental strain. Nitroreductase A thus appears to be a member of the soxRS regulon and probably contributes to the defenses against oxidative stress by minimizing the redox cycling attendant upon the univalent reduction of nitro compounds, quinones, and dyes.

Lucigenin as mediator of superoxide production: revisited

Lucigenin caused a concentration-dependent increase in superoxide production by xanthine oxidase plus xanthine. This was seen, in terms of superoxide dismutase-inhibitable reduction of cytochrome c; in spite of the ability of univalently reduced lucigenin to directly reduce cytochrome c. It follows that in the absence of this interference, by the cytochrome, an even greater increase in superoxide production mediated by lucigenin would have been observed. Clearly lucigenin luminescence should not be relied upon as a method for measurement of, or even for detection of, superoxide.

The familial amyotrophic lateral sclerosis-associated amino acid substitutions E100G, G93A, and G93R do not influence the rate of inactivation of copper- and zinc-containing superoxide dismutase by H2O2

Inactivation of copper- and zinc-containing superoxide dismutase (Cu,ZnSOD) by H2O2 is the consequence of several sequential reactions: reduction of the active site Cu(II) to Cu(I) by H2O2; oxidation of the Cu(I) by a second H2O2, thus generating a powerful oxidant, which may be Cu(I)O or Cu(II)OH or Cu(III); and finally oxidation of one of the histidines in the ligand field, causing loss of SOD activity. Three familial amyotrophic lateral sclerosis (FALS)-associated mutant Cu,ZnSODs, i.e., E100G, G93A, and G93R, did not differ from the control enzyme in susceptibility to inactivation by H2O2. It thus appears that an increased peroxidase activity of the FALS-associated Cu,ZnSOD variants might not be a factor in the development of this disease. This leaves the loss of Zn, and the consequent increase in peroxidase activity, or in nitration activity, as a viable explanation (J. P. Crow et al., 1997, J. Neurochem. 69, 1936-1944), among other possibilities.

Superoxide-dependent peroxidase activity of H48Q: a superoxide dismutase variant associated with familial amyotrophic lateral sclerosis

Approximately 20% of cases of familial amyotrophic lateral sclerosis are caused by dominant mutations in the Cu,Zn superoxide dismutase. One such mutant, in which histidine #48 has been replaced by glutamine (H48Q), exhibits a novel activity. It can react sequentially with O2- and H2O2 to generate a potent oxidant at its active site, possibly Cu(II)-OH, which then can oxidize urate to the corresponding radical. This O2- -dependent peroxidase activity exerted on a substrate peculiar to motor neurons may be the toxic gain of function which leads to the deleterious consequences of this mutation. G93A, G93R, and E100G were also examined and found not to exert this O2- -dependent peroxidase activity.

A mechanism for complementation of the sodA sodB defect in Escherichia coli by overproduction of the rbo gene product (desulfoferrodoxin) from Desulfoarculus baarsii

Overexpression of rbo in Escherichia coli prevents the inactivation of the [4Fe-4S]-containing fumarases that otherwise occurs in the sodA sodB strain. It similarly protects against the increased sensitivity toward H2O2, which is imposed by the lack of SOD A and SOD B. These results would be explained on the basis of scavenging of O-2 within the cells by RBO. This interpretation was supported by measurements of intracellular scavenging of O-2 by the lucigenin luminescence method. Since SOD activity could not be detected in dilute extracts, of the RBO-overexpressing sodA sodB strain, we propose that RBO catalyzes the reduction of O-2 at the expense of cellular reductants such as NAD(P)H. A similar mechanism may apply to other instances of complementation of SOD defects by non-SOD genes.

A potent superoxide dismutase mimic: manganese beta-octabromo-meso-tetrakis-(N-methylpyridinium-4-yl) porphyrin

Variously modified metalloporphyrins offer a promising route to stable and active mimics of superoxide dismutase (SOD). Here we explore bromination on the pyrroles as a means of increasing the redox potentials and the catalytic activities of the copper and manganese complexes of a cationic porphyrin. Mn(II) and Cu(II) octabrominated 5,10,15,20-tetrakis-(N-methylpyridinium-4-yl) porphyrin, Mn(II)OBTMPyP4+, and Cu(II)OBTMPyP4+ were prepared and characterized. The rate constants for the porphyrin-catalyzed dismutation of O2.- as determined from the inhibition of the cytochrome c reduction are k(cat) = 2.2 x 10(8) and 2.9 x 10(6) M(-1) s(-1), i.e., IC50 was calculated to be 12 nM and 0.88 microM, respectively. The metal-centered half-wave potential was E(1/2) = +0.48 V vs NHE for the manganese compound. Cu(II)OBTMPyP4+ proved to be extremely stable, while its Mn(II) analog has a moderate stability, log K = 8.08. Nevertheless, slow manganese dissociation from Mn(II)OBTMPyP4+ enabled the complex to persist and exhibit catalytic activity even at the nanomolar concentration level and at biological pH. The corresponding Mn(III)OBTMPyP5+ complex exhibited significantly increased stability, i.e., demetallation was not detected in the presence of a 400-fold molar excess of EDTA at micromolar porphyrin concentration and at pH 7.8. The beta-substituted manganese porphyrin facilitated the growth of a SOD-deficient strain of Escherichia coli when present at 0.05 microM but was toxic at 1.0 microM. The synthetic approach used in the case of manganese and copper compounds offers numerous possibilities whereby the interplay of the type and of the number of beta substituents on the porphyrin ring would hopefully lead to porphyrin compounds of increased stability, catalytic activity, and decreased toxicity.

Lucigenin luminescence as a measure of intracellular superoxide dismutase activity in Escherichia coli

Lucigenin and paraquat are similar in that each can be taken into Escherichia coli and can then mediate O2.- production by cycles of univalent reduction, to the corresponding monocation radical, followed by autoxidation. Thus, both compounds caused induction of enzymes that are regulated by the soxRS regulon. The lucigenin cation radical has the added property of reacting with O2.-, in a radical-radical addition, to yield an unstable dioxetane, whose decomposition yields light. Superoxide dismutases (SOD), by decreasing [O2.-], inhibit light production and to the same degree inhibit other O2.(-)-dependent reactions in the cell. Lucigenin luminescence was used to show that the levels of SOD in the parental strain provide approximately 95% protection of all O2.(-)-sensitive targets in E. coli. This degree of protection was so close to the limit of 100% that halving the parental level of [SOD], or increasing it 5-fold, had only marginal effects on the intensity of lucigenin-dependent luminescence.

How does superoxide dismutase protect against tumor necrosis factor: a hypothesis informed by effect of superoxide on "free" iron

The manganese-containing mitochondrial superoxide dismutase (MnSOD) is induced by TNF and protects against the necrotic effect of this cytokine. Yet TNF does not increase production of O2- in mitochondria. How is this to be reconciled? TNF is known to increase production of arachidonate, by activation of phospholipase A2 (PLA2). Arachidonate will be converted to the corresponding alkyl hydroperoxide by lipoxygenase. O2- increases "free" iron by oxidizing [4Fe-4S] clusters of dehydratases, such as aconitase. Ferrous iron in turn reacts with alkyl hydroperoxides, in an analogue of the Fenton reaction, to produce alkoxyl radicals: which can initiate the oxidation of polyunsaturated lipids by a free radical chain reaction. MnSOD protects against TNF by decreasing O2- attack on [4Fe-4S] clusters and thus lowering free iron. Inhibitors of PLA2 and of lipoxygenase should also protect by decreasing fatty acyl hydroperoxides and they are known to do so. Cells having little mitochondrial MnSOD, or cells unable to induce that defensive enzyme in response to TNF, will consequently have relatively high levels of "free" iron in that organelle; leading to enhanced lipid peroxidation. Such cells will be preferentially killed by this cytokine.

Lucigenin (bis-N-methylacridinium) as a mediator of superoxide anion production

Lucigenin (bis-N-methylacridinium) (Luc2+) has frequently been used for the luminescent detection of O2-. In fact, the pathway leading to this luminescence requires univalent reduction of Luc2+ to LucH.+ followed by formation of an unstable dioxetane by reaction of LucH.+ with O2-. It is now shown that LucH.+ rapidly autooxidizes and so produces O2-. Luc2+ can thus mediate O2- production in systems, such as glucose plus glucose oxidase, in which there is ordinarily no O2- production. Luc2+ luminescence can thus be used as the basis for assaying superoxide dismutase activity but should not be used for measuring, or even detecting, O2-.

Superoxide dismutase activity in the giant hemoglobin of the earthworm, Lumbricus terrestris

The giant hemoglobin, which occurs in free solution in the blood of earthworms, contains copper and zinc and exhibits superoxide dismutase activity as assessed by competition assays using cytochrome c or nitroblue tetrazolium. On a molar basis, the activity of this hemoglobin was approximately 10% that of the mammalian copper, zinc superoxide dismutase. The dodecamer, which contains the globin chains but lacks the linker subunits, had very little activity when compared to the complete native molecule. This suggests that the superoxide dismutase activity resides in one of the linker subunits. This is the first report of a superoxide dismutase bound to a respiratory pigment.